RNIA 
ERi^ 
BERKELEY,  C,  MIA 


UNIVERSITY  OF  CALIFORNIA 

DEPARTMENT   OF   CIVIL   ENGINEERING 

BERKELEY.  CALIFORNIA 


WORKS   OF  DR.  H.  W.  SCHIMPF 


PUBLISHED   BY 


JOHN  WILEY  &  SONS,  INC. 


A  Manual  of  Volumetric  Analysis. 

For  the  use  of  Pharmacists,  Sanitary  and  Food 
Chemists,  as  well  as  for  Students  in  these 
Branches.  Fifth  edition,  rewritten  and  en- 
larged. 8vo,  xx  +  12  5  pages.  102  figures. 
Cloth,  $4.50  net. 


Essentials  of  Volumetric  Analysis. 

An  introduction  to  the  subject  adapted  to  the 
needs  of  Students  of  Pharmaceutical  Chemistry. 
Second  edition,  rewritten.  Small  8vo,  xi+358 
pages,  6 1  figures.  Cloth,  $1.50  net. 

A    Systematic   Course    of    Qualitative    Chemical 
Analysis  of  Inorganic  and  Organic  Substances 

With  Explanatory   Notes.       Second    Edition. 
8vo,  viii  +  iso  pages.     Cloth,  $1.25  net. 


A    MANUAL 


OF 


VOLUMETRIC  ANALYSIS 

FOR  THE  USE  OF 

PHARMACISTS,   SANITARY    AND  FOOD   CHEMISTS 

AS    WELL    AS   FOR 

STUDENTS  IN  THESE  BRANCHES 


BY 

HENRY  W.   SCHIMPF,    Pn.G.,   M.D. 

Professor  of  Analytical  Chemistry,  Brooklyn  College  of  Pharmacy;    Member  of  the 

American  Chemical  Society;    of  the  American  Association  for  the  Advancement 

of  Science;    of  the  American  Pharmaceutical  Association;    of  the  New  York 

State  Pharmaceutical  Association:  of  the  King's  County  Pharmaceutical 

Society;    of  the  German  Apothecaries'  Society  of  New  York  City; 

Honorary  Member  of  the  Alumni  Association  of  the  Brooklyn 

College  of  Pharmacy,  etc.,  etc. 


With  (One  fjfwttbrtb  anb    tiuui  illustrations 


FIFTH  EDITION,  REVISED  AND  ENLARGED 
SECOND  THOUSAND 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 
LONDON:  CHAPMAN  &  HALL,  LIMITED 


S  3 


Engineerinj 
Libr&ry 
Authority   to  use  for  comment  the  Pharmacopoeia  of  the 

United  States  of  America,  Eighth  Decennial  Revision,  in  this 
volume,  has  been  granted  by  the  Board  of  Trustees  of  the 
United  States  Pharmacopceial  Convention,  which  Board  of 
Trustees  is  in  no  way  responsible  for  the  accuracy  of  any 
translations  of  the  official  weights  and  measures  or  for  any 
statements  as  to  strength  of  official  preparations. 


Copyright,   1893,   1898,   1909 

BY 
HENRY  W.  SCHIMPP 


THE  SCIENTIFIC  PRESS 

ROBERT  DRUMMOND  AND  COMPANY 

BROOKLYN,  N.  Y. 


PREFACE   TO   THE   FIFTH  EDITION 


THE  remarkable  development  of  volumetric  analysis  during  the 
past  decade  has  necessitated  a  complete  revision  of  my  Text-Book 
of  Volumetric  Analysis.  The  present  volume,  which  is  called  "A 
Manual  of  Volumetric  Analysis,"  has  been  much  enlarged  and  is 
almost  entirely  rewritten.  » 

An  effort  has  been  made  to  bring  it  abreast  of  the  times,  in-so-far 
as  methods  of  analysis  are  concerned.  The  book  deals  chiefly  with 
pharmaceutical  and  food  analysis,  and  it  is  hoped  that  chemists 
practising  along  these  lines,  as  well  as  teachers  and  students,  will 
find  it  of  value. 

The  volumetric  processes  described  are  mostly  such  as  have  been 
tried  and  found  of  value. 

The  atomic  weights  used  are  those  of  the  United  States  Phar- 
macopoeia (based  on  H.  =  i);  but  for  the  convenience  of  chemists 
who  prefer  to  work  with  the  atomic  weights  based  on  O=i6,  a  table 
of  the  International  Atomic  Weights  is  introduced  at  the  front  of 
the  book,  where  it  can  easily  be  found.  One  hundred  and  fourteen 
pages  are  devoted  to  alkaloidal  drug  assaying. 

Copious  bibliographical  references  are  given,  and  much  care 
has  been  taken  to  make  the  index  complete  and  comprehensive. 

The  author  has  given  credit  wherever  it  is  due,  and  has  in  no  case 
intentionally  neglected  this  duty. 

In  the  preparation  of  this  edition  the  author  has  consulted,  besides 
those  works  acknowledged  in  the  former  editions,  the  following: 
The  Journal  of  the  American  Chemical  Society,  Chemical  Abstracts, 

American  Journal  of  Pharmacy,  Proceedings  of  the  American  Phar- 

iii 


iy  PREFACE 

maceutical  Association,  Publications  of  the  United  States  Depart- 
ment of  Agriculture,  The  United  States  Pharmacopoeia,  The  Phar- 
maceutical Review,  Fresenius'  Quantitative  Chemical  Analysis, 
Newth's  Chemical  Analysis,  Lyons'  Assay  of  Drugs,  Holland's 
Medical  Chemistry  and  Toxicology.  The  author  also  acknowledges 
his  indebtedness  to  Dr.  Joseph  L.  Mayer,  for  his  assistance  in  revising 
the  chapters  on  Water  and  Milk  analyses. 

H.  W.  S. 


TABLE  OF  CONTENTS 


PAGE 

ABBREVIATIONS ' xvii 

LIST  OF  ELEMENTS  AND  THEIR  ATOMIC  WEIGHTS xix 

TABLE  OF  MULTIPLES  OF  ATOMIC  WEIGHTS,  ETC zx 


PART  I 

CHAPTER  I 


INTRODUCTION 


CHAPTER  II 
GENERAL  PRINCIPLES 4 

CHAPTER  III 
VOLUMETRIC  OR  STANDARD  SOLUTIONS 6 

CHAPTER  IV 

INDICATORS 12 

lonization  Theory,  13.  Chromophoric  Theory,  14.  Glaser's  Classi- 
fication of  Indicators,  17.  Requirements  of  a  Good  Indicator,  18. 
Alizarin,  19.  Azolitmin,  19.  Brazil-wood,  19.  Cochineal,  19.  Congo- 
red,  19.  Corallin,  20.  Eosin,  20.  Gallein,  20.  Hacmatoxylin,  20. 
lodeosin,  21.  Lacmoid,  21.  Litmus,  22.  Luteol,  24.  Methyl-orange, 
24.  Phenacetolin,  26.  Phenolphthalein,  26.  Poirrier  blue,  C4B,  27. 
Potassium  Chromate,  27.  Potassium  Ferricyanid,  27.  Resazurin,  28. 
Rosolic  Acid,  28.  Tropacolin  O.  O.,  29.  Turmeric,  29.  Starch,  29. 

A  Guide  for  the  Selection  of  Indicators,  30. 

CHAPTER  V 

APPARATUS  USED  IN  VOLUMETRIC  ANALYSIS 32 

v 


TABLE    OF    CONTENTS 


CHAPTER  VI 

PA.OE 

ON  THE  USE  OF  APPARATUS 44 

The  Reading  of  Instruments,  46.     The  Calibration  of  Instruments,  49. 


CHAPTER  VII 
WEIGHTS  AND  MEASURES  USED  IN  VOLUMETRIC  ANALYSIS 51 

CHAPTER  VIII 

METHODS  OF  CALCULATING  ANALYSES 53 

Rules  for  Direct  Percentage  of  Estimations,  53.  Factors  or  Coefficients 
for  Calculating  the  Analyses,  55.  Table  of  Normal  Factors,  etc.,  for  the 
Alkalies,  Alkali  Earths,  and  Acids,  57. 

CHAPTER  IX 

SOME  VICARIOUS  VOLUMETRIC  METHODS 58 

Volumetric  Analysis  without  Weights  and  Standard  Solutions,  58. 
Volumetric  Analysis  without  a  Burette,  59. 

CHAPTER  X 

NEUTRALIZATION  ANALYSIS 62 

Alkalimetry,  64.  Preparation  of  Standard  Acid  Solutions,  64.  Estima- 
tion of  Alkali  Hydroxids,  74.  Estimation  of  Alkali  Carbonates,  77. 
Estimation  of  Mixed  Alkali  Hydroxid  and  Carbonate,  81.  Estimation 
of  Mixed  Bicarbonates  and  Carbonates,  82.  Estimation  of  Mixed  Potas- 
sium and  Sodium  Hydroxids,  83.  Estimation  of  Organic  Salts  of  the 
Alkalies,  83.  Table  Showing  Normal  Factors,  etc.,  of  the  Organic  Salts 
of  the  Alkalies,  89.  Estimation  of  Alkali  Metals  in  their  Salts,  89. 
Estimation  of  Alkalies  Combined  with  Volatile  Acids,  oo.  Estimation 
of  Alkalies  Combined  with  Non-volatile  Acids,  90.  Estimation  of  Salts 
of  the  Alkali  Earths,  91.  Preparation  of  Normal  Sodium  Carbonate 
V.  S.,  92.  Estimation  of  Mixed  Hydroxids  and  Carbonates  of  the  Alkali 
Earths,  93.  Table  Showing  Normal  Factors,  etc.,  of  the  Alkali 
Earths,  94. 

Acidimetry,  94.  Preparation  of  Standard  Alkali  Solutions,  96. 
Weighing  Pipettes,  100.  Estimation  of  Inorganic  Acids,  99.  Estimation 
of  Organic  Acids,  104.  Table  of  Acids  which  may  be  Estimated  by 
Neutralization,  106.  Estimation  of  Acids  in  Combination  in  Neutral 
Salts,  108. 

Table  Showing  Quantity  of  Substance  to  be  Taken  for  Analysis  in  Direct 
Percentage  Estimations,  107. 


TABLE    OP    CONTENTS  yii 

CHAPTER  XI 

PAGB 
ANALYSIS  BY  PRECIPITATION no 

Precision  in  Determining  End-reactions  (Rakow),  in.  Preparation  of 
Standard  Silver  Nitrate  V.  S.,  112.  Preparation  of  Standard  Sodium 
Chlorid  V.  S.,  113.  Preparation  of  Standard  Potassium  Sulphocyanate 
V.  S.,  114. 

Estimation  of  Haloid  Salts  and  Haloid  Acids,  115.  Titration  without 
an  Indicator  (Gay-Lussac),  120.  Titration  with  Chromate  Indicator 
(Mohr),  115.  Titration  by  Volhard's  Method,  122. 

Estimation  of  Cyanogen,  125.  Titration  with  Standard  Silver  V.  S.  to 
First  Appearance  of  a  Precipitate  (Liebig),  125.  Titration  with  Standard 
Silver  V.  S.,  using  lodid  as  an  Indicator  (Deniges),  128.  Titration  with 
Standard  Silver  V.  S.,  using  Chromate  as  Indicator  (Vielhaber),  127. 

Estimation  of  Silver  -Salts,  130.  Titration  with  Standard  Sodium 
Chlorid  V.  S.,  130.  Titration  by  Sulphocyanate  Method,  131.  Estima- 
tion of  Metallic  Silver  in  Alloy,  132.  Analysis  of  Certain  Neutral  Salts 
by  Conversion  into  Chlorids,  132. 

Estimation  of  Alkali  lodids  by  Mercuric  Chlorli  V.  S.  (Personne),  134. 

Table  of  Substances  Estimated  by  Precipitation,  135. 

CHAPTER  XII 

ANALYSIS  BY  OXIDATION  AND  REDUCTION 137 

Analysis  by  Means  of  Potassium  Permanganate  V.  S.,  141.  Prepara- 
tion of  Standard  Potassium  Permanganate  V.  S.,  142.  On  the  Use  of 
Empirical  Potassium  Permanganate  V.  S.,  148.  Typical  Analyses  with 
Potassium  Permanganate  V.  S.,  150.  Direct  Titrations,  150.  Estima- 
tion of  Ferrous  Salts,  150.  Estimation  of  Metallic  Iron  in  Ferrum 
Reductum,  152.  Estimation  of  Oxalic  Acid  and  Oxalates,  153.  Esti- 
mation of  Hydrogen  Dioxid,  156.  Estimation  of  Ferric  Salts  after 
Reduction,  162.  Estimation  of  Nitrous  Acid  and  Nitrites,  165.  Residual 
Titrations,  167.  Estimations  in  which  the  Excess  of  Permanganate 
is  Found  by  Residual  Titration  with  Oxalic  Acid,  167.  Estimation  of 
Hypophosphites,  167.  Methods  Involving  a  Precipitation  by  Oxalic 
Acid  and  the  Titration  of  the  Excess  by  Permanganate,  170.  Estimation 
of  Calcium  Salts,  170.  Methods  Involving  a  Reduction  by  Means  of 
Oxalic  Acid  and  the  Titration  of  the  Excess  of  Permanganate,  171.  Esti- 
mation of  Manganese  Dioxid,  171.  Methods  Involving  a  Reduction  by 
Means  of  a  Ferrous  Salt,  and  Titration  of  the  Excess  by  Permanganate, 
172.  Estimation  of  Nitrates  (Pelouze),  172.  Estimation  of  Chromic 
Acid  and  Chromates,  174.  Methods  Involving  an  Oxidation  by  Means  of 
a  Ferric  Salt,  and  Titration  of  the  Resultant  Ferrous  Salt  by  Means  of 
Permanganate,  177.  Estimation  of  Tin  (Lowenthal),  177.  Estimation 
of  Copper  (Fleitmann),  177. 

Analyses  by  Means  of  Potassium  Bichromate  V.  S.,  178.  Preparation 
of  Standard  Potassium  Dichromate  V.  S.,  179.  Estimation  of  Ferrous 


Viii  TABLE    OF    CONTENTS 

PAGE 

Salts,  181.  Table  of  Substances  which  may  be  Estimated  by  Means  of 
Potassium  or  Permanganate  Dichromate,  185. 

Analysis  by  Indirect  Oxidation,  185.  Preparation  of  Standard  lodin 
V.  S.,  186.  Preparation  of  Starch  Indicator,  189.  On  the  Use  of  Sodium 
Bicarbonate  in  Titrations  with  lodin,  190.  Estimation  of  Arsenous 
Compounds,  192.  Estimation  of  Antimonous  Compounds,  195.  Esti- 
mation of  Sulphurous  Acids  and  Sulphites,  197.  Table  of  Substances 
which  may  be  Estimated  by  Means  of  Standard  lodin  V.  S.,  201. 

Estimation  of  Substances  Readily  Reduced,  202.  Determinations  In- 
volving the  Use  of  Sodium  Thiosulphate  (lodometry),  202.  Preparation 
of  Standard  Sodium  Thiosulphate  V.  S.,  203.  Estimation  of  Free  lodin, 
207.  Estimation  of  Free  Chlorin  or  Bromin,  209.  Estimation  of  Hypo- 
chlorites,  210.  Table  Showing  Degrees  Gay-Lussac,  212.  Estimation 
of  Hydrogen  Dioxid,  213 

Distillation  Methods,  214.  Estimation  of  Manganese  Dioxid,  218. 
Estimation  of  Chromic  Acid  and  Chromates,  219.  Estimation  of  Alkali 
lodids,  220. 

Digestion  Methods,  222.  Estimation  of  Chlorates,  Bromates,  and 
lodates,  223.  Estimation  of  Ferric  Salts,  224. 

Reduction  Methods  Involving  the  Use  of  Standard  Arsenous  Acid  V.  S., 
226. 

Preparation  of  Standard  Arsenous  Acid  V,  S.,  227.  Estimation  of  Free 
Halogens,  228.  Estimation  of  Hypochlorites,  230. 

Reduction  Methods  Involving  the  Use  of  Stannous  Chlorid  Solution,  231. 
Estimation  of  Iron,  231.  Estimation  of  Mercuric  Salts  (Laborde),  233. 


PART  II 

CHAPTER  XIII 
ACETIC  ACID  AND  ACETATES.    ACETIC  ACID  TABLE 235 

CHAPTER  XIV 
BORIC  ACID  AND  BORATES 240 

CHAPTER  XV 

CARBONIC  ACID  AND  CARBONATES 244 

Carbonic  Acid  Gas  in  the  Atmosphere,  246. 

Table  Showing  Volume  of  o.ooi  gm.  of  CO2  at  Various  Temperatures, 
249.  Carbonic  Acid  in  Mineral  Waters,  251.  Estimation  of  Small 
Quantities  of  Carbon  Monoxid,  258. 


TABLE    OF    CONTENTS 


CHAPTER  XVI 

PAGE 

CHLORIN,  BROMIN,  AND  IODIN 260 

Chlorates,  Bromates,  and  lodates,  262.     Bromids  or  lodids  by  Direct 
Titration  with  Chlorin  Water,  265. 


CHAPTER  XVII 
CITRIC  ACID  AND  CITRATES : 270 

CHAPTER  XVIII 

CYANOGEN  AND  ITS  COMPOUNDS ayt 

Titration  with  Standard  Silver  Solution,  273.  Titration  with  Standard 
lodin  V.  S.,  274.  Titration  with  Standard  Mercuric  Chlorid  V.  S.,  275. 
Titration  with  Standard  Copper  Solution,  275.  By  the  Modified  Kjeldah! 
Method,  276.  The  lodometric  Method  for  Ferricyanids,  280. 

CHAPTER  XIX 

NITROGEN  AND  ITS  COMPOUNDS 283 

Method  of  Will  and  Varrentrapp,  283.  The  Ruffle  Method,  286.  The 
Kjeldahl  Method  (Modified),  287.  The  Jodlbauer-Kjeldahl  Method, 
290.  Nitric  Acid  and  Nitrates,  294.  The  Harcourt  Method,  296.  The 
Ulsch  Method,  298.  The  Street-Ulsch  Method,  298.  The  Pelouze 
Method,  299.  The  Schlosing  Method,  299.  The  lodometric  Estimation 
(McGowan),  301.  Estimation  of  Nitrous  Acid  by  KMnO4,  303. 
Estimation  of  Nitrous  Acid  lodometric,  303. 

CHAPTER  XX 
OXALIC  ACID  AND  OXALATES , 305 

CHAPTER  XXI 

OXYGEN  AND  THE  PEROXIDS 307 

Estimation  of  Oxygen  Dissolved  in  Water,  307.  Hydrogen  Dioxid, 
309- 

CHAPTER  XXII 

PHOSPHORIC  ACID  AND  PHOSPHATES , 311 

Stolba's  Method,  311.  By  Uranium  Solution,  312.  Segalle-Gluck- 
mann  Method,  312.  Pemberton's  Molybdic  Method,  316.  Pemberton's 
New  Method  (McDonald),  317.  Richardson's  Method,  320.  Estima- 
tion of  Mixed  Disodium  and  Trisodium  Phosphate  (Ahlum),  320. 


X  TABLE   OP    CONTENTS 

CHAPTER  XXIII 

PAGE 

SALICYLIC  ACID  AND  SALICYLATES 324 

CHAPTER  XXIV 

SULPHUR  AND  ITS  COMPOUNDS 326 

Estimation  of  Sulphur  in  Alkali  Sulphid,  327.  Estimation  of  Hydro- 
sulphuric  Acid,  328.  Estimation  by  Permanganate  (Mohr),  328.  Esti- 
mation by  lodin,  329.  Estimation  by  Arsenous  Acid  (Mohr),  329. 
Estimation  by  Silver  Nitrate,  331.  Estimation  of  Sulphurous  Acid  and 
Sulphites,  331.  Estimation  of  Sulphuric  Acid  and  Sulphates,  331. 

CHAPTER  XXV 
ALUMINUM 335 

CHAPTER  XXVI 
AMMONIUM 338 

CHAPTER  XXVII 
ANTIMONY 340 

CHAPTER  XXVIII 
ARSENIC 346 

CHAPTER  XXIX 
BARIUM 353 

CHAPTER  XXX 
BISMUTH 355 

CHAPTER  XXXI 
CALCIUM 359 

CHAPTER  XXXII 
COPPER 362 

CHAPTER  XXXIII 
GOLD 378 


TABLE    OF    CONTENTS  xi 

CHAPTER  XXXIV 

PAGE 

IRON 379 

CHAPTER  XXXV 
LEAD 3»8 

CHAPTER  XXXVI 
MAGNESIUM 394 

CHAPTER  XXXVII 
MANGANESE 397 

CHAPTER  XXXVIII 
MERCURY 408 

CHAPTER  XXXIX 
SILVER 417 

CHAPTER  XL 
STRONTIUM 419 

CHAPTER  XLI 
TIN , 421 

CHAPTER  XLII 
ZINC 425 

PART   III 

SANITARY    ANALYSES     AND    VOLUMETRIC     ANALYSES 
OF    ORGANIC    MEDICINAL    SUBSTANCES 

CHAPTER  XLIII 

SANITARY  ANALYSIS  OF  WATER 437 

Collection  of  Sample,  437.  Color,  437.  Odor,  438.  Reaction,  438. 
Suspended  Matter,  438.  Total  Solids,  438.  Organic  and  Volatile 
Matter  or  Loss  on  Ignition,  439.  Chlorin,  439.  Ammonia,  440. 
Nessler's  Solution,  440.  Albuminoid  Ammonia,  443.  Nitrates,  444. 
Nitrites,  445.  Oxygen-consuming  Power,  446.  Phosphates,  447. 
Hardness,  Temporary  and  Permanent,  448.  Interpretation  of  Results, 


xii  TABLE    OF    CONTENTS 

CHAPTER  XLIV 

PAGE 

MILK 457 

Average  Composition,  457.  Colostrum,  458.  Reaction,  458.  Specific 
Gravity,  458.  Lactometer,  459.  Table  for  Correcting  the  Speciffc 
Gravity  of  Milk  according  to  Temperature,  460.  Adulterations  of 
Milk,  459.  Total  Solids  and  Water,  459.  Fat,  Adam's  Method,  461. 
Werner-Schmid  Method,  462.  Babcock  Method,  462.  Calculation 
Method,  463.  Calculation  of  Per  Cent  of  Added  Water,  464.  Total 
Proteids,  465.  Milk  Sugar,  466. 

CHAPTER  XLV 

BUTTER 467 

Estimation  of  Volatile  Acids  (Reichert),  467.  Estimation  of  Volatile 
Acids  (A.  O.  A.  C.),  467. 

CHAPTER  XLVI 

SOME  TECHNICAL  EXAMINATION  METHODS  FOR  FATS,  OILS,  AND  WAXES  . . .  472 

The  Acid  Value  or  Proportion  of  Free  Fatty  Acids,  472.  The  Saponi- 
fication  Value  (Kattstorfer's  Number),  472.  The  Volatile  Fatty  Acid 
Value  (Reichert's  Number),  474.  The  lodin  Absorption  Number 
(HiibFs  Number),  474  The  Hanus  Method,  476.  The  Wijs  Method, 
477.  Table  Showing  lodin  Absorption  by  the  Three  Methods,  477. 
The  Bromin  Absorption  Number,  478. 

CHAPTER  XLVII 

ANALYSIS  OF  SOAP 480 

Free  and  Combined  Alkali  (Geisler),  480.  Free  Alkali  (Devine),  482. 
Glycerin  (Martin),  482. 

CHAPTER  XLVIII 

ESTIMATION  OF  STARCH  IN  CEREALS,  ETC 484 

Estimation  after  Conversion  into  Glucose  by  Heating  with  HC1,  486. 
Lietz's  Method,  486.  Estimation  after  Inversion  by  Means  of  Diastase, 
487. 

CHAPTER  XLIX 

ESTIMATION  OF  SUGARS 489 

By  Fehling's  Solution,  489.  Various  Methods  of  Determining  End- 
reactions  by  Pavy's  Solution>  493.  By  the  Soxhlet-Fehiing  Method,  494. 
By  the  Permanganate  Method,  495.  Methods  Depending  upon  the 
Reduction  of  Mercury,  Knapp's  Method,  495.  Sachsse's  Method,  496. 


TABLE    OF    CONTENTS  xiii 

CHAPTER  L 

PAGE 

VOLUMETRIC  ESTIMATION  OF  ALKALOIDS 498 

Table  Showing  the  Behavior  of  Some  of  the  Alkaloids  with  Indicators, 
503.  Table  Showing  Factor  for  Various  Alkaloids  when  Titrating  with 

N 

—   Acid  V.  S.,  504.     Estimation  by  Mayer's  Reagent,  505.     Estimation 

10 

of  Alkaloids  by  Wagner's  Reagent,  507.  Estimation  of  Caffeine  by 
Wagner's  Reagent,  509.  Gordin's  Modified  Alkalimetric  Method,  using 
Phenlophthalein  as  Indicator,  510. 

CHAPTER  LI 

VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS 512 

Extraction  of  the  Alkaloids,  512.  Alkaloidal  Assay  by  Immiscible  Solv- 
ents, 512  General  Methods  of  Assaying  Drugs,  515.  Keller  Method, 
515.  Puckner  Method,  516.  Lyons  Method,  516.  Gordin  Method, 
517.  Kebler-Keller  Method,  517.  Gordin  and  Prescott  Method, 

518- 

The  Assaying  of  Crude  Drugs,  520.  Aconite  Root  (Keller),  520. 
Aconite  Leaves:  Keller,  521;  German  Pharmacopoeia,  521.  Aconite 
Root:  U.  S.  P.  VIII,  522;  Gordin,  525;  Kippenberger,  526;  Fromme, 

527- 

The  Mydriatic  Drugs,  527.  Belladonna:  Puckner  (U.  S.  P.  VIII), 
527;  Kebler,  528;  Gordin  and  Prescott,  529.  Hyoscyamus  (Keller),  529. 
Scapola  and  Stramonium,  530.  Cinchona:  U.  S.  P.  (1890),  531;  Gordin, 
532;  Kippenberger,  533;  German  Pharmacopoeia,  533;  Florence,  534; 
Fromme,  535.  Coca:  U.  S.  P.  VIII,  538;  Kebler-Keller,  538;  Lyons, 
538;  Squibbs,  539;  Leger,  541.  Colchicum:  Gordin  and  Prescott,  542; 
Panchaud,  543.  Conium:  Lyons,  544;  U.  S.  P.  VIII,  544.  Hydrastis 
Canadensis:  U.  S.  P.  VIII,  546;  Puckner,  546;  Gordin  and  Prescott, 
547;  Schreiber,  549.  Ipecac:  U.  S.  P.  VIII,  552;  Gordin,  552;  Fromme, 

553- 

Ipecac.  Separate  Estimation  of  Emetine  and  Cephaeline:  Patterson,  553; 
Paul  and  Cownley,  554.  Ipecac  (Frerichs  and  Tapis),  555.  Nux  Vomica, 
556. 

Methods  in  "which  the  "Total  Alkaloids"  is  Determined:  German 
Pharmacopoeia,  556;  Puckner,  557;  Gordin,  558;  Kippenberger,  558. 

Methods  in  which  Strychnine  is  Determined:  Keller,  559;  U.S.  P.,  559; 
Webster  and  Pursel,  561;  Gordin,  561;  The  Ferrocyanid  Methods,  562. 
Opium:  U.  S.  P.  (1890),  564;  U.  S.  P.  VIII,  565;  Lamar,  567;  Mai-  . 
linckrodt,  567;  Parker,  568;  Gordin  and  Prescott,  569;  German  Phar- 
macopoeia, 570;  Stevens,  571;  Ascher,  572;  Picard-Leger,  573;  Tickle, 
574.  Physostigma:  U.  S.  P.  VIII,  576;  Beckurts,  576.  Pilocarpus, 
U.  S.  P.  VIII,  578;  Fromme,  578.  Strophanthus  (Barclay),  579.  Tor 
bacco  (Keller),  580.  Veratrum  (Bredemann),  581.  Wild  Cherry  Bark 
(Stevens),  583. 


Xiv  TABLE    OF    CONTENTS 


CHAPTER  LII 

PAGE 

ASSAY  OF  GALENICAL  PREPARATIONS  ...    584 

Lloyd's  Methods,  584.  Lyons'  Method,  585.  Steiglitz's  Method,  586. 
Thompson's  Method,  586.  Katz's  Method,  586.  Kippenberger' 
Method,  587.  Farr  and  Wright's  Method,  590.  Webster's  Method.  590 
Fluid  Extract  of  Aconite,  592.  Tincture  of  Aconite,  593.  Extract  of 
Belladonna,  593.  Fluid  Extract  of  Belladonna,  594.  Tincture  of  Bella 
donna,  595.  Puckner's  Method,  595.  Lyons'  Method,  595.  Thorn's 
Bismuth-iodid  Method,  598.  Belladonna  Plaster  (U.  S.  P.  VIII),  598. 
German  Pharmacopoeia  Methods,  599.  Fluid  Extract  of  Cinchona,  600. 
Fluid  Extract  of  Coca,  601.  Fluid  Extract  of  Gelsemium  (Sayre),  601. 
Fluid  Extract  of  Hydrastis:  U.  S.  P.  VIII,  602;  Heyl,  603;  Rusting- 
Smeets,  603.  Tincture  of  Hydrastis,  603.  Fluid  Extract  of  Ipecac,  604, 
Extract  of  Nux  Vomica:  U.  S.  P  (1890),  605;  U.  S.  P.  VIII,  606.  Ex- 
tract of  Opium,  608.  Tincture  of  Opium,  609.  Extract  of  Physostigma, 
610.  Fluid  Extract  of  Pilocarpus,  611.  Alkaloidal  Assay  of  Scale  Salts, 
612. 

CHAPTER  LIII 

PHENOL 613 

Preparation  of  Standard  Bromin  V.  S.,  613.  Lloyd's  Hypobromite 
Method,  617.  Waller's  Method,  618.  Crude  Carbolic  Acid,  619. 
Potassium  Permanganate  Method  (Tocher),  620.  Estimation  of  Phenol 
in  Pharmaceutical  Products,  621. 


CHAPTER  LIV 

GLYCERIN 623 

The  Permanganate  Method  (Benedikt  and  Zsigmondy),  623.  Estima- 
tion in  Fats,  624.  Herbig  and  Mangold's  Method,  625.  The  Acetin 
Method  (Benedikt  and  Cantor),  625.  The  Bichromate  Method  (Heh- 
ner),  627.  The  lodic  Acid  Method  (Chaumeil),  628.  Estimation  in 
Fluid.  Extracts,  628. 

CHAPTER  LV 

TANNIN 630 

Lowenthal's  Method,  631.  Fleury's  Method,  634.  Thompson's 
Method,  using  H2O2,  634.  Lorenz's  Method,  using  Safranin,  635. 
Vignon's  Method,  using  Silk,  635.  Tannin  in  Wines,  636.  Tannin  in 
Tea,  636.  Tannin  in  Cloves  and  Allspice,  637. 


CHAPTER  LVI 

FORMALDEHYDE 639 

The  Ammonia  Method:     Legler,  639;     Craig,  640.     The  Ammonium 
Chlorid  Method  (Schiff),  640.     The  Hydrogen  Dioxid  Method  (Blank 


TABLE  OF  CONTENTS  XV 

PAGB 

and  Finkenbeiner),  641.  The  lodiometric  Method:  Ramijn,  642; 
Reuter,  642;  Taylor,  642.  The  Potassium  Cyanid  Method:  Romijn, 
643;  Smith,  643.  The  Phloroglucin  Method  (Clowes  and  Cullens),  644. 

CHAPTER  LVII 
CHLOROFORM  AND  CHLORAL  HYDRATE 646 

CHAPTER  LVIII 

ASSAYING  SURGICAL  DRESSINGS  .  .    . ,. 649 

Carbolic  Acid  Dressings  (Meissinger  and  Vortmann)  649.  Salicylic 
Acid  Dressings,  651.  Boric  Acid  Dressings,  651.  Sublimate  Dressings: 
Denner,  651;  Link  and  Vaswinkel,  652;  Beckurts,  653.  lodoform 
Dressings,  653.  Styptic  Cotton  Dressings  (Parker),  655. 

CHAPTER  LIX 

ESTIMATION  OF  COMPOUND  ETHERS 656 

Spirit  of  Nitrous  Ether,  657. 

CHAPTER  LX 

URINE 660 

Reaction,  660.  Composition,  660.  Specific  Gravity,  66 r.  Total 
Solids,  662.  Chlorids,  663.  Phosphates,  663.  Sulphates,  664.  Total 
Acidity,  665.  Joulie's  Method,  665.  Urea  (Benedict  and  Gephart), 
667.  Uric  Acid,  667.  Uric  Acid:  Bartley's  Method,  668;  lodic  Acid 
Method  (Merck),  670;  lodic  Acid  Method  (Bouillet),  671.  Albumen, 
672.  Blood,  674.  Pus,  674.  Sugar,  674.  Bile,  677. 


PART  IV 
A   FEW   GASOMETRIC   METHODS 

CHAPTER  LXI 

THE  NITROMETER 678 

Charles'  Law,  679.  Boyle's  Law,  680.  Table  of  Factors  for  Tem- 
perature Corrections,  68  r.  Table  of  Factors  for  Barometric  Corrections. 
681. 


XVi  TABLE  OF  CONTENTS 

CHAPTER  LXII 

PAGB 

ASSAY  OF  NITRITES    683 

Spirit  of  Nitrous  Ether,  683.     Amyl  Nitrite,  685.     Sodium  Nitrite,  685. 
Nitric  Acid  in  Nitrates,  686. 

CHAPTER  LXIII 

ASSAY  OF  HYDROGEN  DIOXID 687 

By  Squibb's  Urea  Apparatus,  687.     By  Improvised  Nitrometer,  687. 
The  Hypochlorite  Method,  688.     The  Hypobromite  Method,  689. 

CHAPTER  LXIV 

ESTIMATION  OF  SOLUBLE  CARBONATES 691 

Aromatic  Spirits  of  Ammonia,  691. 

CHAPTER  LXV 
ESTIMATION  OF  UREA  AND  URIC  ACID 693 


ABBREVIATIONS 

A.  J.  Ph American  Journal  of  Pharmacy. 

Am.  Chem.  J American  Chemical  Journal. 

Am.  Drug American  Druggist. 

A.  O.  A.  C Association  of  Official  Agricultural  Chemists. 

Apoth.  Ztg Apotheker  Zeitung. 

Archiv.  Ph Archives  der  Pharmacie. 

At.  wt atomic  weight. 

Ber.  Chem.  Ges Berichte   der  Deutschen   Chemischen   Gesell- 

schaft. 

Bull.  Soc.  Chim. Bulletin  de  la  Societe  Chimique. 

cc cubic  centimeter. 

Chem.  News Chemical  News. 

Chem.  Ztg Chemiker  Zeitung. 

Compt.  rend Comptes  rendus  des  Seances  de  P  Academic  des 

Sciences. 

Chem.  Centralbi Chemisches  Centralblatt. 

Cm centimeter. 

Drug.  Cir Druggists'  Circular. 

Gaz.  Ch.  It Gazzetta  chimica  Italiana. 

gm gram,  i5-43235  grains. 

gr grain. 

J.  A.  C.  S Journal  of  the  American  Chemical  Society. 

J.  S.  C.  I Journal  of  the  Society  of  Chemical  Industry. 

J.  Chem.  Soc Journal  of  the  Chemical  Society. 

J.  f.  prakt.  Ch Journal  fur  praktische  Chemie. 

J.  Anal.  Chem Journal  of  Analytical  Chemistry. 

J.  Pharm.  et  Chim Journal  de  Pharmacie  et  de  Chimie. 

Mg milligrams. 

Mm millimeter. 

Monatsheft Monatshefte  fur  Chemie. 

Ph.  Jour,  and  Trans Pharmaceutical  Journal  and  Transactions. 

Ph.  Centralh Pharmaceutische  Centralhalle. 

Ph.  Rev Pharmaceutical  Review. 


XVlll  ABBREVIATIONS 

Ph.  Post Pharmaceutical  Post. 

Ph.  Ger German  Pharmacopoeia. 

Ph.  Ztg Pharmaceutische  Zeitung. 

T.  S test  solution,  according  to  U.  S.  P. 

Trans.  Brit.  Ph.  Conf Transactions  of  the  British  Pharmaceutical 

Conference. 

U.  S.  P.  VIII United  States  Pharmacopoeia,  8th  Dec.  Re- 
vision. 

V.  S volumetric  solution. 

Zeitschr.  anal.  Chem Zeitschrift  fiir  analytische  Chemie. 

Zeitschr.  f .  Chem Zeitschrift  fur  Chemie. 

Zeitschr.  anorg.  Chem Zeitschrift  fiir  anorganische  Chemie. 

Zeitschr.  angew.  Chem Zeitschrift  fiir  angewandte  Chemie. 

Zeitschr.  physiol.  Chem Zeitschrift  fiir  physiologische  Chemie. 


MULTIPLES    OF    NUMBERS    REPRESENTING    SOME    ATOMIC 
WEIGHTS  AND  COMBINATIONS  IN  FREQUENT  USE. 

(Based  on  H  =  i). 


H 

1 

2 

3 

4 

5 

6 

7 

8 

9 

H 

O 

15.88 

31.76 

47-64 

63.52 

79.40 

95-28 

in.  16 

127.04 

142.92 

0 

OH 

16.88 

33.76 

50.64 

67-52 

84.40 

101  .28 

118.16 

135.04 

151.92 

OH 

H2O 

17.88 

35.76 

53.64 

71-52 

89.40 

107.28 

125.  16 

143-04 

160.92 

H2O 

N 

13-93 

27.86 

41-79 

55-72 

69.65 

83-58 

97-51 

111.44 

125.37 

N 

NHs 

16.93 

33.86 

50-79 

67.72 

84.65 

101.58 

118.51 

135-44 

152.37 

NHs 

NH4 

17-93 

35-86 

53-79 

71.72 

89.65 

107.58 

125.51 

143-44 

161.37 

NH« 

N03 

6i.S7 

123.14 

184.71 

246.28 

307.85 

369.42 

430.99 

492.56 

554-13 

N03 

c 

ii  .91 

23.82 

35-73 

47.64 

59-55 

71-46 

83-37 

95.28 

107-17 

C 

C02 

43-67 

87.34 

131.01 

174-68 

218.35 

262  .02 

305.69 

349.36 

393-03 

C02 

GOs 

59-55 

119.  10 

178.65 

238.20 

297-75 

357-30 

416.85 

476.40 

535.95 

C03 

CN 

25.84 

51.68 

77-52 

103.36 

129.20 

155-04 

180.88 

206.  72 

232.56 

CN 

Cl 

35-iS 

70.36 

105.54 

140.72 

175-90 

211  .08 

246.26 

281.44 

316.62 

Cl 

B 

79-36 

158.72 

238.08 

3I7-44 

396.8o 

476.16 

555-52 

634.88 

714.24 

Br 

I 

125.90 

251.80 

377-70 

503  .  60 

629.50 

755.40 

881.30 

1007.2 

1133-10 

I 

8 

31-83 

63.66 

95-49 

127.32 

I59-I5 

190.98 

222  .8l 

254.64 

286.47 

S 

SO* 

95-35 

190.70 

286.05 

381.40 

476.75 

572.10 

667-45 

762.80 

858.15 

SO4 

P04 

94.29 

188.58 

282.87 

377.i6 

471.45 

565.74 

660.03 

754-32 

848.61 

P04 

Na 

22.88 

45.76 

68.64 

91.52 

114.40 

137.28 

160.16 

183.04 

205.92 

Na 

K 

38.86 

77.72 

116.58 

155-44 

194.30 

233-I6 

272  .02 

310.88 

394-74 

K 

Li 

6.98 

13.96 

20.94 

27.92 

34-90 

42  .  oJ 

48.86 

55.84 

62.82 

Li 

Ba 

136.4 

272.8 

409.2 

545-6 

682.0 

8l8.4 

954-8 

IO9I  .2 

1227.6 

Ba 

Ca 

39-8 

79-6 

128.4 

159.2 

199.0 

238-8 

278.6 

318.4 

358.2 

Ca 

Ag 

107.  12 

214.24 

321.36 

428.48 

535.6 

642.72 

749.84 

856.96 

964.08 

Ag 

Hg 

198.5 

397-0 

595-5 

794-0 

992-5 

II9I  .O 

1389-5 

1588.0 

1786.5 

Hg 

Pe 

55-5 

in  .0 

166.5 

222.  O 

277-5 

333-0 

388.5 

444-0 

499-5 

Fe 

1 

2 

3 

4 

5 

6 

7 

8 

9 

A  LIST  OF  ELEMENTS  OCCURRING  IN  VOLUMETRIC  METHODS, 
THEIR  SYMBOLS,  AND    ATOMIC  WEIGHTS 


Name. 

Atomic 
Weights  * 
Based  on 
H==  i.ooo. 

Atomic 
Weights  t 
Based  on 
O=i6 

Approxi- 
mate 
Atomic 
Weights. 

Aluminum  

Al 

26.0 

27.  1 

27.O 

Sb 

no.  3 

1  2O.  2 

I2O.O 

Arsenic 

As 

74    3 

7C   o 

71;  o 

Barium  

Ba 

1^6   4 

137.4 

136.0 

Bismuth 

Bi 

2o6   4 

208  o 

206  o 

Boron  . 

B 

IO    0 

II  .O 

II    O 

Bromin  

Br 

70   36 

70.  06 

80.0 

Cd 

in  .6 

112  .4 

Til  .O 

Ca 

39.8 

40.1 

40.O 

Carbon    

c 

II    01 

12.  O 

12.  O 

Chlorin  

Cl 

^c.iS 

3C>.4!> 

3^-2 

Chromium 

Cr 

ci    7 

C2    i 

<2    O 

Cobalt  ...                . 

Co 

s8  =;6 

<O   O 

«;8  o 

Copper  .  . 

Cu 

63.1 

63.6 

63.0 

Fluorin 

F 

18  o 

10    O 

10  o 

Gold 

Au 

IQC    7 

107.  2 

1  06.0 

Hydrogen  

H 

I  .OOO 

i  .008 

i  .0 

I 

I2IJ.  Q 

126.97 

126.0 

Fe 

cc.  c 

55.9 

56.0 

Lead    .    .                    . 

Pb 

2<X    3^ 

206.  o 

206.0 

Li 

6.08 

7.03 

7-0 

Mg 

24.18 

24.36 

24.0 

Manganese  ..     .        ............... 

Mn 

^4.6 

">^.o 

cer.o 

Mercury  

Hg 

108.5 

2OO.O 

2OO.O 

Mo 

QC.7 

96.0 

95.O 

Nickel      .                    

Ni 

SS.T 

58.7 

q8.o 

N 

I3.Q3 

I4.OI 

14.0 

o 

15.88 

16.0 

16.0 

Phosphorus  .    .        ......    .....    .    . 

P 

3O.77 

31  .0 

31.0 

Pt 

IO^.  3 

194.8 

194.0 

K 

38.86 

39-i[5 

39.0 

Silver 

Ag 

IO7    12 

IO7.Q3 

107.0 

Sodium  

Na 

22.88 

23.  o< 

23.0 

Sr 

86.94 

87.6 

87.0 

S 

31.83 

32.06 

32.0 

Tin  

Sn 

118.1 

119.0 

118.0 

Zinc  

Zn 

64.0 

65.4 

65.0 

*  Adopted  in  this  book. 

t  International  atomic  weights,  J.  A.  C.  S.,  1907,  par.  in. 


A   MANUAL 

OF 

VOLUMETRIC    ANALYSIS 


PART  I 

CHAPTER  I 
INTRODUCTION 

IN  a  chemical  analysis  the  aim  is  to  determine  the  nature  of  the 
chemical  substances  contained  in  a  given  compound  or  to  ascertain 
their  quantities.  In  the  former  case  the  analysis  is  a  qualitative,  in 
the  latter  a  quantitative,  one. 

The  quantitative  analysis  of  a  substance  may  be  made  either  by 
the  gravimetric  or  the  volumetric  method. 

The  Gravimetric  Method  consists  in  separating  and  weighing 
the  constituents  either  in  their  natural  states  or  in  the  form  of  new 
and  definite  compounds,  the  composition  of  which  is  known  to  the 
analyst.  From  the  weights  of  these  new  compounds  the  analyst  can 
calculate  the  quantities  of  the  original  constituents. 

Example.  To  determine  the  quantity  of  silver  in  a  solution  by 
the  gravimetric  method  we  proceed  as  follows : 

Ten  grams  of  a  solution  containing  silver  in  the  form  of  silver  nitrate 
(AgNO3)  is  placed  into  a  beaker,  and,  after  slightly  acidulating  with 
nitric  acid,  is  treated  with  hydrochloric  acid,  drop  by  drop,  until  no 
further  precipitation  occurs.  The  precipitate  which  consists  of  silver 
chlorid  (AgCl)  is  then  separated  by  filtration,  thoroughly  washed, 
dried,  and  weighed.  Its  weight  is  found  to  be  0.69  gm.  The  calcu- 
lation is  then  made  as  follows:  142.3  gms.  of  silver  chlorid  represents 
107.12  gms.  of  silver  or  168.69  gms.  of  silver  nitrate,  as  the  equation 
shows: 

AgNO3 + HC1 = AgCl + HNO3. 
168.69  142.3 


2  A     ATANIAL    OF    VOLUMETRIC    ANALYSIS 

«..*•*  * 
Therefore,  0.69  gm.  of  silver  chlorid  will  represent 

— — — X 0.69 =0.5 1 9  gm.  of  silver, 

or  —^Xo. 60  =  0.818  gm.  of  silver  nitrate. 

142.3 

The  Volumetric  Method.  This  method  depends  upon  the  use  of 
solutions  (standard  solutions)  which  are  of  known  strength,  and  paying 
attention  to  the  volume  of  such  a  solution  which  must  be  added  to 
the  substance  under  analysis  to  perform  with  it  and  complete  a  certain 
reaction.  Thus,  if  we  conduct  an  analysis  by  means  of  such  a  solution, 
and  can  express  by  chemical  equation  the  reaction  which  takes  place, 
we  can  readily  and  accurately  calculate  the  quantity  present  of  the 
substance  under  analysis. 

Example. — If  a  silver  solution  is  to  be  analyzed  by  this  method 
it  is  treated  with  a  standard  solution  of  sodium  chlorid,  added  slowly 
from  a  burette  until  no  more  silver  chlorid  is  precipitated.  Each  cc. 
of  this  standard  solution  will  precipitate  a  certain  weight  of  silver  as 
silver  chlorid,  and  hence  by  noting  the  number  of  cc.  used  to  complete 
the  precipitation,  the  weight  of  the  silver  in  the  solution  analyzed  is 
easily  ascertained. 

N 
The  —  sodium  chlorid  solution  is  generally  used  for  this  purpose. 

It  is  made  by  dissolving  -^  of  the  molecular  weight  of  the  salt  (in 
grams)  (5.806  gms.)  in  water  sufficient  to  make  1000  cc.  1000  cc. 
of  this  solution  will  precipitate  ^  of  the  atomic  weight  of  silver  (in 
grams)  (10.712  gms.),  and  hence  each  cc.  of  the  sodium  chlorid 
solution  represents  0.010712  gm.  of  metallic  silver,  and  by  mul- 
tiplying this  figure  by  the  number  of  cc.  used,  the  quantity  of  silver 

N 

in  the  solution  is  found.     If  in  the  above  analysis  100  cc.  of  the  — 

10 

sodium  chlorid  solution  were  used,  then  0.010712X100=1.0712  gms. 
of  metallic  silver. 

The  reaction  is  illustrated  by  this  equation : 

Ag      NO3  +   NaCl  =   AgCl  +   NaNO3. 
10)107.12  10)58.06 


N 

i OOP)   10.71 2  gms.          1000)5.806  gms.  =  1000  cc.  —  V.  S. 

10 

0.010712  gm.  0.005806  gm.       =        i  cc.  10  V.  S. 


THE    VOLUMETRIC    METHOD  3 

From  the  examples  given  it  will  be  seen  that  the  gravimetric  opera- 
tions consume  no  little  time,  and  require  the  exercise  of  considerable 
skill.  The  washing  of  the  precipitate  must  be  thoroughly  performed 
in  order  that  it  be  freed  from  all  adhering  matter.  The  drying  also 
is  a  matter  of  some  consequence  and  must  be  performed  in  such  a  manner 
as  to  prevent  the  admixture  of  dust  or  the  decomposition  of  the  pre- 
cipitate by  excessive  heat.  A  very  accurate  balance  is  also  required. 

The  volumetric  operations,  on  the  other  hand,  do  not  require  that 
the  substance  to  be  determined  be  separated  in  the  form  of  a  com- 
pound of  known  composition  and  weighed  in  the  dry  state;  in  fact, 
the  substance  may  be  accurately  estimated  when  mixed  with  many 
others.  It  therefore  obviates  the  necessity  for  the  frequent  separations 
and  weighings  which  the  gravimetric  method  demands,  and  enables 
the  analyst  to  do  the  work  in  a  very  short  time. 

The  instruments  needed  for  volumetric  work  are  few  and  simple, 
and  comparatively  little  skill  is  required.  Furthermore  the  results 
obtained  are  in  most  instances  more  accurate. 


CHAPTER    II 

GENERAL     PRINCIPLES     OF     CHEMICAL     COMBINATION     UPON 
WHICH  VOLUMETRIC  ANALYSIS  IS  BASED 

1.  WHEN  substances  unite  chemically  the  union  always  takes  place 
in  definite  and  invariable  proportions.  Thus  when  silver  nitrate  and 
sodium  chlorid  are  brought  together,  168.69  parts  (by  weight)  of 
silver  nitrate  and  58.06  parts  (by  weight)  of  sodium  chlorid  will 
react  with  each  other,  producing  142.30  parts  of  a  curdy  white  pre- 
cipitate (silver  chlorid). 

These  substances  will  react  with  each  other  in  these  proportions 
only. 

If  a  greater  proportion  of  silver  nitrate  than  that  above  stated  be 
added  to  the  sodium  chlorid,  only  the  above  proportion  will  react, 
the  excess  remaining  unchanged. 

The  same  is  true  if  sodium  chlorid  be  added  in  excess  of  the  above 
proportions.  For  instance,  if  200  parts  of  silver  nitrate  be  mixed 
with  58.06  parts  of  sodium  chlorid  168.69  parts  only  will  react  with 
the  sodium  chlorid,  while  31.31  parts  of  silver  nitrate  will  remain 
unchanged.  Again,  when  potassium  hydroxid  and  sulphuric  acid 
are  mixed  potassium  sulphate  is  formed,  111.48  parts  of  potassium 
hydroxid  and  97.35  parts  of  sulphuric  acid  being  required  for  complete 
neutralization.  These  two  substances  unite  chemically  in  these  pro- 
portions only. 

The  equation  is 

2KOH+ H2S04=  K2S04+  2H20. 

111.48       97.35 

In  other  words,  111.48  parts  of  KOH  will  neutralize  97.35  parts  of 
H2SO4,  and  consequently  97.35  parts  of  H2SO4  will  neutralize  111.48 
parts  of  KOH. 

Oxalic  acid  and  sodium  carbonate  react  upon  each  other  in  the 
proportions  shown  in  the  equation 

H2C2O4  •  2H2O  -f  Na2CO3 = Na2C2O4+  CO2  +  3H2O 
125.10  105.31 


GENERAL    PRINCIPLES    OF    CHEMICAL    COMBINATION      5 

125.10  parts  of  crystallized  oxalic  acid  are  neutralized  by  105.31 
parts  of  anhydrous  sodium  carbonate. 

"2.  Definite  chemical  compounds  always  contain  the  same  elements 
in  exactly  the  same  proportions,  the  proportions  being  those  of  their 
atomic  weights,  or  some  multiple  of  these  weights. 

Thus  sodium  chlorid  (NaCl)  contains  22.88  parts  of  metallic 
sodium  and  35.18  parts  of  chlorin,  these  being  the  atomic  weights 
of  sodium  and  chlorin  respectively. 

Potassium  sulphate  (K2SO4)  contains  twice  38.86=77.72  parts  of 
potassium,  31.83  parts  of  sulphur,  and  four  times  15.88  =  63.52  parts 
of  oxygen. 

Potassium  hydroxid  (KOH)  contains  38.86  parts  of  potassium, 
15.88  parts  of  oxygen,  and  one  part  of  hydrogen.  Hydrochloric  acid 
(HC1)  contains  one  part  of  hydrogen  and  35.18  parts  of  chlorin. 

Upon  these  facts  the  volumetric  methods  of  analysis  are  based. 

It  has  been  shown  that  97.35  gms.  of  sulphuric  acid  will  neu- 
tralize 111.48  gms.  of  potassium  hydroxid;  it  is  therefore  evident 
that  if  a  solution  of  sulphuric  acid  be  made  containing  48.675  gms. 
(half  of  the  molecular  weight)  of  the  pure  acid  in  1000  cc.  that  one  cc. 
of  this  solution  will  neutralize  0.05574  gm.  of  potassium  hydroxid. 
In  estimating  alkalies  with  this  acid  solution  the  latter  is  added  from 
a  burette,  in  small  portions,  until  the  alkali  is  neutralized,  as  shown 
by  its  reaction  with  some  indicator. 

Each  cc.  of  the  acid  solution  required  before  neutralization  is 
complete  indicates  0.05574  gm.  of  KOH,  and  the  number  of  cc.  used 
multiplied  by  0.05574  gm.  gives  the  quantity  of  pure  KOH  in  the 
sample  analyzed. 

One  cc.  of  the  same  solution  will  neutralize  0.03976  gm.  of  sodium 
hydroxid  (NaOH),  0.05265  gm.  of  anhydrous  sodium  carbonate 
(Na2CO3),  etc. 

If  a  solution  of  crystallized  oxalic  acid  be  made  by  dissolving  (half 
of  the  molecular  weight  in  grams)  62.55  g1118-  m  sufficient  water  to 
make  1000  cc.,  we  will  have  a  normal  solution,  the  neutralizing  power 
of  which  is  exactly  equivalent  to  the  above-mentioned  normal  sulphuric- 
acid  solution. 

The  strength  of  acids  is  estimated  by  alkali  volumetric  solutions. 
A  normal  solution  of  potassium  hydroxid  containing  55.74  gms.  in 
the  liter  will  neutralize  exactly  i  liter  of  the  normal  acid  solution; 
i  cc.  of  this  normal  alkali  will  neutralize  0.03618  gm.  of  HC1,  0.06255 
gm.  of  H2C2O4+  2H2O,  or  0.048675  gm.  of  H2SO4,  etc. 


CHAPTER  III 

VOLUMETRIC  OR  STANDARD  SOLUTIONS 

ANY  solution  employed  in  volumetric  analysis  for  the  purpose  of 
estimating  the  strength  of  substances,  that  is,  any  solution  the  chemical 
power  or  titer  of  which  has  been  determined,  is  designated  a  standard 
or  'volumetric  solution.  Such  a  solution  is  said  to  be  "titrated"  (French 
titre  =  title  or  power),  and  is  sometimes  also  called  a  set  solution  or  a 
standardized  solution.  It  may  be  normal,  decinormal,  empirical,  or 
of  any  strength,  so  long  as  its  strength  is  known. 

When  volumetric  analysis  first  came  into  use  the  solutions  were  so 
made  that  each  substance  to  be  estimated  had  its  own  special  volumetric 
solution,  and  this  was  usually  of  such  strength  as  to  give  the  result  in 
percentages.  Thus  a  certain  strength  of  standard  acid  was  employed 
for  potash,  another  for  soda,  and  a  third  for  ammonia,  and  in  testing 
the  acids,  each  had  its  own  special  standard  alkali.  These  solutions 
were  known  as  normal  solutions;  they  are  still  to  some  extent  in  use, 
and  since  solutions  now  designated  as  normal  are  of  an  entirely  differ- 
ent character,  it  is  important  that  no  misconception  should  exist 
when  a  normal  solution  is  spoken  of.  Furthermore,  it  is  to  be  re- 
gretted that  some  authors  still  define  normal  solutions  as  those  having 
the  molecular  weights  in  grams  of  the  active  reagent  in  a  liter,  whereas 
the  normal  solutions  as  now  more  generally  accepted  are  those  having 
the  hydrogen  equivalent,  weighed  in  grams,  in  a  liter;  in  other  words: 

Normal  solutions  are  such  as  contain  in  one  liter  (1000  cc.)  the 
molecular  weight  of  the  active  reagent  expressed  in  grams  and 
reduced  to  the  valency  corresponding  to  one  atom  of  replaceable 
hydrogen  or  its  equivalent. 

Thus  in  univalent  or  monobasic  compounds  the  full  molecular 
weight  in  grams  is  contained  in  a  liter  of  the  normal  solution. 

Example.  Hydrochloric  acid,  HC1,  having  one  replaceable  hydro- 
gen atom,  its  normal  solution  would  contain  the  full  molecular  wreight, 
36.18  grams  in  a  liter.  A  normal  solution  of  potassium  hydroxid 
should  contain  55.74  grams  of  KOH  in  a  liter,  while  that  of 
sodium  hydroxid  should  contain  39.76  grams  of  absolute  NaOH. 

Normal  solutions  of  bivalent  or  dibasic  compounds,  contain  in 

6 


NORMAL   SOLUTIONS  7 

1000  cc.,  one-half  of  the  molecular  weight  in  grams.  Thus,  oxalic 
acid  H2C2O4+2H2O=i25.io,  having  two  replaceable  H  atoms, 
one-half  of  its  molecular  weight  in  grams  =  62.  55  is  contained  in 
a  liter  of  its  normal  solution.  For  the  same  reason  a  liter  of  a 

normal  solution  of  sulphuric  acid  contains  —  —  =  48.675  gms.,  and 
a  liter  of  a  normal  solution  of  sodium  carbonate  Na2CC>3  contains 
=  ^2.655  gms.,  while  in  the  case  of  trivalent  or  tribasic  com- 


pounds one-third  of  the  molecular  weight  in  grams  is  contained  in  a 
liter  of  the  normal  solution. 

Thus  it  will  be  seen  that  one  cc.  of  any  normal  acid  solution  will 
neutralize  one  cc.  of  any  normal  alkali  solution,  because  one  molecule 
of  a  univalent  acid  will  neutralize  one  molecule  of  a  univalent  alkali, 
or  a  half  a  molecule  of  a  bivalent  alkali.  This  is  shown  by  the  equa- 
tions 

HCl+NaOH=NaCl+H2O, 

36.18      30.76 


2)72.36    2)105.31 
36.18         52.655 

The  value  of  a  reagent  as  expressed  by  its  hydrogen  equivalent  is 
readily  seen  in  the  case  of  acids  and  alkalies  by  reference  to  the  chemical 
formula,  but  in  such  standard  solutions  as  potassium  dichromate, 
potassium  permanganate,  sodium  thiosulphate,  and  others,  the  par- 
ticular reaction  in  any  given  analysis  must  be  taken  into  account  in 
making  a  normal  solution;  for  instance,  when  K^C^Oy  is  to  be  used 
as  a  precipitating  agent  its  reaction  is  as  follows  : 


It  is  thus  seen  that  one  molecule  of  K2Cr2O7  will  cause  the  pre- 
cipitation of  two  atoms  of  barium  in  the  form  of  chromate.  Each 
atom  of  barium  is  chemically  equivalent  to  two  atoms  of  hydrogen; 
therefore  one  fourth  of  a  molecule  of  K2Cr2O7  is  equivalent  to  one 
atom  of  hydrogen.  And  therefore  a  normal  solution  of  this  salt,  when 
used  as  a  precipitating  agent,  must  contain  in  one  liter  one  fourth  of 

.         .  292.28 

its  molecular  weight  in  grams;    --  =  73-07 


8  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

If  K2Cr2O7  is  to  be  used  as  an  oxidizing  agent,  the  three  atoms 
of  oxygen  which  it  yields  for  oxidizing  purposes  must  be  taken  into 
account.  When  this  salt  oxidizes  it  splits  up  into  K2O  +  Cr2O3  +  O3. 
The  three  atoms  of  oxygen  combine  with  and  oxidize  the  salt  acted 
upon,  or  they  combine  with  an  equivalent  quantity  of  the  hydrogen 
of  an  acid  and  liberate  the  acidulous  part,  which  then  combines  with 
the  salt.  As  the  equations  show, 

6FeO  +  K2Cr2O7=K2O  +  Cr2O3  +  3Fe2O3    or    (Fe6O9); 

6FeSO4+K2Cr2O7+7H2SO4= 

7H20+K2S04+Cr2(S04)3+3Fe2(S04)3; 


Each  of  these  atoms  of  oxygen  are  equivalent  to  two  atoms  of 
hydrogen.  Thus  O3  is  equivalent  to  H6. 

Hence  a  liter  of  a  normal  solution  of  K2Cr2O7,  when  used  as  an 
oxidizing  agent,  contains  one  sixth  of  its  molecular  weight  in  grams. 

The  same  may  be  said  of  potassium  permanganate  when  used  as 
an  oxidizing  agent. 

2KMnO4  has  five  atoms  of  oxygen  which  are  available  for  oxidizing 
purposes,  and  each  of  these  is  capable  of  taking  two  atoms  of  hydrogen 
from  an  acid  and  liberating  the  acidulous  part.  The  hydrogen  equiva- 
lent of  this  salt  may  therefore  be  said  to  be  one  tenth  of  the  weight  of 
2KMnO4,  and  a  normal  solution  of  this  salt  contains  31.396  gms.  in  a 
liter. 

Sodium  Thiosulphate  (Hyposulphite),  Na2S2O3,  is  another  instance. 
The  molecule  of  this  salt  has  two  atoms  of  sodium,  which  have  re- 
placed two  atoms  of  hydrogen  of  thiosulphuric  acid.  Thus  it  would 
seem  that  a  normal  solution  should  contain  one  half  of  the  molecular 
weight  in  grams.  But  the  particular  reaction  of  this  salt  with 
iodin  is  taken  into  account. 

One  molecule  reacts  with  one  atom  of  iodin,  as  seen  in  the  equa- 
tion 


Since  iodin  is  univalent,  a  molecule  of  the  salt  is  equivalent  to 
one  atom  of  hydrogen. 

A  normal  solution  of  this  salt  therefore  contains  the  molecular 
weight  in  grams  in  a  liter. 


EMPIRICAL    SOLUTIONS'  9 

N 
Decinormal  Solutions,  — ,  are  one  tenth  the  strength  of  normal 

solutions. 

N 
Centinormal  Solutions,  -  — ,  are  one  hundredth  the  strength  of 

normal  solutions. 

N 
Seminormal   Solutions,  — ,  are  one  half  the  strength  of  normal 

solutions. 

N 
Double-normal  Solutions,  — ,    are    twice    the    strength    of    the 

normal. 

Empirical  Solutions.  A  solution  which  does  not  contain  an 
exact  atomic  proportion  of  the  reagent  may  be  employed  as  a  volu- 
metric solution  after  its  strength  or  titer  has  been  determined.  Such  a 
solution  is  said  to  be  empirical,  and  solutions  of  this  sort  are  very 
frequently  used.  To  prepare  solutions  of  exactly  normal  strength  is 
a  tedious  process  and  often  inconvenient.  If  the  solution  is  approxi- 
mately normal  and  its  strength  accurately  determined,  it  may  be  used 
as  it  is.  Again,  in  the  case  of  standard  solutions  of  the  caustic  alkab'es, 
which,  when  not  kept  with  all  precautions,  deteriorate  readily  through 
absorption  of  carbon  dioxid  from  the  air,  as  well  as  through  their 
action  upon  the  glass  containers.  To  restore  the  titer  of  such  solutions 
by  the  introduction  of  more  of  the  alkali  is  an  unnecessary  waste  of 
time,  inasmuch  as  it  is  only  necessary  to  determine  its  exact  strength 
and  then  use  it  as  it  is.  For  instance,  if  an  approximately  normal 
solution  of  potassium  hydroxid  is  on  hand,  its  strength  is  determined 
as  follows: 

Ten  cc.  of  an  exactly  normal  oxalic  or  other  acid  solution  are  put 
into  a  beaker,  and  after  diluting  with  a  little  water,  and  adding  three 
or  four  drops  of  methyl  orange,  the  empirical  potassium  hydroxid 
solution  is  run  in  from  a  burette  until  the  color  of  the  solution  changes 
from  red  to  yellow;  the  number  of  cc.  required  is  then  noted. 

Assuming  that  10.4  cc.  were  required  to  neutralize  the  10  cc.  of 

normal  acid.     Hence  its  strength  is  -  -  or  0.9615,  that  of  a  strictly 

104 

normal  solution,  and  the  number  of  cc.  used  of  it  in  any  estimation 

must  be  multiplied  by  -  -  or  0.9615  and  then  by  the  normal  factor 
104 

for  the  substance  analyzed. 

It  is  a  good  plan  to  have  the  factor  marked  on  the  label  of  the 


10  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

bottle  containing  such  an  empirical  solution.  In  this  case  it  would  be 
Xo.96i5  =  normal. 

Standard  solutions  for  use  in  volumetric  analysis  are  usually  solu- 
tions of  acids,  bases,  or  salts,  and  in  two  cases  elements,  namely,  iodin 
and  bromin. 

A  standard  solution  of  a  base  is  usually  used  for  the  estimation  of 
free  acids. 

A  standard  solution  of  an  acid  is  usually  used  for  the  estimation  of 
a  free  base,  or  the  basic  part  of  a  salt,  the  acid  of  which  can  be  com- 
pletely expelled  by  the  acid  used  in  the  standard  solution.  Example, 
carbonates. 

A  standard  solution  of  a  salt  may  be  used  as  a  precipitant,  or  it 
may  be  used  as  an  oxidizing  or  reducing  agent. 

That  part  of  the  reagent  in  a  standard  solution  which  reacts  with 
the  substance  under  analysis  is  the  active  constituent  of  the  solution. 
As  Ag  in  AgNOs  is  the  active  constituent  of  the  standard  solution  of 
silver  nitrate, 

AgNO3 +  NaCl = AgCl + NaNO3, 

or  Cl  in  NaCl,  is  the  active  constituent  of  the  standard  solution  of 
sodium  chlorid. 

If  the  reagent  is  a  base,  as  KOH,  the  basic  part  K  is  the  active 
constituent.  If  the  reagent  is  an  acid,  the  active  constituent  is  the 
acidulous  part,  as  SO4  in  H2SO4. 

If  the  action  of  the  reagent  is  oxidizing,  then  that  part  of  the  reagent 
which  produces  the  oxidation  is  the  active  constituent. 

The  valence  of  an  acid  is  shown  by  the  number  of  replaceable 
hydrogen  atoms  it  contains.  Thus  HC1  is  univalent,  H2SC>4  is  biva- 
lent, which  means  that  a  molecule  of  HC1  is  chemically  equivalent  to 
one  atom  of  hydrogen,  and  a  molecule  of  H2SO4  is  chemically  equiva- 
lent to  two  atoms  of  hydrogen. 

The  valence  of  a  base  is  shown  by  the  number  of  hydroxyls  it  is 
combined  with.  As,  KOH  is  univalent,  Ca(OH)2  is  bivalent. 

The  valence  of  a  salt  is  shown  by  the  equivalent  of  base  which 
has  replaced  the  hydrogen  of  the  corresponding  acid. 

Thus  NaCl,  in  which  Na  has  replaced  H  of  HC1,  is  univalent. 

K2SC>4,  in  which  K2  has  replaced  H2  of  H2SO4,  is  bivalent. 

In  preparing  standard  solutions  it  must  be  remembered  that  most 
salts  when  dissolved  in  water  reduce  the  temperature,  while  some,  for 
instance  sulphuric  acid  and  alkaline  hydroxids,  cause  a  rise  in  tern- 


RESIDUAL    TITRATION,  RE-TITRATION  11 

perature.  Therefore  the  solutions  should  be  allowed  to  stand  a  while 
so  that  they  may  attain  the  temperature  of  the  air  before  being  measured. 

Furthermore,  most  salts  cause  a  condensation  in  volume  when 
dissolved  in  water;  this  must  also  be  borne  in  mind. 

It  is  always  best  to  weigh  out  a  little  more  of  the  salt  than  the 
amount  required  by  theory;  dissolve  in^  water  less  than  required  for 
the  finished  solution,  determine  its  strength,  and  then  dilute  to  the 
proper  strength.  After  dilution  it  should  always  be  again  carefully 
titrated,  and  proved  normal. 

To  Titrate  a  substance  means  to  test  it  volumetrically  for  the 
amount  of  pure  substance  it  contains.  The  term  is  used  in  preference 
to  "tested  "  or  "analyzed,"  because  these  terms  may  relate  to  quali- 
tative examinations  as  well  as  quantitative,  whereas  titration  applies 
only  to  volumetric  analysis. 

Residual  Titration,  Re-titration,  sometimes  called  Back  Titra- 
tion, consists  in  treating  the  substance  under  examination  with  standard 
solution  in  a  quantity  known  to  be  in  excess  of  that  actually  required; 
the  excess  (or  residue)  is  then  ascertained  by  residual  titration  with 
another  standard  solution. 

Thus  the  quantity  of  the  first  solution  which  went  into  combination 
is  found. 

N 
Example.     Ammonium  carbonate  is  treated  first  with  —  H2SC>4  in 

N 
excess,  and  the  excess  then  found  by  titration  with  —  KOH. 

i 

N 
The  quantity  of  the  —  KOH  used  is  then  deducted  from  the  quantity 

N 
of  —  H2SO4  added,  which  gives  the  quantity  of  the  latter  which  was 

neutralized  by  the  ammonium  carbonate. 


CHAPTER  IV 

INDICATORS* 

IN  volumetric  analysis  the  substance  to  be  analyzed  in  the  state  of 
solution  is  placed  in  a  beaker  and  the  standard  solution  is  added  from 
a  burette  until  a  certain  reaction  is  produced.  The  exact  moment 
when  a  sufficient  quantity  of  the  standard  solution  has  been  added 
is  known  by  certain  visible  changes,  which  differ  according  to  the 
substance  analyzed  and  the  standard  solution  used.  When  such  a 
visible  change  occurs  the  "  end  reaction  "  is  reached. 

The  end  reaction  manifests  itself  in  various  ways,  as  follows: 

1.  Cessation  of  precipitation. 

2.  First  appearance  of  a  precipitate. 

3.  Change  of  color. 

In  some  cases,  however,  the  addition  of  the  standard  solution  to 
the  substance  under  analysis  does  not  produce  either  a  precipitate  or 
a  change  of  color;  in  such  cases  we  must  resort  to  the  use  of  an  in- 
dicator. 

An  Indicator  is  a  substance  which  is  used  in  volumetric  analysis, 
and  which  indicates  by  change  of  color,  or  some  other  visible  effect, 
the  exact  point  at  which  a  given  reaction  is  complete. 

Generally  the  indicator  is  added  to  the  substance  under  examina- 
tion, but  in  a  few  cases  it  is  used  alongside,  a  drop  of  the  substance 
being  occasionally  brought  in  contact  with  a  drop  of  the  indicator. 

Thus  in  estimating  an  alkali  with  an  acid-volumetric  solution  the 
alkali  is  shown  to  be  completely  neutralized  when  the  litmus  tincture 
which  was  added  becomes  faintly  red  or  the  phenolphthalein  colorless. 
Again,  when  haloid  salts  are  estimated  with  nitrate-of -silver  solution, 
chromate  of  potassium  is  added  as  indicator.  A  white  precipitate  is 
produced  as  long  as  any  halogen  is  present  to  combine  with  the  silver, 
and  when  all  is  precipitated  the  chromate  of  potassium  acts  upon  the 
silver  nitrate,  forming  the  red  silver  chromate,  this  color  thus  showing 
that  all  the  halogen  has  been  precipitated. 

*  For  fuller  information  on  indicators  see  A.  I.  Conn's  "  Indicators  and  Test 
Papers,"  John  Wiley  &  Sons,  New  York. 

12 


THE    IONIZATION    OR    DISSOCIATION    THEORY  13 


THE   IONIZATION   OR   DISSOCIATION   THEORY 

When  a  soluble  salt  dissolves  in  water,  its  molecules  split  up  or 
dissociate  more  or  less  completely  into  parts  called  ions.  This  be- 
havior of  substances,  on  going  into  solution,  is  known  as  electrolytic 
dissociation  or  ionization. 

Ions  are  electrically  charged  atoms  or  groups  of  atoms  and  are  of 
similar  composition  to  the  substances  formed  from  the  compound 
when  an  electric  current  is  passed  through  the  solution.  The  electro- 
positive ions  migrate  to  and  collect  around  the  negative  pole  (cathode) 
and  hence  are  called  cathions,  while  the  electro-negative  ions  are  called 
anions,  because  they  concentrate  around  the  positive  pole  or  anode. 
The  dissociation  of  a  compound  into  its  ions  when  an  electric  current 
is  passed  through  its  solution, -although  spoken  of  as  electrolytic  disso- 
ciation, is  really  not  caused  by  the  electric  current,  since  the  dissocia- 
tion into  ions  occurs  at  once  upon  dissolving  the  substance  in  water 
and  without  the  aid  of  an  electric  current,  the  action  of  the  current 
being  the  transportation  of  the  separated  ions  to  the  poles. 

The  dissociation  of  compounds  into  ions  when  dissolved  in  water 
is  illustrated  in  the  following  list: 

Sodium  chlorid  into (Na+)  and  (Cl— ) 

Potassium  nitrate  into (K  + )    ' '    (NO3 — ) 

hydroxidinto (K+)    "     (OH-) 

"         acetate  into (K+)    "    (C2H3O2-) 

Sulphuric  acid  into (H+)    "     (HSO4-) 

or  (H  +  )  and  (H  +  )  and  (SO4) 

The  extent  of  this  dissociation  depends  upon  the  nature  of  the 
substance  and  the  degree  of  dilution;  the  greater  the  dilution  the 
more  complete  the  dissociation.  Furthermore,  strong  acids  and  bases 
dissociate  readily,  even  in  comparatively  concentrated  solutions,  while 
the  weaker  acids  and  bases  are  more  or  less  undissociated  when  dis- 
solved, i.e.,  they  are  not  readily  split  up  into  ions.  Their  salts,  how- 
ever, are  immediately  and  completely  ionzied.  Therefore,  upon  neu- 
tralizing a  weak  acid  or  base  an  ionizable  salt  is  formed.  According 
to  the  theory  of  Arrhenius,  the  reactions  of  analytical  chemistry  are 
chiefly  reactions  between  ions  and  not  between  atoms. 

Strong  acids,  bases,  and  salts  exist  in  solution  not  as  molecules,  but 
chiefly  in  the  form  of  ions.  The  formation  of  silver  cWorid  by  the 


14  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

reaction  between  silver  nitrate  and  sodium  chlorid  takes  place  accord- 
ing to  the  following  equation: 

Ag/ NO3Aq.  +  Na/  ClAq.  =  AgCl  (solid)  +  Na/ NO3 Aq. 

The  state  of  dissociation  being  denoted  by  the  vertical  line  between 

the  ions  of  the  molecules. 

+  + 

This  theory  also  explains  why  K/C1O3  with  Ag/NO3  does  not  form 

AgCl,  in  that  the  reaction  involves  the  ion  C1O3  and  not  the  atom  Cl. 

Theories  of  Indicators.— In  connection  with  the  use  of  indi- 
cators in  neutralization  analyses,  the  question  as  to  the  cause  of  the 
color  changes  is  one  of  considerable  interest. 

Two  distinct  views  are  held.  Of  these  the  lonization  Theory  of 
Oslwald  has  received  almost  universal  acceptance.  According  to  this 
theory  the  color  changes  are  ascribed  to  a  change  in  the  indicator  from 
a  molecular  to  an  ionic  condition.  As  exemplified  in  the  case  of 
phenolphthalein  the  colorless  molecule 

OCOC6H4C-(C6H4OH)2    ......     (I) 

is  dissociated  into  the  red  negative  ion 

OCOC6H4C-(C6H4OH)C6H46 (II) 

In  the  other  and  less  known  view  on  this  subject  (the  Chro- 
mophoric  Theory)*  the  sensitiveness  of  the  indicators  and  its  color 
change  is  ascribed  to  a  change  in  the  constitution  of  the  indicator,  in- 
volving a  chromophoric  group,  under  the  influence  of  hydrogen  and 
hydroxyl  ions.  According  to  this  view  the  color  change  is  due  (in  the 
case  of  phenolphthalein),  to  a  change  of  constitution  from  the  colorless 
lactoid  (I)  with  no  chromophoric  group,  to  the  colored  quinoid  (III) 
with  a  quinone  chromophore. 

(NaOOC.CGH4)(HOC6H4)C:C6H4:0,      .     .     .     (Ill) 

and  that  the  ionization  of  the  sodium  salt  is  merely  a  coincidence  and 
not  the  cause  of  the  color  change. 

Whichever  of  these  views  is  the  correct  one,  remains  for  future 
investigations  to  prove.  That  of  Ostwald  being  most  generally  accepted 
at  the  present  time  is  treated  more  fully  below. 

*  See  Julius  Stieglitz,  Jour.  Am.  Chem.  Soc.,  XXV,  1112  (1903). 


THE   IONIZATION    THEORY   OF  INDICATORS  15 

The  lonization  Theory  of  Indicators.*  The  indicators  used  in 
alkalimetry  and  acidimetry  are  compounds  of  feeble  acid  or  basic  char- 
acter, and  hence  not  prone  to  dissociation  in  solution,  but  when  neu- 
tralized, the  salt  formed  ionizes  the  instant  of  its  formation,  the  ions  so 
liberated  give  rise  to  colors  which  differ  from  those  of  the  original  com- 
pounds. 

Any  feeble  acid  or  base  may  be  utilized  as  an  indicator  if  its  ions 
have  a  color  different  from  that  of  the  un-ionized  compound.  Strong 
acids  or  bases  are  not  suited  as  indicators,  because  they  ionize  while 
in  a  free  state  on  dilution,  and  thus  give  no  color  when  neutralized. 

A  solution  in  which  H  ions  predominate  has  an  acid  reaction,  while 
one  in  which  OH  ions  exist  reacts  alkaline. 

Phenolphthalein  is  a  feebly  acid  indicator,  and  in  its  undissociated 
state  is  colorless.  It  does  not  dissociate  readily  unless  neutralized,  but 
when  sodium  hydroxid  is  added  to  its  solution,  a  sodium  salt  of 
phenolphthalein  is  formed  which  immediately  ionizes  and  the  ions 
liberated  impart  to  the  solution  a  brilliant  red  color.  If  now  some 
acid  is  added  the  sodium  salt  is  decomposed  and  the  acid  phenol- 
phthalein again  set  free,  and  the  solution  becomes  colorless. 

If  a  few  drops  of  phenolphthalein  solution  be  added  to  an  acid 
solution,  ionization  of  the  former  is  prevented  by  the  presence  of  the 
stronger  acid;  if  now  some  sodium  hydroxid  solution  is  added,  the 
OH  ions  of  the  latter  unite  with  the  H  ions  of  the  acid,  and  when  the 
acid  is  completely  neutralized  the  first  drop  of  excess  of  alkali  unites 
with  the  phenolphthalein,  forming  a  salt  which  immediately  ionizes 
and  produces  the  characteristic  red  color  which  shows  the  end  of  the 
reaction. 

In  the  titration  of  a  feeble  acid  the  end  point  is  often  indistinct 
and  is  lacking  in  sharpness;  this  is  because  the  indicator  used  has  a 
greater  tendency  to  ionize  than  the  acid  itself.  In  this  case  the  H 
ions  present  just  before  the  completion  of  the  reaction  are  not  in  suffi- 
cient amount  to  fully  retard  the  ionization  of  the  indicator,  and  hence 
the  latter  dissociates  partly  before  neutralization  is  complete  and  gives 
rise  to  an  indefinite  end-reaction.  Therefore  it  is  necessary  when 
titrating  a  feeble  acid  that  an  indicator  should  be  selected  whose  alkali 
salt  ionizes  with  the  production  of  a  distinct  color  change,  and  whose 
tendency  to  ionize  is  less  than  that  of  the  acid.  Phenolphthalein  is  a 

*  See  Ostwald's  "Lehrbuch  der  Allgemeinen  Chemie,"  1891,  and  "Scientific 
Foundations  of  Analytical  Chemistry,"  1900,  also  Walker's  "Introduction  to 
Physical  Chemistry,"  1899. 


16  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

suitable  indicator  in  this  case,  provided  a  strong  alkali  be  used  for 
titrating. 

Fixed  alkalies  readily  yield  ionizable  salts  with  phenolphthalein, 
but  ammonia  does  not.  The  latter  being  too  weak  a  base  to  yield 
with  so  feeble  an  acid,  a  salt  which  can  withstand  the  hydrolytic  action 
of  the  water  in  dilute  solutions,  and  as  a  consequence  a  larger  excess 
of  the  ammonia  must  be  used  to  overcome  this.  Thus  is  accounted 
for  the  imperfect  color  change  of  phenolphthalein  when  ammonia  or 
its  salts  are  present  and  why  the  color  becomes  visible  only  after  a 
large  excess  of  the  alkali  is  added. 

Paranitrophenol  is  also  an  acid  indicator:  it  exists  in  solution  in 
the  form  of  undissociated  colorless  molecules,  yet  its  electro-negative 
ion  is  intensely  yellow  in  color.  This  compound  has  a  slight  tendency 
to  dissociate  in  dilute  solutions,  but  the  presence  of  a  trace  of  a  stronger 
acid  will  overcome  this  tendency  and  the  solution  remains  colorless. 
If  an  alkali  is,  however,  added,  a  salt  of  paranitrophenol  is  formed 
which  immediately  ionizes  and  exhibits  the  intense  yellow  color  of  its 
liberated  ion.% 

Other  indicators  exhibit  a  color  in  both  the  ionized  and  the  non- 
ionized  state,  but  the  colors  in  both  conditions  are  different,  as  in  the 
case  of  litmus  lacmoid  and  methyl  orange. 

Methyl  orange  is  both  an  acid  and  a  base  and  will  form  ionizable 
salts  with  either  acids  or  alkalies;  its  indicator  characteristics  are, 
however,  due  essentially  to  its  basic  character. 

The  salt  which  methyl  orange  forms  with  acids  dissociates  into  red 
ions;  this,  upon  the  addition  of  an  alkali,  returns  to  its  undissociated 
state,  which  is  yellow.  Because  of  its  weak  basic  character  its  com- 
pound with  acids  is  readily  decomposed  by  alkalies,  but  it  takes  a 
strong  acid  or  a  relatively  large  quantity  of  a  feeble  acid  to  dissociate 
it  in  its  non-ionized  state,  hence  this  indicator  is  very  sensitive  to 
alkalies,  and  much  less  so  to  acids. 

With  reference  to  the  acid  character  of  the  indicator  the  explanation 
of  its  action  is  that  the  non-ionized  indicator  is  red,  while  its  ion  is 
yellow.  Acids  lessen  its  ionization  and  produce  a  red  color,  while 
alkalies  produce  a  highly  ionizable  salt  and  hence  a  yellow  color. 
When  a  weak  but  slightly  ionizable  acid  is  added  to  the  methyl  orange 
solution,  the  H'  ions  of  the  acid  given  up  in  excess  of  the  amount  re- 
quired for  neutralization  are  not  sufficient  to  yield  enough  of  a  non- 
ionizable  salt  to  produce  a  decided  red  color,  hence  a  large  quantity 
of  such  a  weak  acid  is  required  to  give  an  acid  indication.  This  would 


THE   IONIZATION    THEORY  OF   INDICATORS  17 

explain  why  methyl  orange  is  not  suitable  as  an  indicator  for  weak 
acids,  and  why  it  is  very  sensitive  to  alkalies. 

Litmus  is  an  acid  indicator  which  slightly  dissociates  in  solution. 
Its  non-ionized  molecules  are  red,  but  its  negative  ions  are  blue.  If 
it  is  added  to  an  alkali,  a  salt  is  formed  which  at  once  ionizes  and  gives 
a  blue  color.  If  added  to  an  acid,  ionization  is  prevented  and  the  red 
color  of  the  non-ionized  molecules  appears. 

From  the  above  explanations  it  will  be  seen  that  indicators  cannot 
be  indiscriminately  used,  and  that  no  one  indicator  will  be  suitable 
for  every  titration.  Hence  the  indicator  must  be  selected  to  suit  each 
case.  This  selection  is  facilitated  by  reference  to  the  classification 
of  indicators,  according  to  F.  Glaser,  Ztchr.  f.  a.  Chem.,  1899,  273  +  . 

Group  /.  Indicators  Forming  Fairly  Stable  Salts.  The  mem- 
bers of  this  group  comprise  such  indicators  as  are  (i)  of  a  strong  acid 
character  and  which  react  readily  with  weak  bases,  or  (2)  of  a  feeble 
basic  character  and  which  require  a  strong  acid  to  form  a  stable  salt. 
Hence  they  will  be  found  to  be  very  sensitive  to  alkalies,  and  are  useful 
in  the  titration  of  weak  bases,  as  ammonia  and  the  amine  bases,  as 
well  as  strong  bases  and  acids.  The  indicators  of  this  group  are  the 
following,  arranged  in  the  order  of  their  sensitiveness  towards  alkalies: 

(i)  lodeosin,  Resazurin;  (2)  Tropaeolin  OO,  Luteol;  (3)  Methyl 
and  Ethyl  Orange;  (4)  Congo  Red;  (5)  Cochineal;  (6)  Lacmoid. 

Group  II.  Indicators  Possessing  Faint  Acid  Properties  and 
Yielding  Salts  whic  hare  Very  Unstable.  These  are  readily  decom- 
posed by  relatively  feeble  acids,  and  are  in  consequence  very  sensitive 
towards  acids,  slightly  so  towards  alkalies. 

They  are:  (i)  Rosolic  acid;  (2)  Curcuma;  (3)  Phenolphthalein, 
Flavescin;  (4)  Alpha-naphtholbenzein. 

Group  III,  Indicators  Occupying  a  Place  Midway  between  the 
Other  Two  Groups.  They  are  somewhat  stronger  acids  than  those 
of  Group  II,  but  feebler  than  those  of  Group  I. 

They  are  fairly  sensitive  towards  both  acids  and  alkalies,  but  are 
more  sensitive  towards  acids  than  those  of  Group  I  and  less  so  towards 
alkalies.  They  are: 

(i)  Fluorescein,  Phenacetolin;  (2)  Haematoxylin,  Gallei'n,  Alizarin; 
(3)  Litmus;  (4)  Paranitrophenol. 

This  division  of  indicators  into  groups,  as  above,  facilitates  the 
selection  of  an  indicator  suitable  for  the  work  in  hand. 

For  instance,  for  titrating  weak  acids,  a  glance  at  the  groups  will 
show  that  the  members  of  Group  II  are  best  adapted  for  this  purpose. 


18  A    MANUAL  OF    VOLUMETRIC    ANALYSIS 

Again,  weak  bases  will  be  best  titrated  by  the  indicators  of  Group  I. 
Strong  acids  or  bases  may  be  titrated  by  means  of  any  of  the  indicators. 

The  quantity  of  indicator  taken  in  a  titration  is  a  matter  of  con- 
siderable moment.  The  smallest  quantity  which  will  produce  a  distinct 
color  should  be  taken,  but  it  is  equally  important  that  the  quantity 
be  not  too  small  for  the  volume  of  liquid;  for  in  high  dilutions  the 
hydrolytic  action  of  water  asserts  itself,  and  intermediate  tints  will 
result,  which  interfere  with  the  sharpness  of  the  end  color. 

If  too  much  of  the  indicator  is  used  the  sensitiveness  is  lessened, 
because  acid  or  alkali  must  be  added  to  convert  the  indicator  into  a 
salt,  or  when  formed  to  decompose  it;  i.e.,  a  minimum  of  excess  of 
the  titrating  fluid  would  react  with  a  small  portion  only  of  the  indicator 
and  intermediate  tints  would  be  produced,  until  sufficient  of  the 
titrating  solution  has  been  added  to  neutralize  all  of  the  indicator 
present.  This  is  especially  true  when  using  centinormal  solutions. 
20  drops  of  litmus  added  to  10  cc.  of  water  require  from  10  to  14  drops 

N 

of  —  acid  or  alkali  solution  to  produce  a  change  of  color.     Thus 
100 

the  indicator  itself  takes  up  some  of  the  standard  solution,  and  hence 
the  necessity  for  using  as  small  a  quantity  of  the  indicator  as  possible; 
usually  from  3/05  drops  of  the  indicator  may  be  taken  to  each  50  or 
100  cc.  of  the  fluid  titrated. 

The  degree  of  dilution  of  the  substance  titrated  is  also  a  matter  of 
considerable  moment.  In  very  concentrated  solutions  ionization 
does  not  so  readily  occur,  while  too  great  a  dilution  diminishes  the 
reactive  ability  of  the  ions  because  of  their  greater  separation,  and 
also  because  of  the  hydrolytic  dissociation  of  water  itself  into  H*  and 
OH  ions  which  react  acid  and  alkaline  respectively,  and  which  brings 
about  a  premature  dissociation  of  the  indicator. 

The  Requirements  of  a  Good  Indicator,  according  to  H.  A. 
Cripps,  are: 

I.  The  end  reaction  should  be  marked  by  a  prominent  change  of 
color. 

II.  The  smallest  possible  quantity  of  the  reagent  should  be  required 
to  effect  this  change. 

III.  High  tintorial  power,  which  of  itself  assists  in  the  fulfilment 
of  the  second  requirement,  less  of  the  indicator  being  required. 

IV.  The  change  of  color  should  not  be  affected  by  the  impurities 
commonly  present  in  the  substance  under  examination,  nor  by  the 
products  of  the  reaction. 


INDICATORS  19 

In  addition  to  these  requirements  it  is  a  distinct  advantage  if  the 
color  reaction  is  equally  decided  in  alcoholic  as  in  aqueous  liquids. 

The  following  list  includes  the  more  reliable  indicators  in  common 
use  arranged  alphabetically. 

A  lirarin  •      alkalies  =  Red 

Alizarin.     adds    =Yellow 

This  dye  was  first  found  in  the  roots  of  madder  (Rubia  tinctorium) 
but  is  now  also  obtained  synthetically.  A  half  per  cent  solution  in 
alcohol  is  employed  as  an  indicator. 

Azolitmin 

This  is  the  color  principle  to  which  litmus  owes  its  value  as  an 
indicator.  Its  extraction  is  explained  under  litmus.  It  is  a  high- 
priced  article  and  is  in  consequence  seldom  used,  the  purified  litmus 
tincture  being  preferred. 

Brazil  Wood  Test  Solution: 


Boil  50  gms.  of  finely-cut  Brazil-wood  (the  heart-wood  of  Pelto- 
phorum  dubium  (Sprengel)  Briton,  nat.  ord.  Leguminosce)  with  250  cc. 
of  water  during  half  an  hour,  replacing  from  time  to  time.  Allow 
the  mixture  to  cool;  strain;  wash  the  contents  of  the  strainer  with 
water  until  100  cc.  of  strained  liquid  are  obtained;  add  25  cc.  of  alcohol 
and  filter. 

This  indicator  is  especially  serviceable  for  the  titration  of  alkaloids, 
but  it  is  useless  in  the  presence  of  sulphurous  acid,  sulphites,  or  sul- 
phids,  as  these  substances  decolorize  the  solution. 


Cochineal  Test  Solution:    Dalies  . 

acids      =  Yellowish-red 

Cochineal  is  the  dried  female  insect,  Pseudococcus  cacti  Linne. 

The  test  solution  is  made  by  macerating  3  gms.  of  unbroken  cochi- 
neal for  four  days  in  250  cc.  of  a  mixture  of  i  part  of  alcohol  and  3 
parts  of  water,  by  volume. 

It  is  a  very  valuable  indicator,  especially  for  carbonates  of  the 
alkalies  and  alkaline  earths,  because  it  is  not  affected  by  CO2.  It  is 
also  useful  for  titrating  alkaloids,  alkalies,  alkali  earths,  ammonia, 
and  inorganic  acids,  but  is  useless  for  most  organic  acids. 

Congo  Red:     ^I*t     .  / 

Congo  red  is  a  sodium  tetrazo-diphenyl-naphtionate.  It  occurs  in 
commerce  in  the  form  of  lumps  of  a  reddish-brown  color.  It  is  readilv 


20  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

soluble  in  both  water  and  alcohol.  Its  solution  is  exceedingly  sensitive 
to  free  acids  even  in  the  presence  of  acid  salts,  and  is  likewise  very 
sensitive  to  free  carbonic  and  acetic  acids.  It  may  be  employed  for 
estimating  free  mineral  acids,  in  the  presence  of  most  organic  acids. 

The  test  solution  contains  i  per  cent  of  the  dye  and  10  per  cent 
of  alcohol. 

Corallin: 


This  is  a  coloring-matter  obtained  from  coal-tar  and  contains 
rosolic  and  para-rosolic  acids.  The  test  solution  is  made  by  dissolving 
one  gram  in  10  cc.  of  alcohol  and  adding  water  to  make  100  cc. 
It  is  recommended  for  titrating  free  ammonia. 

p      •     .      alkalies  =  Green  fluorescence 
acids     =  Yellow 

Tetra-bromo-resorcin-phthalein.  —  This  is  made  by  adding  bromin 
to  a  solution  of  fluorescein  in  glacial  acetic  acid.  Crystals  gradually 
separate,  which  may  be  purified  by  conversion  into  a  potassium  salt 
and  precipitated  with  an  acid. 

The  composition  of  this  substance  is 


OP  11  pin  •      alkalies  =  Bright  red 

uaiiem.     adds    =Palebrown 

Anthracene  violet  or  pyrogallol-phthalein  was  proposed  by  M. 
Dechan  for  use  as  an  indicator. 

It  is  prepared  by  heating  a  mixture  of  one  part  of  phthalic  an- 
hydrid  and  two  parts  of  pyrogallol,  and  finally  recrystallizing  in  a 
similar  way  to  phenolphthalein. 

It  is  described  as  a  dark  reddish  crystalline  solid,  possessing  a 
greenish  luster.  It  is  nearly  insoluble  in  water,  but  readily  soluble  in 
alcohol.  In  commerce  it  is  frequently  found  as  a  paste,  mixed  with 
water. 

It  forms  a  red  coloration  with  alkalies,  which  is  changed  to  yellowish 
brown  on  addition  to  an  acid  in  excess. 

It  is  said  to  be  more  delicate  towards  alkalies  than  phenolphthalein, 
and  may  be  used  in  its  stead  for  titrating  many  of  the  alkaloids.  It 
may  be  used  in  the  presence  of  ammonia  or  its  salts.  It  indicates 
sharply  with  the  organic  acids.  A  solution  in  rectified  spirit  i-iooo  is 
generally  employed. 


Haematoxylin: 

A  peculiar  principle  obtained  from  logwood,  having  the  compo- 
sition  CieH^Og,  and  crystallizing  with  one  or  three  molecules   of 


INDICATORS  21 

water.  It  is  an  efflorescent  yellowish-rose  colored  substance,  but 
when  pure  is  said  to  be  colorless,  reddening  on  exposure  to  light. 

It  is  soluble  in  hot  water  or  alcohol.  Its  alcoholic  solution  is  largely 
used  as  an  indicator  in  the  titration  of  alkaloids,  for  which  it  is  con- 
sidered the  indicator  par  excellence. 

The  solution  is  prepared  by  dissolving  one  gram  of  the  well- 
crystallized  material  in  100  cc.  of  alcohol.  In  titrating  use  about 
three  drops  of  this  solution. 

Its  color  reaction  with  carbonates  and  bicarbonates  is  interesting. 
When  added  to  a  solution  of  alkali-bicarbonate  the  reaction  requires 
many  seconds,  and  results  in  a  gradually  deepening  carmine-red  which 
is  permanent,  while  in  the  case  of  soluble  carbonates  the  reaction  is 
instantaneous,  a  purple-red  which  changes  rapidly  through  cherry, 
eosin-red  to  orange.  The  reaction  with  ammonium  carbonate  is 
similar  to  that  with  bicarbonate. 

lodeosin: 

Tetra-iodo-fluarescein  Erythrosm  B.  This  indicator  is  useful  for 
minute  quantities  of  alkali,  as  for  instance,  such  as  may  be  dissolved 
out  from  glass  on  contact  with  water.  It  is  used  in  connection  with 
highly  dilute  standard  solutions  only.  The  iodeosin  solution  is  made 
by  dissolving  .002  gm.  of  the  indicator  in  1000  cc.  of  pure  ether.  Titra- 
tion with  this  indicator  is  carried  out  by  introducing  50  to  100  cc. 
of  the  liquid  to  be  titrated  into  a  stoppered  bottle  and  adding  10  to 
20  cc.  of  the  ethereal  indicator  solution  and  setting  aside  after  shaking. 
The  ethereal  layer  as  well  as  the  fluid  will  be  colorless  if  the  latter 
is  neutral,  but  if  traces  of  alkali  are  present  the  rose-red  tint  passes 
into  the  aqueous  liquid  leaving  the  ether  colorless.  If  the  fluid  is 
acid,  the  ethereal  layer  is  yellow.  If  preferred,  4  or  5  drops  of  a  i-  10,000 
aqueous  solution  of  the  indicator  may  be  added  to  the  liquid,  and 
ether  then  added.  lodeosin  is  particularly  useful  in  titration  of  alka- 
loids, especially  those  of  weak  basicity,  as  emetine,  atropine,  mor- 
phine, etc. 

Lacmoid: 


Lacmoid  is  somewhat  allied  to  litmus,  but  differs  from  it  in  many 
respects.  It  is  a  product  of  resorcin,  and  may  be  prepared  by  heating 
gradually  to  no°C.  a  mixture  of  100  parts  of  resorcin,  5  parts  of 
sodium  nitrite,  and  5  parts  of  water.  After  the  violent  reaction  mod- 
erates it  is  heated  to  120°  C.  until  ammonia  ceases  to  be  evolved. 


22  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

The  residue  is  then  dissolved  in  warm  water  and  the  lacmoid  pre- 
cipitated therefrom  by  HC1;  the  free  acid  is  then  removed  by  washing 
and  the  residue  dried. 

This  constitutes  the  commercial  lacmoid,  which  is  not  in  a  suffi- 
ciently pure  state  to  be  used  as  an  indicator;  its  purification  is  effected 
according  to  Forster  by  treating  the  powder  with  boiling  water,  and 
acidulating  the  resulting  blue  solution  with  hydrochloric  acid.  After 
a  few  hours  the  precipitate  is  collected  and  washed  with  a  little  cold 
water  and  carefully  dried,  or  it  is  dissolved  in  alcohol  and  the  solution 
evaporated.  Even  after  careful  purification,  lacmoid  solution  may 
still  exhibit  a  violet  tinge,  which  is  a  disturbing  factor  in  accurate  work. 
To  remedy  this  defect,  Forster  suggests  the  addition  of  5  gms.  of 
beta-naphthol  green  to  3  gms.  of  purified  lacmoid  dissolved  in  700  cc. 
of  water  and  300  cc.  of  alcohol.  A  sample  of  lacmoid  which  is  only 
sparingly  soluble  in  water  should  be  rejected.  The  purer  the  article 
the  more  readily  does  it  dissolve  in  water. 

Lacmoid  paper  is  prepared  by  dipping  slips  of  calendered  unsized 
paper  into  the  blue  or  red  solution  and  drying  them. 

Lacmoid  is  slightly  affected  by  carbonic-acid  gas.  It  may  be  used 
cold  for  the  alkaline  and  earthy  hydroxids,  arsenites,  and  borates,  and 
the  mineral  acids.  The  carbonates  and  bicarbonates  of  the  alkalies  and 
alkali  earths  are  titrated  hot  with  this  indicator. 

Many  of  the  metallic  salts,  such  as  the  sulphates  and  chlorids  of 
iron,  copper,  and  zinc,  which  are  more  or  less  acid  to  litmus,  are  neu- 
tral to  lacmoid;  therefore  free  acids  in  such  solutions  may  be  estimated 
by  its  aid. 

Lacmoid  paper  reacts  alkaline  with  the  chromates  of  potassium  or 
sodium,  but  neutral  with  the  dichromates,  so  that  a  mixture  of  the  two 
or  of  chromic  acid  and  dichromate  may  be  titrated  by  its  aid. 

Litmus 

A  pigment  obtained  by  the  fermentation  of  certain  lichens,  prin- 
cipally from  Roccella  tinctoria  and  R.  fuciformis,  but  also  from  other 
species  of  lichen. 

It  occurs  in  commerce  in  small,  friable,  light  cakes  or  cubes,  of  a 
violet  color. 

The  coloring  principles  of  litmus  are  azolitmin,  erythrolitmin,  and 
erythrolein.  The  first,  which  is  the  most  important,  is  soluble  in  water, 
but  insoluble  in  alcohol.  The  other  two  are  readily  soluble  in  alcohol, 
but  only  sparingly  soluble  in  water. 


INDICATORS  23 

The  U.  S.  P.  process  for  making  litmus  test  solution  consists  in 
exhausting  coarsely  powdered  litmus  with  boiling  alcohol. 

The  residue  is  then  digested  with  about  an  equal  weight  of  cold 
water  so  as  to  dissolve  the  excess  of  alkali  present. 

The  blue  solution  thus  obtained,  after  being  acidulated,  may  be 
used  to  make  red  litmus-paper.  Finally,  the  residue  is  extracted  with 
about  five  times  its  weight  of  boiling  water  and  the  solution  filtered. 

The  filtrate  is  preserved  as  test  solution  in  wide-mouthed  bottles, 
stoppered  with  loose  plugs  of  cotton  to  exclude  dust,  but  to  admit  air. 

When  kept  in  closed  vessels  litmus  solution  gradually  loses  color, 
but  this  returns  upon  exposure  to  air  and  consequent  absorption  of 
oxygen. 

The  fermentation  to  which  the  loss  of  color  is  due  may  be  prevented 
by  saturating  the  solution  with  NaCl  or  by  the  addition  of  thymol  or 
phenol. 

The  British  Pharmacopoeia  recommends  to  boil  the  litmus  in 
powder  with  three  successive  portions  of  rectified  spirit,  and  then  to 
digest  the  residue  in  distilled  water  and  filter,  the  object  of  these  steps 
in  the  process  being  to  get  rid  of  the  greater  portion  of  erythrolitmin 
and  erythrolein,  which  are  soluble  in  alcohol.  Then  by  treating  the 
residue  with  water  a  larger  proportion  of  azolitmin  is  dissolved,  and 
the  solution  is  contaminated  with  very  little  of  the  other  two  prin- 
ciples. 

Litmus  test  solution  should  be  of  such  strength  that  3  drops  added 
to  50  cc.  of  water  will  impart  to  the  latter  a  distinct  color.  If  one  drop 

N 

of   —  acid  or  alkali  solution  be  added  to  this  the  color  should  change 
10 

to  red  or  blue. 

Litmus  may  be  used  in  a  very  large  number  of  titrations.  It  is  of 
value  in  the  titration  of  most  mineral  acids  and  of  a  few  organic  acids, 
e.g.,  benzoic  and  oxalic.  It  is  also  useful  in  the  titration  of  alkaline 
hydroxids  when  the  latter  are  free  from  carbonates. 

But  for  carbonates,  bicarbonates,  etc.,  a  reliable  end  reaction  can 
only  be  obtained  by  boiling  the  solution  during  the  titration,  in  order 
to  dispel  the  liberated  CO2. 

Free  CC>2  has  an  acid  reaction  with  litmus,  and  interferes  very  much 
with  the  finding  of  the  end  reaction. 

Litmus  may  be  used  for  ammonia  and  for  borax.  It  is  of  no  use 
for  phosphoric  or  arsenic  acid,  nor  for  sulphurous  acid,  phosphates, 
or  arsenates,  because  the  change  of  tint  is  too  gradual. 


24  A    MANUAL    O'F    VOLUMETRIC    ANALYSIS 

It  is  unsatisfactory  in  titrating  many  organic  acids,  e.g.,  tartaric 
and  citric,  but  may  be  used  for  oxalic  or  benzoic,  as  before  stated. 

Sometimes  it  is  required  to  perform  a  titration  with  litmus  at  night. 
Gas  or  lamp  light  is  not  adapted  for  showing  the  reaction  satisfactorily, 
but  by  using  a  monochromatic  light,  such  as  the  sodium  flame,  a  very 
sharp  line  of  demarcation  may  be  found. 

The  operation  should  be  conducted  in  a  dark  room,  using  a  piece  of 
platinum  foil  sprinkled  with  salt  or  a  piece  of  pumice-stone  saturated 
with  a  solution  of  salt,  heated  in  a  Bunsen  flame. 

The  red  color  then  appears  perfectly  colorless,  while  the  blue  appears 
like  a  mixture  of  ink  and  water. 


alkalies 

acids      =  Colorless. 

Chemically  it  is  an  oxy-chlor-diphenyl-quinoxalin.  It  was  sug- 
gested as  an  indicator  by  Autenrieth. 

The  solution  for  the  purposes  of  an  indicator  is  prepared  by  dis- 
solving i  part  in  100  parts  of  alcohol.  Of  this,  four  drops  are  sufficient 
for  50  cc.  of  fluid  to  be  titrated. 

In  sensitiveness,  luteol  exceeds  both  litmus  and  phenolphthalein. 
It  is  more  sensitive  toward  ammonia  than  Nessler's  solution.  10  cc. 
of  a  solution  containing  one  drop  of  ammonia  water  per  liter,  is  colored 
yellow  immediately  upon  adding  luteol,  whereas  with  Nessler's  solu- 
tion it  takes  quite  some  time  before  a  reaction  is  obtained. 

Methyl  Orange: 

Poirrier's  Orange  III,  Tropaeolin  D,  Helianthin,  Mandarin-orange, 
Para  -sulpho  -benzeneazo-di  methyl  an  il  in  . 

This  is  prepared  by  the  action  of  diazo-sulphanilic  acid  upon 
dimethylanilin;  the  acid  so  formed  is  converted  into  a  sodium  or 
ammonium  salt,  purified  by  reprecipitation  with  HC1,  and  again  con- 
verted into  a  sodium  or  ammonium  salt.  If  prepared  carefully  and 
from  the  purest  materials,  it  is  a  bright  orange-red  powder,  perfectly 
soluble  in  water  and  slightly  in  alcohol;  but  it  is  often  found  in  com- 
merce as  a  dull  orange-brown  powder,  often  not  completely  soluble  in 
water.  Many  conflicting  statements  have  been  made  by  operators  as 
to  the  value  of  methyl  orange  as  an  indicator,  which  have  tended  to 
bring  this  indicator  into  disrepute. 

Sutton  has  examined  many  specimens,  but  has  not  found  any  in 
which  the  impurities  sensibly  affected  its  delicate  action.  He  claims 


INDICATORS  25 

that  the  common  error  is  the  use  of  too  much  indicator,  and  that  some 
eyes  are  more  sensitive  to  a  change  of  tint  than  others. 

Methyl  orange  is  no  doubt  a  very  good  indicator,  but  practice 
with  it  must  be  had  in  order  to  obtain  good  results.  The  author  has 
found  one  sample  which  had  a  beautiful  orange  color,  but  which  was 
absolutely  useless  as  an  indicator. 

A.  H.  Allen  describes  as  follows  the  characters  and  tests  of  a  good 
article: 

1.  Aqueous    solution,    not    precipitated    by    alkalies.     (Orange    I 
becomes  red-brown;  orange  II  brownish-red.) 

2.  Hot  concentrated  aqueous  solution  yields  with  HC1  microscopic 
acicular  crystals  of  the  free  sulphonic  acid,  soon  changing  to  small 
lustous  plates  or  prisms  having  a  violet  reflection.      (Orange  I  gives 
yellow-brown  color  or  flocculent  precipitate;    orange  II  brown-yellow 
precipitate.) 

3.  Dissolves  in  concentrated  H2SO4  with  a  reddish  or  yellowish- 
brown  color,  which  on  dilution  becomes  fine  red. 

4.  BaCl2  yields  a  precipitate. 

5.  CaCl2  yields  no  precipitate.     (Orange  I  gives  a  red  precipitate.) 

6.  Pb(C2H3O2)2  yields  an  orange-yellow  precipitate. 

7.  MgSO4  in  dilute  solutions  precipitates  the  coloring  matter  in 
microscopic  crystals. 

Methyl-orange  T.  S.  is  made  by  dissolving  i  gm.  of  methyl  orange, 
in  1000  cc.  of  water.  Add  to  it  carefully  diluted  sulphuric  acid  in 
drops  until  the  liquid  turns  red  and  just  ceases  to  be  transparent. 
Then  filter. 

The  great  value  of  this  indicator  consists  in  the  fact  that  it  is  not 
affected  by  carbonic-acid  gas,  sulphureted  hydrogen,  or  silicic,  oleic, 
stearic,  and  many  other  acids. 

It  answers  well  for  ammonia,  but  it  is  useless  for  most  of  the  organic 
acids.  Phosphoric  and  arsenic  acids  are  rendered  neutral  to  methyl 
orange  when  only  one  third  of  the  acid  has  combined  with  the  base, 
the  end  reaction  being  well  defined.  (Phenolphthalein  indicates  neu- 
trality when  two  thirds  of  acid  are  combined.) 

This  indicator  should  not  be  employed  when  titrating  with  standard 
solutions  which  are  weaker  than  decinormal,  nor  should  it  be  used  in 
any  hot  titrations,  nor  in  excessive  quantities.  Two  drops  are  sufficient 
for  50  cc.  of  the  fluid  to  be  titrated,  or  just  enough  to  give  it  a 
faint  tint. 


26  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

«    ,.  /  carbonates  =Red 

Phenacetolin:         l11  \  hydroxids   =  Yellow 

acids  =  Yellow 

This  indicator  is  prepared  by  heating  together  for  several  hours 
equal  molecular  weights  of  phenol,  glacial  acetic  acid,  and  sulphuric 
acid  in  a  vessel  provided  with  a  reflux  condenser.  The  product  is  then 
thoroughly  washed  with  water  to  remove  excess  of  acid  and  dried  for 
use.  It  is  only  very  slightly  soluble  in  water,  but  dissolves  readily  in 
alcohol,  forming  a  greenish-browrn  solution. 

The  solution  yields  with  alkali  hydroxids  a  scarcely  perceptible 
pale  yellow,  but  with  normal  carbonates  of  the  alkalies,  sulphids,  and 
with  ammonia  it  gives  a  decided  pink  color;  with  bicarbonates  a 
more  intense  pink,  while  with  acids  a  golden  yellow. 

This  indicator  is  useful  for  estimating  the  amount  of  alkali  or 
alkali  earth  hydroxids  in  the  presence  of  carbonate,  unless  the 
hydroxid  is  present  in  too  small  quantity.  Ammonia  must  not  be 
present.  The  titration  is  carried  out  by  adding  the  acid  until  a  faint 
red  color  appears;  this  indicates  that  the  alkali  hydroxid  or  the  lime 
has  been  neutralized.  The  further  addition  of  the  acid  intensifies  the 
red  until  the  carbonate  present  in  the  mixture  is  neutralized,  when  a 
golden  yellow  color  appears.  The  proportion  of  alkali  hydroxid  must 
be  far  in  excess  of  the  carbonate  in  order  to  obtain  reliable  results; 
furthermore,  considerable  practice  is  required  in  the  use  of  this  indi- 
cator in  order  to  accustom  the  eye  to  the  color  changes. 

A  convenient  strength  of  solution  is  15:00. 

Phenolphthalein  (C20H14O4): 

Preparation. — Five  parts  of  phthalic  anhydrid  (C8H4O3),  10  parts 
of  phenol  (CeHeOH),  and  4  parts  of  H2SO4  are  heated  together  at  120° 
to  130°  C.  for  several  hours.  The  product  is  then  boiled  with  water, 
and  the  residue,  which  consists  of  impure  phenolphthalein,  is  dissolved 
in  dilute  soda  solution  and  filtered.  By  neutralizing  this  solution  the 
phenolphthalein  is  precipitated  and  may  be  purified  by  crystallization 
from  alcohol;  or  the  alcoholic  solution  may  be  boiled  with  animal 
charcoal,  filtered,  and  the  phenolphthalein  reprecipitated  by  boiling 
water. 

Uses. — Phenolphthalein  is  a  very  valuable  indicator;  it  is  extremely 
sensitive,  and  exhibits  a  well-marked  and  prompt  change  from  colorless 
to  pink,  and  vice  versa.  A  few  drops  of  the  solution  of  the  indicator 
show  no  color  in  neutral  or  acid  liquids,  but  the  faintest  excess  of 
alkali  produces  a  sudden  change  to  red. 


INDICATORS  27 

It  may  be  employed  in  the  titration  of  mineral  and  organic  acids 
and  most  alkalies,  but  it  is  not  suited  for  the  titration  of  ammonia  or 
its  salts.  It  is  very  sensitive  to  CO2,  and  therefore  in  estimating  car- 
bonates the  liquid  must  be  boiled,  as  with  litmus.  It  is  inapplicable 
for  borax,  except  in  the  presence  of  glycerin,  because  the  color  gradu- 
ally fades  away  as  the  acid  is  added.  One  great  advantage  which 
phenolphthalein  possesses  is  that  its  indications  may  be  clearly  reid 
in  many  colored  liquids;  another  is  that  it  may  be  used  in  alcoholic 
liquids  or  in  mixtures  of  alcohol  and  ether,  and  therefore  many  organic 
acids  which  are  insoluble  in  water  may  be  accurately  titrated  by  its 
help. 

Phenolphthalein  T.  S.  is  a  one  per  cent  solution  in  diluted  alcohol. 

-II  «r  /  carbonates  =  Blue 

Poirrier  Blue  (C4B):   alkah  I  hydroxids  =Red 

acids  =  Blue 

This  indicator,  which  is  closely  allied  to  Gentian  Blue  in  properties, 
is  obtained  by  the  action  of  sulphuric  acid  on  triphenylrosanilin.  It 
is  a  blue  powder  with  a  coppery  luster.  It  dissolves  in  water  and  in 
alcohol,  yielding  blue  solutions.  KOH  and  NaOH  change  the  color 
to  red,  but  ammonia  decolorizes  it.  It  is  employed  as  an  indicator  in 
aqueous  solution  1:500.  This  indicator  is  exceedingly  sensitive  to 
acids.  Borax  and  boric  acid  give  a  blue  color;  in  the  titration  of  boric 
acid  the  red  color  does  not  appear  until  the  acid  is  completely  neu- 
tralized. This  indicator  is  recommended  for  the  titration  of  hydro- 
cyanic acid,  toward  which  it  is  especially  sensitive,  the  alkaline  cyanids 
are  alkaline  in  reaction  to  most  indicators,  but  C±B  does  not  show  an 
alkaline  reaction  until  the  HCN  is  completely  neutralized,  and  a  minute 
excess  of  the  alkali  hydroxid  has  been  added.  €46  is  of  the  char- 
acter of  a  weak  acid  and  its  salts  are  very  unstable;  they  are  decomposed 
by  water  alone  when  in  very  great  dilution,  therefore  the  indicator 
must  be  used  in  sufficient  quantity,  The  addition  of  a  few  drops  of 
alcohol  facilitates  the  color  change,  which  is  indeed  a  very  sharp  one. 

Potassium-chromate  Test-solution  is  used  in  the  titration  of 
haloid  salts  with  silver-nitrate  solution.  It  indicates  that  all  the  halogen 
has  combined  with  the  silver  by  producing  a  red-colored  precipitate 
(silver  chromate). 

Potassium-ferricyanid  Test-solution  is  used  in  the  estimation  of 
ferrous  salts  with  potassium-dichromate  solution.  It  gives  a  blue  color 
to  a  drop  of  the  solution  on  a  white  slab  as  long  as  any  iron  salt  is 
present  which  has  not  been  oxidized  to  ferric. 


28  A    MANUAL    OF   VOLUMETRIC   ANALYSIS 

Pp«o7iirin«     alkalies  =  B/M« 

Kesazurm.    adds    =Red 

This  is  a  new  indicator  for  alkalimetry,  proposed  by  Crismer.  It 
is  prepared  as  follows:  Dissolve  4  gms.  of  resorcin  in  300  cc.  of  an- 
hydrous ether  and  add  40  to  45  drops  of  nitric  acid  (sp.  gr.i.25)  satu- 
rated with  nitrous  anhydrid.  Allow  the  mixture  to  stand  in  a  cold 
place  for  two  days,  whereupon  a  deposit  of  blackish  crystals,  having 
a  reddish-brown  reflection,  will  be  formed  in  the  bottom  of  the  vessel. 
The  supernatant  clear  red  liquid  is  decanted  and  the  crystals  washed 
with  ether  until  the  washings  show  a  blue  color  with  ammonia-water. 

Resazurin  (C^HyNOJ  is  slightly  soluble  in  water,  more  so  in 
alcohol  and  freely  soluble  in  acetic  ether.  It  produces  a  blue  solution 
with  water,  alkalies,  and  alkali  carbonates,  which  are  turned  red 
upon  the  addition  of  a  slight  excess  of  acid.  To  use  this  indicator  in 
alkalimetry,  Crismer  recommends  the  following  solution:  Resazurin 

N 

0.2  gm.  dissolved  in  40  cc.  of   —  ammonia  solution,  and  made  up  to 

10 

1000  cc.  with  distilled  water. 

This  is  deep  blue  in  color  and  keeps  well.  Two  or  three  drops  are 
sufficient  to  color  200  cc.  of  liquid. 

This  indicator  is  not  suited  for  the  titration  of  nitric  acid  or  mono- 
basic organic  acids,  and  it  is  not  very  sensitive  to  carbonic  acid.  It  is, 
however,  extremely  sensitive  to  alkalies.  If  the  solution  is  acidulated 
to  a  rose-red  color  and  heated  in  a  white  glass  flask,  the  solution  will 
turn  blue  through  the  alkaline  reaction  of  the  dissolved  glass  before 
the  boiling-point  is  reached. 

This  indicator  is  especially  useful  for  borax. 

Rosolic  Acid  (C20H1403): 

This  compound  is  also  called  Aurin  and  Coralline,  and  is  prepared 
as  follows: 

A  mixture  of  phenol  and  sulphuric  acid  is  placed  upon  a  water- 
bath,  and  oxalic  acid  gradually  added,  waiting  each  time  till  the  evo- 
lution of  gas  ceases,  and  using  less  oxalic  acid  than  is  required  to  attack 
all  the  phenol. 

In  this  process  the  oxalic  acid  is  decomposed  into  CO,  CO2,  and 
H2O.  The  CO  immediately  reacts  with  the  phenol  and  forms  rosolic 
acid,  as  the  following  equation  shows: 


INDICATORS  29 

Commercial  rosolic  acid  is  a  mixture  of  several  derivatives,  among 
them  the  above,  methylaurin  C2oH16O3  and  others.  Commercial 
poeonin  (also  known  as  Aurin  R.)  [chiefly  Ci9Hi4O3]  may  be  used  in 
place  of  rosolic  acid. 

Rosolic  acid  is  soluble  in  diluted  alcohol.  Its  color  is  pale  yellow, 
not  changed  by  acid,  but  turns  violet  red  with  alkalies. 

It  is  an  excellent  indicator  for  the  mineral  acids  and  strong  bases, 
weak  ammoniacal  solutions,  oxalic  acid,  and  other  organic  acids, 
except  acetic. 

The  test  solution  is  made  by  dissolving  i  gm.  of  the  commercial 
rosolic  acid  in  10  cc.  of  diluted  alcohol,  and  then  adding  enough  water 
to  make  100  cc. 


n  (OO)  : 

This  is  used  in  the  form  of  a  solution  containing  .5  gm.  to  1000  cc. 

Turmeric  Tincture.  Digest  any  convenient  quantity  of  ground 
curcuma-root  (from  Curcuma  longa  Linne,  nat.  ord.  Scitaminece) 
repeatedly  with  small  quantities  of  water,  and  throw  this  liquid  away. 
Then  digest  the  dried  residue  for  several  days  with  six  times  its  weight 
of  alcohol,  and  filter. 

Turmeric  Paper.  Impregnate  white,  unsized  paper  with  the 
tincture  and  dry  it. 

The  color  principle  of  turmeric  is  curcumin.  It  is  seldom  used  in 
volumetric  analysis,  except  in  the  form  of  turmeric  paper.  For  high' 
colored  solutions  curcumin  gives  no  reaction  with  acids,  but  becomes 
brown  with  alkalies.  There  is  another  color  principle  in  turmeric 
besides  curcumin,  which  is,  however,  useless  in  that  it  is  indifferent 
to  alkalies;  it  is  soluble  in  water,  and  is  extracted  by  digestion  with 
water,  after  which  the  curcumin  is  dissolved  out  with  alcohol. 

Turmeric  paper  is  especially  useful,  because  of  its  peculiar  reaction 
with  boric  acid,  with  which  it  develops  a  brown  color  after  drying, 
and  which  color,  when  touched  with  caustic  soda  solution,  is  changed 
to  dark  green. 

Starch  Solution.  This  is  useful  as  in  indicator  in  iodometric 
estimations.  It  is  sensitive  to  the  extent  of  detecting  iodin  in  solu- 
tions containing  less  than  i  in  2,000,000.  It  gives  with  iodin  a  dark- 
blue  color.  Starch  solution  decomposes  readily  and  should,  therefore, 
be  freshly  prepared  when  needed.  A  decomposed  solution  gives  a 
brownish-red  or  greenish  color  instead  of  a  blue.  If  too  much  of  the 
indicator  is  used  the  color  produced  is  nearly  black.  About  i  cc.  should 


30 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


be  employed  ordinarily  in  titrations,  and  the  operation  should  be 
conducted  at  the  ordinary  temperature,  as  the  application  of  heat 
destroys  the  color.  It  is  further  recommended  that  the  starch  indicator 
be  introduced  only  toward  the  end  of  the  reaction.  The  solution  is 
prepared  by  mixing  i  gm.  of  potato  starch  with  10  cc.  of  cold  water 
and  then  adding  sufficient  boiling  water,  with  constant  stirring  to 
make  200  cc.  of  a  thin  jelly-like  solution.  The  addition  of  two  or  three 
drops  of  mercuric  chlorid  T.  S.  will  tend  to  prevent  decomposition, 
though  it  is  always  best  to  prepare  the  solution  when  wanted.  Arrow- 
root starch  may  be  used  instead  of  potato  starch. 

A   GUIDE   FOR   THE   SELECTION   OF   INDICATORS. 

Hydroxids  and  Carbonates  of  the  Alkalies  and  Alkali  Earths.  In  the 
titration  of  carbonates  with  standard  acid  solutions  there  is  an 
evolution  of  CC>2  which  reacts  acid  with  most  indicators.  Those 
indicators  which  are  so  affected  cannot  be  employed  in  the  presence 
of  carbonates  except  in  hot  titrations,  the  CO2  being  dispelled  by 
boiling. 

Those  which  are  not  so  affected  may  be  used  in  cold  titrations  and 
are  therefore  preferred,  as  heat  affects  many  of  the  indicators.  Alkali 
hydroxids  almost  invariably  contain  small  amounts  of  carbonate,  hence 
the  selection  of  an  indicator  for  either  hydroxids  or  carbonates  is  to 
be  made  from  the  same  list. 


Indicators  not  affected  by  CO2. 

(Cold  Titrations). 
Methyl  Orange: 

Useful  for  hydroxids  of  alkali  and 
alkali  earths  and  carbonates  of  al- 
kalies. Large  quantities  of  neutral 
salts  must  be  absent;  standard 
oxalic  acid  is  useless. 
Phenacetolin: 

Large  quantities  of  NH3  salts  must 
not  be  present. 
Congo  Red: 

Large  quantities  of  salts  of  fixed 
alkalies  NH3,  Ca,  Ba,  and  Mg  must 
not  be  present. 
Cochineal: 

Not  reliable  in  presence  of  salts  of 
Fe,  Al,  or  Cu. 
lodeosin: 

Useful,     especially     for      minute 
quantities  of  alkali. 
Gallein. 


Indicators  affected  by  CO2. 

(Hot  Titrations). 
Litmus. 
Phenolphthalein : 

Useless   if  "NH3  or  its  salts   are 
present. 


Lacmoid: 

Used  in  residual  titrations  for  car 
bonate. 
Luteol. 
Resazurin. 
Rosolic  Acid. 


INDICATORS 


31 


Ammonia  (NH3) 

Luteol. 
Litmus. 
Gallein. 

Congo  Red, 

Methyl  Orange, 

RosolicAcid  other  alkalies  must 

L      be  absent. 
Cochineal: 

In  absence  of  salts  of  Fe,  Al,  and 
Cu. 


Ammonium  Carbonate 
(Residual  Titration). 


Litmus. 
Phenacetolin. 
Methyl  Orange. 
Rosolic  Acid 
Phenolphthalein. 
Gallein. 


Inorganic  Acids 


(H2S04,  HC1,  HN03). 
Phenolphthalein : 

Useless  in  presence  of  H3AsO3. 
Congo  Red: 

Not  reliable  for  HNO2  or  H2SO4. 
Litmus. 
Lacmoid: 

Not  reliable  for  HNO3,  HNO2  or 

H2S. 

Rosolic  Acid. 
Methyl  Orange: 

Not  reliable  for  HNO2. 
Cochineal. 
Resazurin: 

Not  reliable  for  HNO,. 
Luteol. 
Gallein. 


(H3P04). 
Congo  Red. 
Methyl  Orange: 

Neutralized  to  NaH2PO4. 
Cochineal: 

Neutralized  to  NaH3PO4. 
Phenolphthalein: 

Neutralized  to  Na2HPO4. 
(H2S03). 

Rosolic  Acid. 
Methyl  Orange. 
(H3B03). 

Phenolphthalein : 

After  the  addition  of  glycerin. 
Ferric  Salicylate  Solution. 
Litmus. 

Turmeric  Paper. 
Poirrier's  Blue,  C4B, 


Organic  Acids 

Phenolphthalein  (all). 

Rosolic  Acid  (except  Acetic,  Citric  and  Tartaric). 

Congo  Red  (Acetic  Acid  only). 

Gallein. 

Litmus  (Oxalic  and  Benzoic  only). 


CHAPTER  V 


APPARATUS  USED  IN  VOLUMETRIC  ANALYSES 

The  Burette  is  a  graduated  glass  tube  which  holds  from  25  to 
100  cc.  and  is  graduated  in  fifths  or  tenths  of  a  cc.,  and  provided  at 
the  lower  end  with  a  rubber  tube  and  pinch- cock.  The  use  of  this 
instrument  is  to  accurately  measure  quantities  of  standard  solutions 

used  in  an  analysis.  It  is  in  an  upright 
position  when  in  use,  and  the  flow  of  the 
solution  can  be  regulated  so  as  to  run  out 
in  a  stream  or  flow  in  drops  by  pressing 
the  pinch-cock  between  the  thumb  and 
forefinger.  The  quantity  of  solution  used 
can  be  read  from  the  graduation  on  the 
outside  of  the  tube.  This  is  the  simplest 
and  most  common  form  of  burette,  and  is 
known  as  Mohr's  (Fig.  i). 

The  use  of  the  pinch-cock  in  Mohr's 
burette  may  be  dispensed  with  by  intro- 
ducing into  the  rubber  tube  a  small  piece 
of  glass  rod,  which  must  not  fit  too 
tightly.  By  firmly  squeezing  the  rubber 
tube  surrounding  the  glass  rod  a  small 
canal  is  opened,  through  which  the  liquid 
escapes.  A  very  delicate  action  can  in 
this  way  be  obtained,  and  the  flow  of  the 
liquid  is  completely  under  the  control  of 
the  operator.  (See  Fig.  2.) 

The  greatest  drawback  to  this  burette 
is  that  it  cannot  be  used  for  permanganate  or  other  solutions  that  act 
upon  the  rubber. 

This  defect  can  be  overcome  by  the  use  of  a  burette  having  a  glass 
stop-cock  in  place  of  the  rubber  tubing  and  pinch-cock.  This  form 
has  the  additional  advantage  of  being  capable  of  delivering  the  solu- 
tion in  drops  while  both  hands  of  the  operator  are  disengaged  (Fig.  3). 

32 


FIG.  i. 


THE  BURETTE 


33 


Another  good  arrangement  is  that  in  which  the  tap  is  placed  in  an 
oblique  position,  so  that  it  will  not  easily  drop  out  of  place  (Fig.  4). 

These  glass  stop-cock  burettes  should  be  emptied  and  washed 
immediately  after  use,  especially  if  soda  or  potassa  solution  has  been 
used ;  for  these  act  upon  the  glass  and  often  cause  the  stopper  to  stick 
so  firmly  that  it  cannot  be  turned  or  removed  without  danger  of  breaking 
the  instrument. 

The  most  satisfactory  form  of  glass  stop-cock  is  that  shown  in 


FIG.  4. 


FIG.  3. 


FIG.  5. 


FIG.  6. 


Fig.  6.  Other  forms  of  burettes  are  Mohr's  Foot  Burette,  with  rubber 
ball  (Fig.  5).  There  is  a  hole  in  the  rubber  ball,  and  by  placing  the 
thumb  over  the  hole  and  gently  squeezing,  the  flow  of  the  liquid  may 
be  regulated. 

Bink's  Burette  (Fig.  7  is  used  by  holding  in  the  hand  and  inclining 
sufficiently  to  allow  the  quid  to  flow,  then  placing  in  an  upright  posi- 
tion, and  reading  when  the  surface  of  the  liquid  has  settled. 

Gay-Lussac's  (Fig.  8)  must  also  be  inclined  when  used.  A  wooden 
foot  is  generally  provided,  into  which  this  burette  is  placed  to  rest  in 


34 


A    MANUAL    OF    VOLUMETRIC   ANALYSIS 


an  upright  position.  By  inserting  a  tightly  fitting  cork  into  the  open 
end  and  passing  through  this  cork  a  small  bent  glass  tube,  the  flow 
of  the  solution  from  the  exit-tube  can  be  nicely  regulated  by  blowing 
through  the  small  glass  tube.  The  necessity  for  inclining  the  burette 
is  thus  obviated.  (See  Fig.  9). 

The  burette  shown  in  Fig.   10  with  the  spindle-shaped  spout  is 
used  in  the  same  manner  as  Bink's.     It  is  claimed  for  the  dilated  spout 


FIG.  7. 


FIG.  8. 


FIG.  9. 


that  it  more  readily  admits  of  the  delivery  of  single  drops  and  prevents 
the  too  sudden  back-dropping  of  the  solution  upon  returning  the 
burette  to  the  upright  position. 

The  three  latter  burettes  being  held  in  the  hand  when  in  use,  there 
is  a  chance  of  increasing  the  bulk  of  the  fluid  by  the  heat  of  the  hand, 
thus  leading  to  errors  in  measurement. 

Geissler's  Foot  Burette  (Fig.  u)  needs  no  further  description. 

A.  Hesse  has  constructed  the  burette  shown  by  Fig.   12,  which 


THE  BURETTE 


35 


facilitates  the  accurate  standardization  of  normal  test  solution.  The 
upper  constricted  part  of  the  burette,  having  a  capacity  of  0.5  cc.,  is 
divided  into  T-^  cc.,  the  next  following  main  portion,  as  usual,  into  j\ 
cc.,  and  the  lower  constricted  portion  again  in  T fa  cc.  for  a  distance 
of  2  cc.  (from  the  49  to  51  cc.  mark).  These  graduations  enable  the 
practically  absolute  measurement  of  the  solutions  required  when  it  is 
known  that  the  total  volume  required  for  a  titration  reaches  to  within 
the  limit  of  the  constricted  lower  portion  (to  between  49  and  51  cc.), 
this  having  previously  been  ascertained  experimentally.  The  upper 


FIG.  10. 


FIG.  ii. 


FIG. 


constricted  portion  is  surmounted  by  a  cup-shaped  expansion,  to 
prevent  overflow  in  case  the  liquid  is  allowed  to  enter  too  rapidly. 

When  a  number  of  estimations  are  to  be  made  in  which  the  same 
volumetric  solution  is  employed,  the  arrangement  shown  in  Figs.  13 
and  14  is  very  serviceable. 

A  T-shaped  glass  tube  is  inserted  between  the  lower  end  of  the 
burette  and  the  pinch-cock  and  connected  by  a  rubber  tube  with  a 
reservoir  containing  the  volumetric  solution.  The  tube  which  commu- 
nicates with  the  reservoir  is  provided  with  a  pinch-cock,  which  when 
open  allows  the  solution  to  flow  into  and  fill  the  burette  in  so  gradual 
a  manner  that  no  bubbles  are  formed.  The  burette  is  emptied  in  the 
usual  manner. 


36 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


E.  &  A.  Automatic  Burette  (Fig.  15).  This  is  used  for  the  same 
purpose  as  the  foregoing.  It  is  provided  with  a  side  tube  for  con- 
nection with  reservoir,  and  has  an  over-flow  reservoir  which  pre- 
vents its  being  filled  to  above  the  zero  mark.  The  three-way  stop-cock 
is  so  arranged  that  if  turned  one  way  the  inlet  is  opened  and  the 


FIG.  13. 


FIG.  14. 


liquid  from  the  reservoir  flows  into  and  fills  the  burette.  If  turned 
the  other  way  the  inlet  is  closed  and  the  outlet  is  opened  and  the 
burette  may  be  emptied.  If  the  handle  of  the  stop-cock  is  turned 
half-way  round,  both  openings  are  closed. 

There  are  many  other  forms  of  automatic  burettes. 

When  working  with  solutions  which  are  readily  altered  by  contact 


THE  BURETTE 


37 


with  air,  as  for  example,  stannous  chlorid,  potassium,  sodium,  or 
barium  hydroxid  or  ammonia,  an  arrangement  like  that  depicted  in 
Fig.  1 6  is  very  serviceable.  In  this  the  upper  end  of  the  burette  is 
connected  with  the  reservoir  by  means  of  a  rubber  tube,  thus  making 
an  air-tight  combination  between  the  burette  and  the  reservoir.  Its 


FIG.  15. 


FIG.  i 6. 


utility  may  be  further  enhanced  by  providing  the  reservoir  with  a  soda- 
lime  tube  or  some  other  suitable  absorption  tube. 

Another  form  of  apparatus,  and  one  which  is  better,  is  shown  in 
Fig.  17.  In  this  both  the  burette  and  the  reservoir  are  provided  with 
tubes  containing  soda-lime  to  insure  a  protection  against  the  admission 
of  CO2  and  moisture  from  the  air. 

Fig.  1 8  depicts  a  form  which  answers  the  same  purpose,  as  does 
also  the  instrument  represented  in  Fig.  19.  The  latter  having  the 


38 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


advantage  in  being  provided  with  an  automatic  zero-point  burette, 
and  in  that  the  solution  does  not  need  to  be  passed  through  rubber 
tubes  except  at  the  outlet.  It  is  furthermore  provided,  like  the  preced- 


FIG.  17. 


FIG.  18. 


ing  one,  with  an  absorption  tube  placed  between  the  rubber  bulbs  and 
the  reservoir,  as  well  as  with  one  at  the  top  of  the  burette. 

Pinch-cocks  used  with  Mohr's  burettes  are  of  various  kinds.  See 
Figs.  20,  21,  and  22. 

The  screw-pinch  cock,*  Fig.  22,  is  a  very  useful  device;  it  may 
be  used  like  the  ordinary  pinch-cock  by  pressure  with  the  fingers 

*  W.  v.  Hergdendorf. 


PIPETTES 


39 


upon  a-a,  when  a  rapid  flow  is  desired,  or  the  nut-screw  (b)  may  be 
so  adjusted  as  to  allow  a  slower  flow  or  to  deliver  the  solution  in 
drops,  thus  giving  the  operator  the  freedom  of  both  hands  for  other 
work. 

Burette-supports  are  of  various  forms.  One  of  the 
best  for  one  or  two  burettes  is  shown  in  Fig.  23.  It  is 
made  of  iron,  can  stand  firmly  upon  an  uneven  surface, 
and  does  not  easily  tip  over.  The  burettes  are  fastened 
to  it  by  means  of  clamps,  illustrated  in  Figs.  24  and  25. 

A  revolving  burette -holder  for  eight  burettes  is  shown 
in  Fig.  26.  Burette-supports  are  also  made  with  white 
porcelain  base,  which  enables  the  operator  more  readily 
to  see  the  change  of  color  in  the  liquid  titrated. 

Pipettes  are  of  two  kinds — those  which  are  marked 
to  deliver  one  quantity  only,  and  those  which  are 
graduated  on  the  stemlike  burettes.  Their  use  is  to 
measure  out  portions  of  solutions  with  exactness. 

Pipettes  are  filled  by  applying  the  mouth  to  the 
upper  end  and  sucking  the  liquid  up 
to  the  mark,  then  by  closing  the  upper 
opening  with  the  forefinger  the  liquid 
is  prevented  from  running  out,  but 
may  be  delivered  in  drops  or  allowed 
to  run  out  to  any  mark  by  lessening 
the  pressure  of  the  finger  over  the 
opening. 

In  using  the  pipettes  of  the  first 
class  (Fig.  27)  the  finger  is  raised  and 
the  instrument  allowed  to  empty  itself 
entirely.  A  drop  or  two,  however, 
usually  remains  in  the  lower  portion 
of  the  instrument,  which  may  be 
blown  out.  By  inclining  the  pipette 
and  placing  the  point  against  the  side  FlG-  19- 

of  the  vessel  which  is  to  receive  the  liquid,  the  instrument  may  be 
emptied  more  satisfactorily. 

Pipettes  of  the  second  class  (Fig.  28)  are  never  emptied  completely 
when  in  use.  The  flow  of  the  liquid  is  regulated  by  the  pressure 
of  the  finger  over  the  upper  opening,  and  stopped  at  the  desired 
point. 


40  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 


FIG.  24. 


FIG.  23. 


FIG.  25. 


PIPETTES 


41 


A  very  convenient  form  of  pipette  is  one  which  has  attached  to  its 
upper  end  a  piece  of  rubber  tubing,  into  which  a  short  piece  of  glass 
rod  has  been  inserted.  By  squeezing  the  rubber  surrounding  the  glass 
rod  firmly  between  the  fingers,  a  canal  is  opened  and  the  liquid  can. 
be  drawn  up  into  the  pipette  by  suction  with  the  lips  and  run  out  again 


soo,a 


lid  C.C. 


FIG.  26. 


FIG.  27. 


By  removing  the  pressure  the  canal  closes  and  the  flow  of  the  liquid 
is  stopped  at  any  point  (Fig.  29). 

The  Nipple  Pipette  is  very  convenient  for  measuring  small  quan- 
tities of  liquids,  such  as  i  or  2  cc.  (Fig.  30). 

When  a  volatile  or  highly  poisonous  solution  is  to  be  measured 
it  is  not  advisable  to  suck  it  up  with  the  mouth.  The  pipette  in  this 
case  is  filled  by  dipping  it  into  the  liquid  contained  in  a  long,  narrow 


42 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


vessel,  until  the  liquid  reaches  the  proper  mark  on  the  pipette,  then 
closing  the  upper  opening  and  withdrawing.  When  this  is  done  the 
liquid  which  adheres  to  the  outside  of  the  pipette  should  be  dried  off 
before  the  measured  liquid  is  delivered. 

A  French  firm  has  introduced  pipettes  provided  with  suction 
pumps,  shown  in  various  forms  by  Fig.  31,  which  possess  the  ad- 
vantage over  the  ordinary  forms  provided  with  a  compressible  rubber 
bulb,  that  the  liquid  can  with  perfect  ease  be  drawn  up  to  the  desired 


FIG.  28. 


FIG.  29.         FIG.  30. 


FIG.  31. 


point  on  the  scale,  and  with  absolute  accuracy  maintained  at  the  same 
height  as  long  as  may  be  desired. 

The  Measuring-flask  is  a  vessel  made  of  thin  glass  having  a 
narrow  neck,  and  so  constructed  as  to  hold  a  definite  amount  of  liquid 
when  filled  up  to  the  mark  on  the  neck.  These  flasks  are  of  various 
sizes,  holding  100,  250,  500,  1000  cc.,  etc.,  but  are  generally  called 
"Liter  Flasks."  (Fig.  32.) 

Giles'  Liter  Flask,  Fig.  33,  is  of  noo  cc.  capacity,  graduated  at 
1000  cc.  and  noo  cc.  Its  value  in  making  normal  solutions  is  obvious, 


THE   TEST  MIXER 


43 


by  taking  one-tenth  more  of  the  reagent  than  would  be  used  for  making 
one  liter  of  normal  solution,  and  then  filling  up  to  the  noo  cc.  mark, 
there  is  obtained  100  cc.  of  solution  for  ascertaining  exact  liter,  leaving 
exactly  one  liter  for  correction. 

Another  very  convenient  liter  flask  is  that  shown  in  Fig.  34, 
which  was  designed  by  A.  Gaske  The  neck  of  the  flask  is  somewhat 
prolonged  and  graduated  from  1000  cc.  down  to  920  cc.  This  arrange- 
ment permits  any  quantity  of  liquid  to  be  removed  for  standardization, 
and  the  balance  made  up  to  the  volume  desired. 

Liter  flasks  are  used  for  making  volumetric  solutions. 


FIG.  32. 


FIG.  33. 


FIG.  34. 


FIG.  35. 


Those  which  have  the  mark  below  the  middle  of  the  neck  are  to 
be  preferred,  because  the  contents  can  be  more  easily  shaken. 

Liter  flasks  are  sometimes  made  with  two  marks  on  the  neck  very 
near  together;  the  lower  one  is  the  liter  mark.  If  the  flask  is  required 
to  deliver  a  liter,  it  must  be  filled  to  the  upper  mark;  the  difference 
between  the  two  measures  being  the  equivalent  of  the  liquid  which 
remains  in  the  flask,  adhering  to  the  sides. 

The  Test  Mixer,  or  Graduated  Cylinder  (Fig.  35),  is  for  measur- 
ing and  mixing  smaller  quantities  of  solutions.  They  are  made  of 
different  sizes,  holding  100,  250,  500,  and  1000  cc.,  and  graduated  in 
fifths  or  tenths  of  a  cc. 


CHAPTER  VI 
ON  THE  USE  OF  APPARATUS 

IT  is  important  that  all  apparatus  used  in  volumetric  analysis  should 
be  perfectly  clean.  Even  new  apparatus  should  be  cleansed  by  pass- 
ing dilute  hydrochloric  acid  through  them  and  then  rinsing  with  dis- 
tilled water. 

If  the  burette,  pipette,  or  other  instrument  is  even  slightly  greasy, 
the  liquid  will  not  flow  smoothly,  and  drops  of  the  liquid  will  remain 
adhering  to  the  sides,  thus  leading  to  inaccurate  results. 

Greasiness  may  be  removed  with  dilute  soda  solution.  If  this 
fails  the  instrument  should  be  allowed  to  remain  for  some  little  time 
in  a  solution  containing-  sulphuric  acid  and  potassium  dichromate, 
which  will  radically  remove  all  traces  of  grease. 

The  burette  or  other  measuring  instruments  should  never  be  filled 
with  volumetric  solution  without  first  rinsing,  even  if  the  burette  be 
perfectly  dry. 

It  is  well  to  wash  the  inside  of  the  instrument  with  two  or  three 
small  portions  of  the  solution  with  which  it  is  to  be  filled. 

The  burette  may  be  filled  with  the  aid  of  a  funnel,  the  stem  of 
which  should  be  placed  against  the  inner  wall  of  the  burette,  so  that 
the  solution  will  flow  down  the  side  and  thus  prevent  the  formation 
of  bubbles. 

The  burette  should  be  filled  to  above  the  zero  mark,  and  the  air- 
bubbles,  if  there  are  any,  removed  by  gently  tapping  with  the  finger. 

A  portion  of  the  liquid  should  then  be  allowed  to  run  out  in  a 
stream  so  that  no  air-bubbles  remain  in  the  lower  part  of  the  burette. 
In  the  glass  tap  burette  it  can  be  easily  seen  if  any  air  is  present,  but 
in  the  pinch-cock  burette  it  is  sometimes  necessary  to  take  hold  of 
the  rubber  tube  between  the  thumb  and  forefinger  and  gently  stroke 
upward.  Or  the  glass  nip  at  the  lower  end  of  the  burette  may  be 
pointed  upward,  and  the  pinch-cock  opened  wide  so  that  a  stream  of 
the  liquid  will  pass  through  and  force  out  any  air  that  may  be  inclosed. 

If  the  titration  is  to  be  conducted  at  a  high  temperature,  as  in 
the  estimation  of  carbonates,  when  litmus  is  used  as  the  indicator,  or 

44 


ON  THE   USE  OF  APPARATUS  4o 

in  the  estimation  of  sugar  by  copper  solution,  a  long  rubber  tube  should 
be  attached  to  the  lower  end  of  the  burette.  The  boiling  can  then 
be  done  at  a  little  distance,  and  the  expansion  of  the  liquid  in  the 
burette  avoided.  The  pinch-cock  is  fixed  about  midway  on  the  tube. 

Hart  calls  attention  to  the  fact  that  if  the  fluid  in  a  burette  or  pipette 
be  run  out  rapidly  at  one  time  and  slowly  at  another,  different  amounts 
of  fluid  are  obtained. 

This  is  due  to  the  adhesion  of  the  fluid  to  the  inner  sides  of  the 
instrument,  and  reading  before  it  has  settled  down.  It  is  therefore 
advisable  always  to  deliver  burettes  slowly,  as  more  constant  results  are 
then  obtained. 

Solutions  which  are  measured  by  means  of  pipettes  should  be  dilute, 
since  concentrated  solutions  adhere  to  glass  with  different  degrees  of 
tenacity,  and  hence  the  amount  of  fluid  delivered  is  slightly  less  than 
that  measured. 

The  temperature  of  the  solutions  measured  should  be  taken  into 
account,  since  all  liquids  are  affected  by  change  of  temperature,  ex- 
panding and  contracting  as  the  temperature  is  increased  or  reduced. 

This  change  of  volume  in  the  case  of  standard  solutions  does  not 
exactly  correspond  to  that  in  pure  water;  in  fact  some  of  them  differ 
widely.  The  correction  of  the  volume  of  a  standard  solution  for  the 
temperature  by  the  expansion  coefficient  of  water  is  not  entirely  satis- 
factory, but  in  the  case  of  very  dilute  solutions  this  may  be  done. 

Casamajor  (C.  N.,  xxxv.  160)  gives  the  following  figures  showing 
the  relative  contraction  and  expansion  of  water  below  and  above  15°  C.: 

Degree  C.  Degree  C. 

8  —  .000590  17  +  .000305 

9-. 000550  18+.  000473 

10— .000492  19+.  000652 

1 1  —  .000420  20  +  .000841 

12  — .000334  21  -f.  001039 

13  — .000236  22 +  .OOI246 

14— .000124  23 +  .001462 

15  — normal  24 +.001 686 

16+. 000147  25  +  . 001919 

By  means  of  these  numbers  it  is  easy  to  calculate  the  volume  of 
liquid  at  15°  C.  corresponding  to  any  volume  observed  at  any  tempera- 
ture between  8°  C.  and  25°  C.  If  25  cc.  of  solution  had  been  used 
at  20°  C.,  the  table  shows  that  i  cc.  of  water  passing  from  15°  to  20° 


46 


A    MANUAL   OF   VOLUMETRIC  ANALYSIS 


is  increased  to  1.000841  cc.  Therefore,  by  dividing  25  cc.  by  1.000841, 
the  quotient,  24.97  cc-  is  obtained,  which  represents  the  volume  at  15°  C. 
corresponding  to  25  cc.  at  20°  C. 

These  corrections  are  of  value  only  for  very  dilute  solutions  and 
for  water,  but  useless  for  concentrated  solutions.  Slight  variations  of 
atmospheric  pressure  may  be  disregarded. 

ON  THE   READING   OF   INSTRUMENTS 

In  narrow  vessels  the  surface  of  liquids  is  never  level.  This  is 
owing  to  the  capillary  attraction  exerted  by  the  sides  of  the  vessel  upon 
the  liquid,  drawing  the  edge  up  and  forming  a  saucerlike  concavity 


FIG.  36. 


FIG.  37. 


FIG.  38. 


(Fig.  36).  All  liquids  present  this  concave  surface  except  mercury, 
the  surface  of  which  is  convex. 

This  behavior  of  liquids  makes  it  difficult  to  find  a  distinct  level, 
and  in  reading  the  measure  either  the  upper  meniscus  (a)  or  the  lower 
meniscus  (b)  may  be  used  (Fig.  37). 

The  most  satisfactory  results  are  obtained  if  the  lowest  point  of 
the  curve  (b)  is  used,  especially  with  light-colored  solutions.  But  if 
dark-colored  or  opaque  solutions  are  measured,  it  is  necessary  to  use 
the  upper  meniscus  (a)  for  reading. 

In  all  cases  the  eye  should  be  brought  on  a  level  with  the  surface 
of  the  liquid  in  reading  the  graduation. 

The  eye  is  very  much  assisted  by  using  a  small  card,  the  lower 
half  of  which  is  black  and  the  upper  half  white.  This  card  is  held 
behind  the  burette,  the  dividing  line  between  white  and  black  being 


ERD MAN'S  FLOAT 


47 


about  an  eighth  of  an  inch  below  the  surface  of  the  liquid.  The  eye 
is  then  brought  on  a  level  with  it,  and  the  lower  meniscus  can  be  dis- 
tinctly seen  as  a  sharply  defined  black  line  against  the  white  background 
(Fig.  38). 

Erdman's  Float,   Fig.  39,  is  an  elongated  glass  bulb,  which  is 
weighted  at  its  lower  end  with  mercury,  to  keep  it  in  an  upright  posi- 


FIG.  39. 


FIG.  40. 


FIG.  41. 


tion  when  floating.  It  is  of  such  diameter  that  it  will  slide  easily  up 
and  down  inside  of  a  burette.  There  is  a  ring  at  the  top  by  which 
it  can  be  lifted  in  or  out  by  means  of  a  bent  wire.  Around  its  center 
a  line  is  marked.  At  this  line  instead  of  at  the  meniscus  the  reading 
is  taken. 

These  floats  are  sometimes  provided  with  a  thermometer,  and  they 
then  register  the  temperature  as  well  as  the  volume. 

Others  are  provided  with  projecting  points  along  the  sides,  the 


48  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

object  of  which  is  to  prevent  it  from  adhering  to  the  walls  of  the 
burette.     See  Fig.  40. 

For  the  purpose  of  facilitating  the  reading,  special  forms  of  burettes 
are  constructed  which  are  provided  with  a  dark  longitudinal  stripe  on 
a  white  enameled  background  (Fig.  41);  the  reflection  of  the  dark 
stripe  with  the  meniscus  produces  the  peculiar  appearance  shown  in 
Fig,  42.  The  narrowest  point  is  at  the  middle  of  the  meniscus,  and 


FIG.  42.  FIG.  43- 

by  reading  from  this  point  very  accurate  measurements  are  obtained. 
The  same  effect  can  be  produced  by  holding  behind  an  ordinary  burette 
a  white  flexible  card  having  a  heavy  black  longitudinal  stripe,  about 
one-eighth  inch  in  width. 

Another  form  of  burette  designed  for  the  purpose  of  facilitating 
reading  is  that  provided  with  white  enameled  sides,  leaving  a  strip 
of  clear  glass  in  front  and  back  (Fig.  43).  This  form  is  especially 
adapted  for  use  with  dark-colored  liquids  such  as  iodin  and  perman- 
ganate. 


CALIBRATION  OF  INSTRUMENTS  49 

CALIBRATION   OF   INSTRUMENTS 

Burettes  are  made  from  tubes  of  nearly  uniform  width.  They 
are  filled  with  distilled  water  at  15°  C.*  (59°  F.)  to  the  o  mark,  and 
then  25,  50,  or  100  cc.  run  out,  and  another  mark  made  at  the  surface 
of  the  liquid.  The  distance  between  these  two  marks  is  then  divided 
into  25,  50,  or  100  equal  parts,  and  the  spaces  again  subdivided  into 
fifths  and  tenths.  Now  it  is  very  rarely  possible  to  obtain  tubes  of 
exactly  the  same  caliber  throughout,  and  the  divisions  made  as  above 
do  not  always  represent  exactly  what  they  are  intended  to. 

If  the  tube  is  wider  at  one  point  the  divisions  at  that  point  will 
contain  more,  and  if  it  is  narrower  they  will  contain  less,  than  they 
should. 

Hence  before  using  a  new  burette,  or  in  fact  any  other  measuring 
instrument,  it  is  essential  that  the  error,  if  any,  should  be  determined. 
This  is  done  as  follows: 

Fill  the  burette  to  the  o  mark  with  distilled  water  at  15°  C.  (59°  F.) 
and  run  out  10  cc.  at  a  time  into  a  small  weighed  flask,  and  weigh 
after  each  addition  of  10  cc. 

Each  10  cc.  should  weigh  exactly  10  gms.,  and  every  deviation 
found  should  be  noted  and  taken  into  consideration  in  using  the  instru- 
ment. 

Example 

Flask  weighed  20.0000  grams. 

"     -fiocc.       "        30.1005 

11       +  20CC.  "  40.0499 

"  -{-30CC.  "  49.8000  " 
"  +  4occ.  "  59.9700  " 
"  +5occ.  "  70.0100  " 

Thus  the  ist  10  cc.  weighed  10.1005  grams. 

2d  10  cc.       "         9-9494       " 

3d  10  cc.  "  9-7501  " 
4th  10  cc.  "  10.1700  " 
5th  10  cc.  "  10.0400  " 

Having  obtained  these  data,  a  table  like  the  following  may  be 
constructed  and  kept  in  some  convenient  place  where  it  can  be  readily 

*  Instead  of  15°  C.  (59°  F.)  the  temperature  25°  C.  (77°  F.)  is  recommended 
in  the  U.  S.  P.,  because  this  more  nearly  approaches  the  ordinary  temperature  of 
the  atmosphere  in  temperate  climes. 


50 


A    MANUAL   OF   VOLUMETRIC  ANALYSIS 


consulted  whenever  the  burette  it  represents  is  being  used.     It  is  not 
necessary  to  carry  the  figure  beyond  the  second  decimal  place. 


No  of  cc. 
as  read  on 
Burette. 

No.  of  cc. 
as 
Corrected. 

No.  of  cc. 
as  read  on 
Burette. 

No.  of  cc. 
as 
Corrected. 

No.  of  cc. 
as  read  on 
Burette. 

No.  of  cc. 
as 
Corrected. 

I 

I.  01 

14 

14.06 

27 

26.79 

2 

2.  O2 

15 

15.05 

28 

27.76 

3 

3-°3 

16 

16.04 

29 

28-73 

4 

4.04 

i? 

17.03 

3° 

29.70 

5 

5-°5 

18 

18.02 

3i 

30.71 

6 

6.06 

19 

19.01 

32 

31-72 

7 

7.07 

20 

20.00 

33 

32-73 

8 

8.08 

21 

20.97 

34 

33-74 

9 

9.09          i|                22 

21-94 

35 

34-75 

10 

10.10 

23 

22.91 

36 

35-76 

ii 

11.09 

24 

23.88 

37 

56-77 

12 

12.  08 

25 

24.85 

38 

37-78 

13 

13.07 

26 

25.82 

39 

38-79 

There  should  be  no  greater  deviation  than  0.15  cc.  A  burette 
which  deviates  more  is  best  not  used.  In  the  foregoing  table  there  is 
a  deviation  of  0.30  cc.  at  one  point. 

In  order  to  test  the  accuracy  of  a  pipette,  fill  to  the  mark  with 
distilled  water  at  15°  C.  (59°  F.);  empty  into  a  previously  weighed 
flask,  weigh  again  and  thus  determine  the  weight  of  the  water  measured, 
i  gram  is  equal  to  i  cc. 

Liter  flasks  are  tested  as  follows: 

The  flask,  perfectly  dry  and  clean,  is  counterpoised  on  a  balance 
capable  of  turning  with  .005  when  carrying  about  2000  grams;  it  is 
then  filled  to  the  mark  with  distilled  water  at  15°  C.  (59°  F.),  and  the 
increase  in  weight  should  be  exactly  the  number  of  grams  as  the  cc. 
indicated  at  the  mark. 


CHAPTER  VII 

WEIGHTS    AND    MEASURES     USED    IN    VOLUMETRIC    ANALYSIS 

THE  metric  or  decimal  system  is  used  in  this  country  and  on  the 
continent  in  Europe,  but  in  England  the  grain  system  is  sometimes 
used. 

The  unit  of  weight  in  the  metric  system  is  the  gram  (gm.). 

A  gram  of  distilled  water  at  its  maximum  density,  4°  C.  (39°  F.), 
measures  one  cubic  centimeter  (cc.). 

A  kilogram  is  1000  gms. 

A  liter  is  1000  cubic  centimeters. 

Volumetric  instruments  are  graduated  in  the  metric  system,  but 
not  at  4°  C.  If  they  were,  it  would  necessitate  the  carrying  out  of  all 
volumetric  operations  at  that  temperature,  and  it  would  be  impossible 
to  do  careful  volumetric  work,  except  for  two  or  three  months  of  the 
year,  unless  troublesome  calculations  for  the  correction  of  volume 
were  made. 

For  this  reason  the  temperature  of  15°  C.  (59°  F.)  was  taken  as 
the  standard,  and  at  this  temperature  most  volumetric  instruments 
are  graduated.  In  making  very  careful  examinations  the  work  should 
be  done  at  this  temperature. 

It  is,  however,  not  imperative  that  this  temperature  or  any  other 
definite  temperature  be  taken  as  the  standard.  The  U.  S.  P.,  8th  Dec. 
Revis.,  recommends  the  temperature  of  25°  C.  (77°  F.)  to  be  employed 
for  graduating  all  instruments  used  in  volumetric  work,  because  this 
more  nearly  approaches  the  ordinary  temperature  of  the  atmosphere 
in  chemical  laboratories  in  this  country.  Whichever  temperature  is 
adapted  the  whole  set  of  measuring  instruments  must  be  graduated 
at  the  same  temperature,  and  the  titrations  must  of  course  always  be 
conducted  with  the  standard  solutions  at  the  temperature  at  which 
the  instruments  are  graduated  or  very  near  it. 

One  gram  of  distilled  water  at  15°  C.  measures  one  cc.  as  used  in 
volumetric  analysis. 

The  true  cc.  weighs  at  15°  C.  only  0.999  Sm-  In  the  grain  system 
used  in  England,  10,000  grains  is  taken  as  the  standard  of  measure- 
Si 


52  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

ment.  Sutton,in  his  "Handbook  of  Volumetric  Analysis,"  proposes  that 
ten-grain  measures  be  called  a  decem,  or  for  shortness  dm.;  this  term 
corresponding  to  the  cubic  centimeter,  and  bearing  the  same  relation 
to  the  io,ooo-grain  measure  that  the  cubic  centimeter  does  to  the 
liter.  A  io,ooo-grain  measure  contains  10,000  fluid  grains,  or  1000 
decems.  The  flasks  used  in  working  by  this  system  are  graduated 
to  hold  10,000,  5000,  2500,  and  1000  grain  measures.  Burettes  are 
graduated  in  3oo-grain  measures  with  i -grain  divisions,  600  grains 
in  i  or  2  grain  divisions,  noo  grains  in  5  or  10  grain  divisions,  etc. 
The  system  based  upon  the  imperial-gallon  measure  of  70,000 
grains  is  still  to  some  extent  in  use.  In  this  the  decimillem  (7  grains) 
bears  the  same  relation  to  the  pound  (7000  grains)  that  the  cubic 
centimeter  does  to  the  liter,  or  the  decem  to  the  io,ooo-grain  measure. 


CHAPTER  VIII 
METHODS  OF  CALCULATING  ANALYSES 

N 
EACH  cc.  of  a  —  univalent  volumetric  solution  contains  y^W  of  the 

molecular  weight  in  grams  of  its  reagent,  and  will  neutralize  -njW 
of  the  molecular  weight  of  a  univalent  substance,  or  -JQ^  of  the 
molecular  weight  of  a  bivalent  substance. 

N 
Each  cc.  of  a  —  bivalent  volumetric  solution  contains  5-^-5^  of  the 

molecular  weight  in  grams  of  its  reagent,  and  will  neutralize  or 
combine  with  -g-oW  °f  the  molecular  weight  of  a  bivalent  salt,  or  T^Vff 
of  the  molecular  weight  of  a  univalent  salt. 

N 
A   —  is  only  -fa  the  strength  of  a  normal  solution  and  will  neutralize 

only  fa  the  quantity  of  salt,  etc. 

Normal  and  decinormal  solutions  of  acids  should  neutralize, 
respectively,  normal  and  decinormal  solutions  of  alkalies,  volume  for 
volume.  Decinormal  solution  of  silver  nitrate  and  decinormal  solution 
of  hydrochloric  acid  or  sodium  chlorid  should  combine,  volume  for 
volume,  etc. 


RULES    FOR    DIRECT   PERCENTAGE    ESTIMATIONS 

i.  With  normal  solutions  fa  or  fa  (according  to  its  atomicity) 
of  the  molecular  weight  in  grams  of  the  substance  is  weighed  for 
titration,  and  the  number  of  cc.  of  the  V.  S.  required  to  produce  the 
desired  reaction  is  the  percentage  of  the  substance  whose  molecular 
weight  has  been  used. 

Thus,  if  sodium  hydroxid  (NaOH,  39.76)  is  to  be  examined  by 
titration  with  normal  acid  solution  fa  of  its  molecular  weight  in 
grams  (3.976  gms.)  is  weighed  out,  and  each  cc.  of  the  normal  acid 
solution  required  for  its  neutralization  represents  i  per  cent  of  pure 
sodium  hydroxid. 

53 


54  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

If  sodium  carbonate  (Na2CO3,  105.31)  is  to  be  titrated  ^0  of  its 
molecular  weight  in  grams  (5.265  gms.)  is  taken. 

2.  With  decinormal  solutions  -j-J-^  or  -y-J-g-  of  the  molecular  weight 
in  grams  of  the  substance  to  be  analyzed  is  taken,  and  the  number 
of  cc.  will,  in  like  manner,  give  the  percentage. 

The  following  equations  will  serve  to  explain  more  fully: 

N 
Sodium  hydroxid  with  —  sulphuric  acid 

2NaOH     +     H2S04     =     Na2SO4     +     2H2O. 

2X39-76=  79-52  2)97.35 

10)39.76  48.67=10  1000  cc. 

3.976  gms.  =  to    100  cc. 

Thus  the  quantity  taken,  3.976  gms.,  is  the  quantity  of  pure  sub- 
stance which  is  neutralizable  by  100  cc.  of  the  normal  acid  solution, 
and  therefore  each  cc.  of  the  latter  represents  T^  of  the  quantity 
taken,  or  i  per  cent. 

N 
Sodium  carbonate  with  —  sulphuric  acid: 


20)105.31  48.67=10  1000  cc. 

5.265  gms.  =to    100  cc. 

N 

With  —  sulphuric  acid: 
10 

2NaOH+H2SO4  =  NaSO4+2H2O. 

2X39-76=  79-52         2)97.35 
100)^9.76  48.67=10  1000  cc. 

.3976  gms.  =to    100  cc. 

In  the  case  of  a  trivalent  substance  as  citric  acid  -fa  of  the  molecular 
weight  in  grams  is  taken  for  analysis  when  a  normal  solution  is  em- 
ployed and  -$%-$  when  a  decinormal  solution  is  used. 

In  other  words,  when  it  is  desired  that  each  cc.  of  the  standard 
solution  should  represent  i  per  cent  of  the  substance  upon  which  it 
acts,  the  rule  is  to  take  for  analysis  as  much  of  the  substance  as  is 
represented  by  100  cc.  of  the  standard  solution. 

In  the  case  of  substances  whose  percentage  of  purity  is  high,  it  is 
advisable  to  take  smaller  quantities,  in  order  to  avoid  the  use  of 


FACTORS  OR  COEFFICIENTS  FOR  CALCULA TING  ANALYSES     55 

excessive  quantities  of  standard  solution.  Thus  sulphuric  acid,  which 
contains  92.5  per  cent  of  absolute  sulphuric  acid,  would  require  under 
the  above  conditions  92.5  cc.  of  normal  alkali  solution. 

In  the  case  of  this  acid,  if  4.867  gms.  are  taken  for  analysis,  each 
cc.  of  a  normal  alkali  solution  would  represent  i  per  cent  of 
H2SO4. 

If  half  of  this  quantity,  i.e.,  2.4335  gms-  are  taken  for  analysis, 
each  cc.  of  the  normal  alkali  will  represent  2  per  cent  of  H2SO4,  and 
thus  less  of  the  standard  solution  will  be  required.  Again,  if  0.4867 
gm.  be  taken,  each  cc.  of  the  standard  alkali  will  represent  10  per  cent 
of  H2S04. 

In  the  case  of  liquids  where  it  is  not  always  convenient  to  weigh 
off  the  exact  quantity  required  for  titration  by  the  direct  percentage 
method,  the  liquid  is  diluted  to  a  convenient  degree  with  water,  and 
then  a  quantity  of  this  diluted  liquid  (representing  the  weight  re- 
quired of  the  substance)  is  measured  for  analysis. 

Example.  A  sulphuric  acid  solution  of  specific  gravity  1.826  is 
to  be  analyzed.  2  cc.  are  accurately  measured  and  diluted  to  100  cc. 
and  then  12.97  cc.  of  this  solution  (representing  0.4867  gm.  of  the 
acid)  are  taken  for  analysis. 

N 
Each  cc.  of  —  NaOH  V.  S.  required  in  the  titration  represents 

N 
10  per  cent   of   absolute  H2SO4.      If  —  NaOH  V.  S.   is  employed, 

each  cc.  will  represent  i  per  cent.  To  determine  the  amount  of  the 
diluted  liquid  to  be  taken  we  proceed  as  follows: 

Two  cc.  of  sulphuric  acid  specific  gravity  1.826  weigh  3.752  gms., 
therefore  the  100  cc.  of  diluted  acid  contain  this  weight,  and  i  cc. 
of  the  same  contains  0.03752  gm. 

If  0.03752  gm.  are  contained  in  i  cc.,  then  0.4867  gm.  are  con- 
tained in  how  many  cc.  ? 

gm.       cc.  gm. 

.03752  :  i  : :  .4867  :  x, 

x=  12.97  cc. 

Factors  or  Coefficients  for  Calculating  the  Analyses.  It 
frequently  occurs  that  from  the  nature  of  the  substance,  or  from  its 
being  in  solution,  this  percentage  method  cannot  be  conveniently 
followed. 

The  best  way  to  proceed  in  such  a  case  is  to  find  the  factor. 


56  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  first  step  in  all  cases  is  to  write  the  equation  for  the  reaction 
which  takes  place  between  the  substance  under  analysis  and  the  solu- 
tion used. 

For  instance,  a  solution  of  caustic  potash  is  to  be  examined,  a  — 
solution  of  sulphuric  acid  being  used. 

2KOH  +  H2SO4  =  K2SO4  +  2H2O. 
2)111.48          2)97-35 

55.74  48.67=10  1000  cc —  acid. 

i 

N 

•°S574  gm.      .04867=10        i  cc.— acid. 

i 

N 
The  factor  for  KOH  when  —  solution  is  used  is  .05574  gm.,  that 

N  N 

being  the  quantity  neutralized  by  each  cc.  of  the  —  acid.     If  — acid 

were  used  the  factor  would  be  .005574  gm. 

The  number  of  cc.  of  the  acid  used  to  produce  the  desired  result, 
when  multiplied  by  the  factor,  gives  the  quantity  in  grams  of  KOH 
in  the  solution  taken. 

Example.     If  10  grams    of    caustic-potash    solution    were    taken, 

N 
and  40  cc.  of  —  acid  were  required,  the  10  gms.  of  solution  contained 

.05574  gm.  X4o=2.2296  gms.  of  pure  KOH. 

To  find  the  percentage,  the  following  formula  may  be  used. 

QXioo 
~W~ 

()=the  quantity  of  pure  substance  found  by  calculation; 

W=  weight  of  substance  taken. 

If  the  above  example  is  taken,  we  have 

2.2296X100  m 

=  22.296%. 

10 

Or  the  calculation  may  be  made  by  proportion. 

The  quantity  of  the  substance  taken  is  always  the  first  term,  and 
the  quantity  of  pure  substance  found,  the  second  term. 

The  following  rule  is  easily  remembered:  As  the  quantity  taken 
is  to  the  quantity  found,  so  is  100  to  x,  the  percentage  of  pure  substance 
in  the  sample. 


FACTORS  OR  COEFFICIENTS  FOR  CALCULATING  ANALYSES     57 

Three  terms  of  the  equation  being  given,  the  fourth  is  found  by 
multiplying  the  means  and  dividing  the  product  by  the  given  extreme. 
By  applying  this  rule  to  the  above  case  we  have 

10  :  2.2296  : :  100  :  x.        #=22.296%. 


TABLE  SHOWING  THE  NORMAL  FACTORS,   ETC.,  OF  THE  ALKALIES,  ALKALINE 
EARTHS,  AND  ACIDS. 


Substance. 

Formula. 

Molecular 
Weight. 

Normal 
Factor.* 

Quantity  of 
Substance 
to  be  takent 
so  that  each 

cc.  of  y  V.S. 

will  indicate 
i  per  cent. 

Sodium  hydroxid  

NaOH 

3Q   76 

0.03076 

gram. 
3.076 

Sodium  carbonate     . 

Na2CO3 

IOC     71 

O    OC26C 

e    26  C 

Sodium  bicarbonate.  

NaHCO3 

82   43 

0.08343 

8    34.3 

Potassium  hydroxid 

KOH 

CC    74 

O   OCC74 

5C74. 

Potassium  carbonate  

K,CO, 

137    27 

o  06863 

6  863 

Potassium  bicarbonate 

KHCO3 

QQ   41 

O    OQO4I 

Ammonia  (gas) 

NH3 

l6   Q3 

o  01603 

i  603 

Ammonium  carbonate,  normal.  .  .. 
Ammonium  carbonate,  commercial 
Lime     ..                       

(NH4)2C03 
N3HUC205 
CaO 

95-41 
156.01 

cc  68 

0.0477 
O.O52 

o  02784 

4.77 

5-2 
2    784. 

Calcium  hydroxid 

Ca(OH)2 

73    c6 

o  03678 

7.    678 

Calcium  carbonate. 

CaCO3 

QQ      7  C 

o  04067 

4067 

Nitric  acid  

HNO3 

62    C7 

o  o62C7 

6    2C7 

Hydrochloric  acid        . 

HC1 

36  18 

o  03618 

3  618 

Sulphuric  acid  

H2SO4 

Q7    7C 

o  04867 

4   867 

Oxalic  acid   crystallized 

H2C2O4-2H2O 

1  25  10 

o  062  cc 

6    2CC 

Acetic  acid  

HC,H,O2 

Co    eg 

O    OCQCS 

C  oc8 

*  This  is  the  coefficient  by  which  the  number  of  cc.  of  normal  solution  used  is  to  be 
multiplied  in  order  to  obtain  the  quantity  of  pure  substance  present  in  the  material 
examined. 

t  This  is  the  quantity  of  substance  to  be  taken  in  direct  percentage  estimations.     Each 

cc.  of  —  acid  or  alkali  V.  S.  employed  will  then  indicate  i  per  cent:  in  the  case  of  many 

of  these  substances  it  will,  however,  be  better  to  take  smaller  quantities  so  that  less  of 
the  standard  solution  be  required.     Thus  if  one  half  of  the  quantity  be  taken  each  cc. 

of  the  -  V.S.  will  represent  2  percent,  if  r\j  of  the  quantity  be  taken  each  cc.  will  represent 

10  per  cent.     If,  however,  —  solutions  be  used  and  A  of  the  quantity  indicated  in  the 
10 

table  be  taken  each  cc.  will  indicate  i  per  cent. 


CHAPTER  IX 

SOME  VICARIOUS  VOLUMETRIC  METHODS 
VOLUMETRIC   ANALYSIS    WITHOUT   WEIGHTS   AND    STANDARD   SOLUTIONS 

THIS  is  a  matter  of  curiosity  rather  than  of  value,  but  under  certain 
circumstances  it  might  prove  useful.  The  way  in  which  this  is  carried 
out  is  best  explained  by  an  example. 

Suppose  we  wish  to  determine  the  proportion  of  pure  sodium 
chlorid  in  an  impure  specimen  of  salt.  A  portion  of  the  latter  is 
placed  upon  one  pan  of  a  balance  and  exactly  counterpoised  by 
placing  on  the  other  pan  sufficient  of  the  pure  sodium  chlorid.  The 
samples  are  then  dissolved  in  water  and  each  titrated  with  a  solution 
of  silver  nitrate  of  unknown  strength  and  the  calculation  made  as 
follows : 

If  the  pure  salt  required  60  cc.  of  the  silver  solution,  and  the 
impure  specimen  45  cc.,  then 

60  :  45  ::  100  :  x;    *=75, 

the  percentage  of  pure  sodium  chlorid  in  the  salt  analyzed. 

This  process  it  will  be  seen  can  be  applied  only  to  such  substances 
of  which  pure  specimens  can  be  had,  though  in  some  instances  a  pure 
specimen  of  some  other  salt  may  be  used  as  a  substitute,  and  the 
result  obtained  by  calculation. 

For  instance,  suppose  it  is  required  to  estimate  sodium  carbonate, 
and  we  have  only  pure  calcium  carbonate  on  hand  to  use  as  a  weight. 
Equal  weights  are  taken  and  each  titrated  with  an  acid  solution. 
It  is  now  necessary  to  find  out  how  many  cc.  of  acid  solution  would 
be  required  if  pure  sodium  carbonate  were  used,  instead  of  pure  cal- 
cium carbonate,  as  a  counterpoise  The  molecular  weights  of  calcium 
carbonate  and  sodium  carbonate  are  100  *  and  106  *  respectively, 

and  thus  sodium  carbonate  would  require  — -,  the  amount  of  acid 
solution  as  calcium  carbonate. 

*  Approximate  molecular  weights. 


VOLUMETRIC  ANALYSIS   WITHOUT  A    BURETTE  59 

We  will  assume  that  the  calcium  carbonate  required  60  cc.  of  the 
acid  solution  and  the  impure  sal  soda  40  cc.  60  X — 7  =  56.6,  the 

number  of  cc.  which  an  equal  weight  of  sodium  carbonate  will  require. 
Then 

56.6  :  40  : :  100  :  x;   #=70.67, 

the  percentage  of  pure  sodium  carbonate  in  the  specimen  analyzed. 

With  the  exercise  of  a  little  ingenuity  the  method  may  be  extended 
to  a  number  of  substances. 

Koningh  and  Peacock  have  devised  a  method  by  which  the  same 
endris  attained  without  the  aid  of  a  pure  substance  as  a  standard. 

If  impure  sodium  chlorid  is  to  be  examined,  an  equal  weight  of 
silver  nitrate  is  taken  and  dissolved  in  sufficient  water  to  make  100  cc. 
of  solution;  this  is  placed  in  a  burette,  and  the  sodium  chlorid  titrated 
after  the  latter  is  dissolved. 

Assuming  that  10  cc.  of  the  silver  solution  were  required,  then 

169.7*  :  58.4*  : :  10  :  x\  #=3.44%. 

In  the  estimation  of  sodium  carbonate  an  equal  weight  of  oxalic 
acid  is  taken  and  used  in  the  same  manner. 


VOLUMETRIC   ANALYSIS   WITHOUT  A   BURETTE 

The  standard  solutions  are  weighed  instead  of  measured.  This 
method  is  often  resorted  to  where  great  accuracy  is  desired,  for  varia- 
tions in  temperature  do  not  influence  the  result.  It  is,  however,  a 
slow  process. 

The  standard  solution  is  placed  in  a  suitable  flask  (see  Fig.  44), 
and  the  whole  weighed  on  a  delicate  balance.  The  solution  is  then 
carefully  run  into  the  beaker  containing  the  substance  to  be  analyzed, 
and  when  the  end  reaction  is  obtained  the  flask  is  again  weighed,  and 
the  difference  in  weight  is  the  amount  of  solution  used.  The  standard 
solution  should  of  course  be  standardized  by  weight. 

A  weight  burette  is  shown  in  Fig.  45,  which  is  of  a  very  ser- 
viceable kind.  It  was  designed  by  C.  T.  Heycock,f  and  consists  of 

A,  an  ordinary  glass  cylinder  of  about  70  cc.  capacity,  very  roughly 
divided  into  cc.'s.  These  divisions  are  not  absolutely  necessary,  but 

*  Approximate  molecular  weights. 

t  Described  at  1899  meeting  of  the  Brit.  Ph.  Conf. 


60 


A   MANUAL   OF   VOLUMETRIC   ANALYSIS 


they  will  be  found  useful  in  the  case  of  a  second  titration  of  the  same 
solution. 

B,  a  hollow  stopper  with  a  small  perforation  opposite  C,  which  is 
a  glass  tube  used  to  admit  the  air  when  drawing  out  the  solution  without 
taking  out  the  stopper. 

E,  an  ordinary  glass  stop-cock. 
D,  a  wire  loop  for  hanging  the  burette  upon 
the  Kook  above  the  balance  pan. 

A  standard  weight  solution  is  made  by  taking 
a    flask,   carefully    drying    and    weighing    it,    the 
standard  substance  being  introduced  into  this  from 
a   stoppered   weighing    tube,    water    added    to   ap- 
proximate volume  desired  and  solution  effected. 

The  whole  is  then  weighed.  The  weight  of  dry  flask 
being  known,  the  weight  of  the  solution  is  found. 

The  strength  of  this  solution  is  then  x  grams  of 
standard  substance  in  x  grams  of  the  solution, 
which,  therefore,  becomes  the  standard  solution. 

The  standard  solution  is  placed  in  one  of  these 
burettes  and  another  rilled  with  the  solution  to  be 
estimated. 

Both  are  carefully  weighed,  a  certain  quantity 
from  one,  say  10.15  grams,  is  run  into  a  suitable 
vessel,  in  which  the  titration  can  be  made  and 
again  weighed.  The  titration  is  then  carried  out 
and  the  weight  of  the  second  burette  found.  It  is 
seen  that  a  definite  weight  of  the  solution  of  un- 
known strength  requires  a  definite  weight  of  stand- 
ard solution,  which  in  its  turn  contains  a  definite 
weight  of  standard  substance.  FlG-  45- 

From  these  data  the  strength  of  the  unknown  solution  is  obtained. 
It  is  essential  when  weighing  both  the  empty  flask  and  burettes  that 
each  is  approximately  counterpoised  by  a  similar  vessel  upon  the 
opposite  scale-pan,  so  that  the  same  amount  of  air  is  displaced  on 
both  sides,  and  thus  corrections  for  variations  of  temperature  and 
pressure  are  avoided. 

As  in  ordinary  volumetric  work  different  substances  will  require 
slightly  different  manipulation;  for  instance,  it  will  frequently  be 
found  convenient  to  weigh  sufficient  only  of  the  standard  substance 
for  each  titration. 


VOLUMETRIC  ANALYSIS   WITHOUT   A    BURETTE  61 

The  advantages  of  this  method  of  using  weight  burettes  are: 

1.  Changes  of  temperature  during  the  experiment,  and  differences 
of  temperature  of  the  two  solutions  exert  no  influence  upon  the  result. 

2.  The  errors  arising  from  inaccurate  calibration  of  the  ordinary 
burette  are  entirely  eliminated. 

3.  The   question  as   to  whether  all  pipettes  are  made  to  contain 
or  deliver  a  certain  volume,  and,  if  the  latter,  whether  the  last  drop 
should  be  blown  out,  is  entirely  avoided. 

4.  Supposing  the  weight  of  these  burettes  is  carried  to  the  third 
decimal  place,  it  is  equivalent  to  having  a  burette  accurately  graduated 
to  one-thousandth  part  of  a  cubic  centimeter. 

5.  The  difficulty  of  reading  correctly  and  uniformly  the  ordinary 
burettes  owing  to  the  unevenness  of  the  meniscus,  or  from  the  fact 
of  some  of  the  solution  adhering  to  the  sides  of  the  burette,  particu- 
larly noticeable   in   dark  solutions  such  as  potassium  permanganate, 
or  from  other  causes,  is  obviously  overcome. 


CHAPTER  X 

ANALYSIS  BY  NEUTRALIZATION 

THIS  is  based  upon  the  fact  that  acids  are  neutralized  by  alkalies 
and  alkalies  by  acids. 

The  strength  of  an  acid  is  estimated  by  the  quantity  of  alkali 
that  is  required  to  neutralize  it.  This  process  is  called  acidi- 
metry. 

The  strength  of  an  alkali  is  found  by  the  quantity  of  an  acid  that 
is  required  to  neutralize  it.  This  process  is  called  alkalimetry.  The 
stronger  the  acid,  the  more  alkali  is  required,  and  vice  versa. 

A  substance  is  said  to  be  alkaline  when  it  turns  red  litmus  blue; 
phenolphthalein,  red;  turmeric,  brown,  etc.  Acid,  when  it  turns  blue 
litmus  red;  red  phenolphthalein,  colorless,  etc. 

The  principal  alkaline  substances  are  the  hydroxids  and  car- 
bonates of  sodium,  potassium,  and  ammonium,  and  the  hydroxids 
and  oxids  of  calcium,  barium,  and  strontium,  and  the  alkaloids. 

When  an  acid  is  brought  in  contact  with  an  alkali  combination 
takes  place,  and  a  neutral  salt  is  formed.  This  combination  takes 
place  in  definite  and  invariable  proportions;  thus:  If  111.48  parts  of 
potassium  hydroxid  are  mixed  with  97.35  parts  of  absolute  sulphuric 
acid  the  alkali  as  well  as  the  acid  will  be  neutralized.  If  only  80 
parts  of  the  acid  have  been  added  the  mixture  would  still  be  alkaline, 
for  it  requires  97.35  parts  of  the  acid  to  neutralize.  If  more  than  97.35 
of  the  acid  have  been  added,  the  mixture  would  consist  of  potassium 
sulphate  and  free  sulphuric  acid.  The  reaction  is  thus  illustrated: 

2KOH  +  H2SO4  =   K2SO4+2H2O. 


2K=77-72  H2=    2.00 

_.      ;/  '  sulphate 

20=31.76  8=31.83 

2H=     2.00  04=63.52 


111.48          97.35 

Sodium   hydroxid   will   unite   with  oxalic  acid  in  the  proportion 

62 


ANALYSIS  BY   NEUTRALIZATION  63 

of  79.52  parts  by  weight  of  the  former  and  125.10  parts  by  weight 
of  the  latter,  as  the  equation  shows. 

2NaOH+H2C2O4-  2H2O  =  Na2 


Na=45.y6  6H'=  6.00 
20=31.76  C2=  23.82 
2H'=  2.00  6O=  95.28 


79.52  125.10 

Ammonia  water  unites  with  hydrochloric  acid  as  per  the  equation, 

NH4OH+HC1=NH4C1+H2O. 

34.81       36.18 

Sodium  carbonate  with  hydrochloric  acid, 

Na2C03  +  2HCl=2NaCl+H20  +  C02. 
105.31        72.36 

Upon  a  careful  perusal  of  the  foregoing  equations  it  will  be  seen 
that  since  definite  weights  of  acids  neutralize  definite  weights  of  alka- 
lies, the  quantity  of  a  certain  alkali  in  solution  can  be  easily  deter- 
mined by  the  quantity  of  an  acid  solution  of  known  strength  required 
to  neutralize  it,  and  vice  versa. 

Referring  to  the  first  equation  we  see  that  97.35  gms.  of  H2SO4 
neutralize  111.48  gms.  of  KOH.  If  we  prepare  a  normal  solution  of 
H2SO4  we  take  half  the  molecular  weight,  97.35  =  48.675  gms.  to 
1000  cc.  Half  the  molecular  weight  is  taken  because  sulphuric  acid 
is  a  bivalent  acid.  1000  cc.  of  this  solution  will  neutralize  55.74  gms. 
of  KOH;  hence  i  cc.  will  neutralize  0.05574  gm.  of  KOH. 

Thus  if  10  gms.  of  a  solution  of  KOH  be  treated  with  the  above 
normal  solution  of  H2SO4,  and  it  is  found  that  25  cc.  of  the  acid  solu- 
tion are  required  to  nutralize  the  alkali  solution,  the  latter  contains 
25  X  0.05574=  i.  39  +gm.  of  pure  KOH. 

Since  the  acid  and  alkali  as  well  as  the  neutral  salt  which  is  formed 
are  colorless,  and  no  visible  change  takes  place  during  the  reaction, 
it  is  necessary  to  add  some  substance  which  by  change  of  color  will 
show  when  the  neutralization  is  complete.  Such  a  substance  is  known 
as  an  indicator. 

In  the  case  of  sodium  hydroxid  with  oxalic  acid  (see  the  second 
equation),  we  find  that  125.10  gm.  of  crystallized  oxalic  acid  neu- 


64  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

tralizes  79.52  gms.  of  NaOH.  Oxalic  acid  like  sulphuric,  is  bivalent, 
therefore  a  normal  solution  of  it  contains  half  the  molecular  weight 
in  grams,  i.e.,  62.55  gms.  in  1000  cc. 

1000  cc.  will  neutralize  39.76  gms.  of  NaOH; 
i  cc.  will  neutralize    0.03976  gm.  of  NaOH. 

The  neutralizing  power  of  all  normal  acids  is  exactly  the  same, 
because  they  all  contain  in  1000  cc.  the  molecular  weight  in  grams  of 
the  acid,  in  the  case  of  univalent  acids  and  half  of  the  molecular  weight 
in  grams  of  bivalent  acids. 

Thus  i  cc.  of  any  normal  acid  will  neutralize  0.05574  gm.  of  KOH 
or  0.03976  gm.  of  NaOH  or  yoVff  °f  tne  molecular  wreight  of  any 
other  univalent  alkali,  or  ^W  °f  *ne  molecular  weight  of  an  alkali 
earth,  the  latter  being  bivalent.  In  like  manner  all  decinormal  solu- 
tions have  a  like  neutralizing  power,  their  neutralizing  equivalence 
is  one-tenth  that  of  normal  solutions. 

Thus  i  cc.  of  a  decinormal  acid  will  neutralize  0.005574  gm.  of 
KOH  or  0.003976  gm.  of  NaOH,  etc. 

ALKALIMETRY 

Preparation  of  Standard  Acid  Solutions.  It  is  possible  to  carry 
out  the  titration  of  most  alkalies  by  means  of  one  standard  acid  solu- 
tion, but  the  same  standard  acid  is  not  equally  applicable  in  all  cases; 
furthermore,  the  standard  acids  are  frequently  employed  for  other 
volumetric  operations  than  neutralization,  and  therefore  it  is  advis- 
able to  have  a  variety. 

The  standard  oxalic  acid  solution  is  preferred  by  some,  because 
of  the  ease  with  which  it  may  be  prepared,  provided  a  pure  oxalic 
acid  is  to  hand.  It  does  not,  however,  keep  very  long,  is  unreliable 
for  use  with  methyl  orange,  and  is  inapplicable  for  the  titration  of 
alkali  earths,  because  it  forms  insoluble  compounds  with  these 
metals.  Standard  hydrochloric  acid  is  the  most  desirable  for  alkali 
earths,  because  it  forms  soluble  compounds  with  them;  its  dis- 
advantage, however,  is  in  its  volatility  and  its  consequent  useless- 
ness  in  hot  titrations.  Standard  sulphuric  acid  is  preferred  by  most 
analysts  as  being  the  best  general  standard.  A  pure  acid  can  be 
gotten  without  difficulty  and  the  standard  solution  made  from  it  is 
unaffected  by  boiling,  and  can  therefore  be  used  in  hot  as  well  as 
in  cold  titrations;  it  reacts  sharply  with  the  indicators  and  it  keeps 


NORMAL  HYDROCHLORIC   ACID  65 

its  titer  indefinitely.  It  is,  however,  not  suited  for  the  titration  of 
alkali  earths,  because  it  forms  insoluble  compounds  with  them 
which  precipitate,  and  are  very  annoying  to  the  operator.  In  the 
preparation  of  standard  solutions  the  greatest  care  should  be  exer- 
cised in  order  that  the  product  be  absolutely  accurate.  The  slightest 
inaccuracy  in  the  strength  of  a  standard  solution  will  result  in  relative 
errors  in  the  analysis.  It  is  customary  to  prepare  one  standard 
solution,  and  then  from  this  to  adjust  various  others.  For  example, 
a  normal  oxalic  acid  may  be  made  first,  and  by  means  of  this  a 
normal  alkali  solution,  which  in  turn  may  be  utilized  for  the  adjusting 
of  other  standard  acid  solutions. 

N 
Normal  Oxalic  Acid  (H2C2O4-2H2O=  125.10;  —  V.S.=62.55  gms. 

in  1000  cc.) 

Dissolve  62.55  g1118-  °f  pure  oxalic  acid  (see  below)  in  enough 
water  to  make  at  or  near  15°  C.  exactly  1000  cc. 

Pure  oxalic  acid,  crystallized,  is  in  the  form  of  colorless,  trans- 
parent, clinorhombic  crystals,  which  should  leave  no  residue  when 
ignited  upon  platinum  foil.  It  is  completely  soluble  in  14  parts  of 
water  at  15°  C.  If  the  acid  leaves  a  residue  on  ignition  it  should  be 
purified  by  recrystallization,  as  directed  in  the  U.  S.  P. 

N 
i  cc.  of  —  oxalic  acid  V.  S.  is  the  equivalent  of 

Ammonia  gas,  NH3 0.01693 

Potassium  hydroxid,  KOH 0.05574 

Sodium  hydroxid,  NaOH 0.03976 

Potassium  permanganate,  KMnC>4 0-031396 

Manganese  dioxid,  MnO2 0.043  *8 

Calcium  hydroxid,  Ca(OH)2 0.03678 

/N  \ 

Decinormal  Oxalic  Acid  ( —  V.  S.= 6.255  gms-  m  I00o  cc-j 

Dissolve  6.255  gms.  of  pure  oxalic  acid  in  enough  water  to  make 
at  or  near  15°  C.  exactly  1000  cc. 

Normal  Hydrochloric   Acid    (HCl  =  36.i8;   —  V.  S.= 36.18  gms. 

in  1000  cc.) 

Mix  130  cc.  of  hydrochloric  acid  of  sp.gr.  1.163  with  enough  water 
to  measure,  at  or  near  15°  C.,  1000  cc. 

Of  this  liquid  (which  is  still  too  concentrated)  measure  carefully 
into  a  flask  or  beaker  10  cc.,  add  a  few  drops  of  phenolphthalein  T.  S., 


66 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


N 
and  gradually  add  from  a  burette  —  potassium  hydroxid  V.  S.  until 

a  permanent  pale  pink  tint  is  produced.     Note  the  number  of  cc.  of 

N 

—  potassium   hydroxid  solution   consumed,  and   then  dilute   the  acid 

so  that  equal  volumes  of  this  and 

the  -KOH  V.  S.  neutralize  each 
i 

other. 

Example.  Assuming  that  the 
10  cc.  of  the  acid  solution  re 

N 
quired    12    cc.    of   the    —  KOH, 

each  10  cc.  of  the  acid  must  be 
diluted  to  12  cc.,  or  the  whole 
of  the  remaining  acid  in  the  same 
proportion. 

After  the  dilution  a  new  trial 
should  be  made.  10  cc.  of  the 
acid  V.  S.  should  require  exactly 
10  cc.  of  the  alkali. 

This  method  is  fairly  satis- 
factory if  an  accurately  stand- 
ardized normal  potassium  hy- 
droxid solution  is  at  hand;  the 
latter,  however,  always  contains 
a  small  quantity  of  carbonate, 
hence  methyl  orange  would  be 
more  desirable  as  an  indicator. 
Other  methods  of  standardizing  acid  solutions  are: 

Standardization  by  Means  of  the  Specific  Gravity.  As  the 
result  of  a  careful  experimental  study  of  the  various  methods  proposed 
for  the  standardization  of  acid  volumetric  solution,  E.  C.  Worden 
and  John  Morton,*  conclude  that  the  most  accurate  and  most  easily 
performed  method  is  that  of  taking  the  specific  gravity  in  accurately 
calibrated  pycnometers. 

Standardization  by  Means  of  Borax.  This  is  a  ready  and  accu- 
rate method  for  the  standardization  of  the  strong  mineral  acids.  The 


FIG.  46. 


J.  Soc.  Chem.  Ind.  24-178. 


VOLUME!  RIC  STANDARDIZATION  BY  SILVER   NITRATE     67 

commercial  borax  is  purified  by  recrystallization,  the  crystals  are  then 
finely  pulverized  and  shaken  with  water  at  25°  to  30°  C.  The  satu- 
rated solution  is  rapidly  filtered  and  set  aside  to  crystallize  at  a  low 
temperature.  The  crystals  so  obtained  are  quite  pure,  and  after 
draining  are  thoroughly  dried  between  sheets  of  filtering  paper,  but 
without  the  use  of  heat  or  an  exsiccator. 

The  neutralization  takes  place  according  to  the  following  equation: 


379.40  72.36 

Thus, 

189.7      gms-  °f  borax  neutralize  38.18  gins,  of  HC1; 

N 
189.7      gms.  of  borax  neutralize  1000  cc.  —  HC1; 

\" 
1.897  gms.  of  borax  neutralize      10  cc.  —  HC1. 

These  figures  form  the  basis  for  the  standardization,  which  is 
conducted  as  follows: 

1.897  gm.  of  borax  is  accurately  weighed,  dissolved  in  40  cc.  of 
water,  three  drops  of  methyl  orange  *  added,  and  then  the  hydro- 
chloric acid  solution  to  be  standardized  is  run  in  from  a  burette  until 
a  red  color  appears.  If  for  example  9  cc.  are  required,  then  each 
9  cc.  of  the  acid  solution  must  be  diluted  to  10  cc.,  or  the  whole  of 
the  remaining  solution  in  the  same  proportion.  After  the  dilution 
a  new  trial  should  be  made,  and  then  the  1.897  gms-  of  borax  should 
require  exactly  10  cc.  of  the  acid  solution.  If  more  than  10  cc.  are 
required  the  acid  solution  is  too  weak. 

Volumetric  Standardization  by  Means  of  Silver  Nitrate,    The 

N 
standardization  of  —  hydrochloric  acid  may  be  effected  also,  by  means 

of  decinormal  silver  nitrate  solution,  as  per  the  equation 

HC1+  AgN03  =  AgCl-f  HN03. 

36.18      168.69 

36.18    gms.  of  HC1       =  1000  cc.  of  —  V.  S. 

N 

16.869  g1115-  of  AgNO3=  1000  cc.  of  —  V.  S. 

10 

*  Phenolphthalein  and  litmus  are  inapplicable  here. 


68  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

N 
If  10  cc.  of  the  hydrochloric -acid  solution  are  titrated  with  —  AgNO3 

and  108  cc.  of  the  latter  are  required,  then  each  10  cc.  must  be  diluted 

to  ( )  =  10.8  cc.     If  less  than  100  cc.  of  the  —  silver  nitrate  were 

\  10  /  10 

required,  the  acid  solution  is  too  dilute  and  a  stronger  solution  should 
be  prepared.  After  dilution,  a  second  trial  should  be  made,  and  the 
10  cc.  of  acid  solution  should  then  require  for  complete  precipitation 

N 
exactly  100  cc.  of  —  AgNCV 

Another  way  is  to  take  exactly  1.687  gms.  of  pure  silver  nitrate, 
dissolve  it  in  50  cc.  of  water,  and  then  add  the  hydrochloric  acid 
solution  from  a  burette.  Exactly  10  cc.  should  be  required.  If  less 
is  required  the  solution  must  be  diluted  accordingly. 

Gravimetric  Standardization  by  Means  of  Silver  Nitrate.*  In 
this  method  the  titer  of  the  hydrochloric  acid  solution  is  deter- 
mined by  weighing  the  silver  chlorid  which  a  definite  volume  of  it 
precipitates. 

20  cc.  of  the  hydrochloric  acid  solution  to  be  standardized  are 
transferred  to  a  250  cc.  Erlenmeyer  flask,  provided  writh  a  well -fitting 
rubber  stopper.  50  cc.  of  a  5  per  cent  solution  of  silver  nitrate  are 
then  added,  the  stopper  inserted,  and  the  flask  shaken.  Then  suffi- 
cient of  the  silver  solution  is  to  be  added  to  complete  the  pre- 
cipitation. This  should  be  added  in  portions  of  less  than  i  cc.  each 
and  the  flask  stoppered  and  shaken  after  each  addition.  A  large 
excess  of  silver  nitrate  must  be  avoided,  because  it  possesses  a  slight 
solvent  action  upon  the  precipitate  and  necessitates  much  washing 
for  its  complete  removal. 

The  precipitate  is  then  allowed  to  settle,  and  the  supernatant  liquid 
when  quite  clear  carefully  decanted  through  a  Gooch  crucible.  The 
precipitate  is  then  twice  or  thrice  shaken  with  pure  water,  faintly 
acidulated  with  nitric  acid,  and  each  time  carefully  decanted  into 
the  crucible  as  before,  and  finally  the  precipitate  is  transferred  (by 
the  use  of  a  "spritz  "  bottle  with  pure  water)  to  the  crucible  and  dried 
in  the  air-bath  at  130°  to  150°  C.  to  constant  weight. 

HC1+  AgN03  -  AgCl+ HN03. 

36.18  142.3 


*  Hopkins,  J.  A.  C.  S.,  1901,  p.  727. 


STANDARDIZATION  BY   MEANS  OF   CALC-SPAR  69 

N 
142.3  gms.  of  AgCl  =  36.18  gms.  of  HC1  =1000  cc.  of  —  V.  S.; 

N 
1.423  gms.  of  AgCl=  0.3618  gm.  of  HC1  =     10  cc.  of  —V.  S.; 

N 
2.846  gms.  of  AgCl=  0.7236  gm.  of  HC1  =     20  cc.  of  —  V.  S. 

Supposing  that  the  precipitate  weighed  3.557  gms.,  then  the  solu- 
tion is  too  strong  and  must  be  diluted  thus  : 

N 
If  2.846  gms.  of  AgCl  =  o.723i  gm.  of  HC1  or  20  cc.  of  its  —  V.  S., 


then  3.557  gms.  of  AgCl  will  represent 


Therefore,  each  20  cc.  of  the  hydrochloric  acid  solution  must  be 
diluted  to  25  cc.  in  order  to  make  it  a  normal  solution. 

Standardization  by  Means  of  Calc-spar.  Very  accurate  results 
may  be  attained  by  the  use  of  pure  crystallized  calcium  carbonate 
(Calc-spar,  Iceland-spar),  which,  unlike  an  alkali,  is  not  subject  to 
hygroscopic  variation. 

CaCO3  +   2HC1  =  CaCl  +  H2O  +  CO2. 
2)99-35          2)72-36  N 

49.675  gms.  =  36.18  gms.=  iooo  cc.  of  —  V.  S. 

N 
Referring  to  the  equation  it  will  be  seen  that  1000  cc.  of  —  HC1 

i 

will  dissolve  49.675  gms.  of  pure  CaCO3.  100  cc.  will  therefore 
dissolve  4.9675  gms. 

Weigh  out  6  gms.  of  calc-spar  in  small  pieces  in  a  tared  flask  and 
add  100  cc.  of  the  acid  solution  to  be  standardized.  When  the  reaction 
has  ceased,  pour  off  the  solution,  wash  the  residual  in  the  beaker  with 
water,  pour  off,  and  after  drying  the  contents  of  the  flask,  weigh  and 
deduct  from  original  weight.  The  difference  represents  the  CaCO3 
which  was  dissolved. 

Supposing  that  this  difference  be  5.65  gms.,  then  the  acid  solution 
is  too  strong  and  must  be  diluted  so  that  100  cc.  will  dissolve  no  more 
than  4.9675  gms.  of  CaCOs. 


70  A    MANUAL  OF    VOLUMETRIC  ANALYSIS 

The  amount  of  dilution  required  in  this  case  may  be  calculated 
thus: 


That  is,   each  100  cc.  of  the  acid  solution   must  be  diluted  to 

N 
measure  113.74  cc.  This  being  the  quantity  of  -      HC1  V.  S.   which 

will  dissolve  5.65  gms.  of  CaCO3.  J.  P.  Catford  (Chemist  and 
Druggist),  suggests  the  use  of  marble.  The  frequent  presence  of 
magnesium  in  this,  the  saturating  power  of  whose  carbonate  is  greater, 
weight  for  weight,  than  calcium  carbonate,  makes  it  necessary  to 
determine  the  quantity  present  in  the  sample.  If  this  quantity  is 
small,  the  slight  excess  in  saturating  power  is  usually  counterbalanced 
by  the  presence  of  traces  of  silica,  and  the  sample  may  be  weighed 
as  CaCO3. 

Half  Normal  Hydrochloric  Acid  (HC1==  36.18.      -  V.  S.  =  i8.o6 

gms.  in  1000  cc.). 

N 
Normal  Sulphuric  Acid  (H2SO4=  97.35.    —  V.  S.  =  48.675  gms. 

in  1000  cc.). 

Mix  carefully  30  cc.  of  pure  concentrated  sulphuric  acid  (sp.gr. 
1.835)  with  enough  water  to  make  about  1050  cc.,  and  allow  the  liquid 
to  cool  to  about  15°  C. 

N 
Titrate  10  cc.  of  this  liquid  in  the    manner  described  under  — 

hydrochloric  acid,  and  dilute  it  so  that  equal  volumes  of  the  acid 
and  the  alkali  will  neutralize  each  other. 

The  standardization  of  normal  acid  solutions  may  also  be  effected 
by  the  use  of  pure  anhydrous  sodium  carbonate. 

Standardization  by  Means  of  Sodium  Carbonate.  Pure  anhy- 
drous sodium  carbonate  may  be  obtained  by  heating  to  dull  redness 
a  few  grams  of  pure  sodium  bicarbonate  for  about  fifteen  minutes. 
The  resulting  carbonate  is  practically  free  from  impurity. 

The  sodium  bicarbonate  loses  on  ignition  one  half  of  its  carbonic 
acid  gas: 


The  bicarbonate  should,  however,  be  tested  before  igniting,  and 
if  more  than  traces  of  chlorid,  sulphate,  or  thiosulphate  are  found, 


STANDARDIZATION   BY    THE  IODOMETRIC  METHOD      71 

these  may  be  removed  by  washing  a  few  hundred  grams,  first  with 
a  saturated  solution  of  sodium  bicarbonate,  and  afterward  with  dis-  ) 
tilled  water.  . 

0.5265  gm.  of  the  pure  anhydrous  sodium  carbonate  is  accurately 
weighed  and  dissolved  in  about  20  cc.  of  water  in  a  flask  and  a  few 
drops  of  methyl  orange  T.  S.  added  as  indicator.  The  acid  to  be 
"set"  or  ''standardized  "  is  then  run  into  the  sodium  -carbonate  solu- 
tion until  a  permanent  light-red  color  is  produced.  It  should  require 

N 
exactly  10  cc.  of  the  —  acid  solution. 

If  8  cc.  of  the  acid  solution  are  consumed  to  bring  about  the  re- 
quired result,  then  every  8  cc.  must  be  diluted  to  10  cc.,  or  the  whole 
of  the  remaining  solution  must  be  diluted  in  this  proportion: 


2)105.31  2)98 

52.65  gms.        4 

0.5265  gm.  =to 


52.65  gms.        49  =to  1000  cc.  —  V.  S.; 


This  method  may  be  employed  as  well  for  the  standardization 
of  hydrochloric  or  of  oxalic  acid. 

Standardization  by  the  lodometric  Method.  —  This  is  a  very 
accurate  method  and  depends  upon  the  fact  that  when  diluted  min- 
eral acids  (as  HC1  or  H2SO4)  are  brought  in  contact  with  a  mixture 
of  potassium  iodid  and  iodate,  an  equivalent  amount  of  iodin  is  set 
free,  as  shown  by  the  equation 

3H2S04    +    5KI+KI03  61         +    3K2S04+3H20. 


3*  97-35=  292-°5  6X125.90=755.4 

N 
The    liberated   iodin    is    estimated   by  means   of  —  sodium  thio- 

sulphate  solution. 

The  reaction  between  iodin  and  sodium  thiosulphate  is 

I2+2Na2S2O3.5H2O=2NaI+Na2S4O6-f  ioH2O. 
251.80         492.92 

5  cc.  (accurately  measured)  of  the  acid  solution  to  be  standard- 
ized are  diluted  with  50  cc.  of  water.     2  gms.  of  pure  potassium  iodid 


72  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

and  0.3  gm.  of  neutral  potassium  iodate  are  added,  and  the  solution 

N 

then  carefully  titrated  with  —  sodium   thiosulphate   until  the  color 

10 

of  the  iodin   disappears,  or  if  starch   is  used   as   indicator  until  the 

N 

blue  of  starch  iodid  is  destroyed.     The  quantity  of  —  sodium  thio- 
sulphate used  is  divided  by  10*  and  the  acid  solution  then  diluted 

N 

so  that  each  5  cc.  will  measure  one  tenth  the  quantity  of  —  thiosul- 
phate used 

Assuming  that  60.6  cc.  of  the  sodium  thiosulphate  solution  were 
used,  then  each  5  cc.  of  the  acid  solution  must  be  diluted  to  measure 
6.06  cc. 

Standardization  by  Ammonium  Sulphate  Method.f  This 
consists  in  neutralizing  the  acid  solution  with  pure  ammonium 
hydroxid,  evaporating  and  weighing  the  resulting  ammonium  sul- 
phate. 25  cc.  of  the  sulphuric  acid  solution  to  be  standardized  are 
carefully  measured  into  a  platinum  evaporating  dish.  Ammonium 
hydroxid  is  then  added  in  slight  excess  and  the  solution  evaporated 
to  dryness  on  a  water-bath  and  the  residue  finally  dried  to  constant 
weight  at  120°  C. 

H2SO4+2NH4OH=(NH4)2SO4+2H2O. 

97-35  I31-21 

N 
48-675        gms.  or  1000  cc.  —  H2SC>4=  65.605    gms.  (NH4)2SO4; 

N 
4-^675     gms.  or    100  cc.  —  H2SO4=  6.5605  gms.  (NH4)2SO4; 

N 
1.216875  gms.  or      25  cc.  —  H2SC>4=   i.64o  +  gms.  (NH4)2SC>4. 

Supposing  the  residue  of  ammonium  sulphate,  weighs  1.84  gms., 
then 

1.84 


1.64 


X  25  =  28.04  cc. 


N  N 

*  This  converts  the  quantity  of  —  thiosulphate  into  cubic  centimeters  of  — 

10  i 

N 
thiosulphate,  which  is  equivalent  to  —  acid. 

t  M.  Wenig,  Ztschr.  angew.  Chem.,  1892,  p.  204. 


STANDARDIZATION   OF   SULPHURIC   ACID  73 

Therefore  each  25  cc.  of  the  acid  solution  must  be  diluted  to 
28.04  cc- 

This  method  is  simple,  accurate,  and  rapid. 

Standardization  of  Sulphuric  Acid  by  Means  of  Barium  Chlorid 
(Gravimetric).  10  cc.  of  the  acid  solution  to  be  standardized  is  diluted 
to  about  100  cc.  and  boiled  in  a  covered  beaker.  To  this  boiling 
solution  is  slowly  added  a  solution  of  pure  barium  chlorid  until  pre- 
cipitation ceases.  The  precipitate  is  then  thoroughly  washed  with 
hot  water,  and  when  all  traces  of  barium  chlorid  have  been  removed 
it  is  dried  and  weighed.  Suppose  the  precipitate  of  barium  sulphate 
weighed  1.24  gms.  This  corresponds  to  0.5208  gm.  of  H2SO4  in  the 
10  cc.  As  there  should  be  only  0.48675  gm.  present,  we  then 
measure  off  4867.5  cc.  and  dilute  it  with  water  to  the  measure  of  5208  cc. 
to  make  a  normal  solution. 

Or  we  may  calculate  in  this  way,  1.15875  gms.  of  barium  sulphate 
corresponds  to  10  cc.  of  the  normal  sulphuric  acid,  therefore  1.24  gms. 
of  barium  sulphate  will  correspond  to  10.7  cc.  of  the  normal  acid. 
Each  10  cc.  must  therefore  be  dilute  to  10.7  cc.  or  each  100  to  107  cc. 

X  i. 24=  10.7. 


1-15875 
The  reaction  is  thus  illustrated 

H2SO4     +     BaCl2       =       BaSO4     +     2HC1. 

48.675    gms.=  iooo  cc.  —  V.  S.=  115.875  gms. 

N 
0.48675  gm.=      10  cc.  —  V.  S.=     1.15875  gms. 

Standardizatition  of  Sulphuric  Acid  may  also  be  Affected  by 
Means  of  the  Specific  Gravity.  Besides  normal  sulphuric  acid  the 
U.  S.  P.  also  employs 

N 
Half  normal  —  sulphuric  acid, 

N 

Tenth  normal  —  sul-bhuric  acid, 
10 

N 
and  Fiftieth  normal  —  sulphuric  acid. 


74  A   MANUAL   OF    VOLUMETRIC   ANALYSIS 


ESTIMATION   OF   ALKALI   HYDROXIDS 

N 
Potassium  and    sodium    hydroxids    are    usually   titrated  with   — 

sulphuric  or  hydrochloric  acid;  they  are,  however,  so  prone  to  absorb 
carbon  dioxid  out  of  the  air  that  they  are  seldom  free  from  carbonate, 
and  hence  the  selection  of  an  indicator  is  a  matter  of  some  importance. 
Phenolphthalein  or  litmus  may  be  employed,  but  it  is  then  advisable 
to  boil  the  solution  while  titrating,  in  order  to  drive  off  the  liberated 
carbon  dioxid,  because  the  latter  gives  an  acid  reaction  with  phenol- 
phthalein  and  litmus  and  thus  causes  an  end-reaction  tint  to  appear 
before  neutralization  is  complete.  It  is  better,  usually,  to  employ  an 
indicator  which  is  not  affected  by  carbon  dioxid.  Methyl  orange  is 
mostly  preferred;  cochineal  and  Congo  red  are  also  useful.  These 
indicators  are  especially  serviceable  in  the  presence  of  carbonates  in 
that  they  are  not  affected  by  carbon  dioxid,  and  can  therefore  be 
employed  in  direct  titrations  without  the  use  of  heat. 

The  quantity  of  carbonate  in  a  recent  sample  of  sodium  or  potas- 
sium hydroxid  is  so  small  usually  that  it  is  customary  to  disregard 
it  and  to  report  the  total  alkalinity  as  hydroxid. 

A  definite  quantity  of  the  sample  (from  0.5  gm.  to  i  gm.  of  the 
solid  or  an  equivalent  of  a  solution)  is  taken  for  analysis,  dissolved 
in  30  to  50  cc.  of  water  in  a  white  porcelain  dish  or  a  beaker  placed 
over  a  white  surface,  and  a  few  drops  of  a  suitable  indicator  added. 

The  vessel  is  then  placed  beneath  a  burette  containing  the  standard 
acid  solution  and  the  latter  run  in,  drop  by  drop,  until  the  last  drop 
just  causes  the  color  to  change.  The  solution  should  be  rotated  or 
stirred  after  each  addition  of  the  standard  acid. 

The  alkali  hydroxids  are  so  exceedingly  hygroscopic  that  they 
take  up  water  from  the  air  while  being  weighed;  it  is  therefore  diffi- 
cult to  make  a  direct  weighing  with  any  degree  of  accuracy. 

The  best  way  is  to  take  a  small  piece  of  the  sample  (about  i  gm.), 
place  it  immediately  in  a  tared  stoppered  flask  and  take  the  weight 
accurately.  It  is  then  dissolved  in  water,  transferred  to  the  porcelain 
dish  or  beaker  and  titrated. 

Potassium  Hydroxid  (KOH=55.74). 

An  accurately  weighed  portion  (preferably  less  than  i  gm.),  is 
placed  in  a  small  beaker,  dissolved  in  50  cc.  of  water,  three  drops  of 

N 
methyl  orange  added,  and  the  titration  begun  with  —  sulphuric  acid 


SODIUM  HYDROXID  75 

and  continued  until  the  yellow  color  of  the  solution  is  changed  to 
red.  Then  the  burette  is  carefully  read  to  see  how  much  of  the  acid 
solution  was  used.  The  number  of  cc.  of  the  latter  are  multiplied 
by  the  normal  factor  for  KOH  (0.05574)  and  the  result  is  the  quantity 
of  pure  KOH  in  the  sample  taken  for  analysis. 
The  following  equation  illustrates  the  reaction: 

2KOH  +   H2S04  =   K2S04  +   2H20 
2)111.48        2)97.35  N 

55.74  gms.     48.675  gms.,  quantity  in  1000  cc.  of  —  acid  V.  S.; 

°-°5574  gm-  (tne  factor  for  KOH),  quantity  neutralized  by 

rN 

i  cc.  of  —  acid, 
i 

N 

Thus  1000  cc.  of  —  H2SO4  V.  S.  containing  48.675  gms.  of  abso- 
lute H2SO4  will  neutralize  55.74  gms.  of  KOH.  Therefore  each 

N 
cc.  of  —  H2SO4  V.  S.  will  neutralize  0.05574  gm.  of  pure  KOH. 

Example.  In  the  above  analysis  let  it  be  assumed  that  0.915  gm. 
of  potassium  hydroxid  were  taken  and  that  15.3  cc.  of  the  standard 
acid  were  required  to  neutralize  it,  then  0.05574  gm.Xi5.3  =  o.8528  gm. 
the  quantity  of  pure  KOH  in  the  0.915  gm.  taken. 

The  percentage  is  then  calculated  in  this  way: 

0.915  :  0.8528  : :  100  :  x-  #=93.2  +  per  cent. 

0.8528X100 

—  =  03.2. 
0.915 

Sodium  Hydroxid  (Na  OH =3 9. 76). 

This  is  estimated  in  exactly  the  same  manner  as  described  for 
potassium  hydroxid,  the  following  equation  being  applied: 

2NaOH   +   H2SO4   =   Na2SO4   +   2H2O 

2)79.52  2)97.35 

39.76  gms.        48.675  gms.=  1000  cc.  —  V.  S. 

i 

N 

•°3976  gm-  i  cc.  ~  V.  S. 

The  factor. 

The  official  solutions  of  potassium  and  of  sodium  hydroxid  are 
estimated  in  this  same  manner,  10  cc.  may  be  taken  for  analysis, 
diluted  with  20  cc.  of  water. 


76  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

Ammonia  Water  (NH3  .  H2O). 

3  cc.  of  ammonia  water  are  put  into  a  stoppered  weighing  bottle 
and  the  weight  taken.     40  cc.  of  water  are  then  added  and  the  solu- 

N 
tion   titrated   with   —  sulphuric  acid.     As   indicator,   litmus,    methyl 

orange,  or  rosolic  acid  may  be  used.  Phenolphthalein  is  useless  for 
titrating  ammonia  and  even  methyl  orange  and  rosolic  acid  are  un- 
suitable in  the  presence  of  much  salts  of  ammonium.  Because  of  the 
volatile  character  of  ammonia  its  solutions  readily  lose  strength  upon 
exposure.  It  is  therefore  best  to  measure  a  quantity  into  a  weighing 
bottle  and  find  its  weight  as  directed  for  potassium  hydroxids.  If 
the  specific  gravity  of  the  ammonia  solution  is  known,  the  weight 
of  a  given  volume  is  easily  calculated,  it  being  only  necessary  to  mul- 
tiply the  volume  in  cc.  by  the  sp.gr.  Thus,  if  the  sp.gr.  of  an  am- 
monia solution  is  0.9585  and  the  volume  taken  is  3  cc.,  the  weight  of 
the  3  cc.  is  3X0.9585  =  2.8755  gms. 

N 
In  the  titration  with  —  sulphuric  acid  each  cc.  of  the  latter  rep- 

resents 0,01693  gm-  °f  NH3  as  shown  by  the  equation 


2)33.86  2)_97.35_  N 

16.93  gms.     «       48.675  gms.  =  1000  cc.  —  V.  S. 

N 
.01693  gm.  =        i  cc.  —  V.  S. 

Factor. 

N 
If   16.9  cc.   of   —  acid  were  required  in   the  above  assay,  then 

0.01693  gm.  X  16.9=  0.287  1  +  gm.,  tne  quantity  of  pure  NH3  in  the 
3  cc.  (2.8755  gms.)  of  ammonia  water  taken. 

The  percentage  is  found  as  follows  : 

If  3  cc.  of  ammonia  water  weighing  2.8755  gms-  contain  0.2871  +  gm. 
of  NH3,  100  gms.  of  ammonia  water  will  contain  X  gm.  of  NH3 

0.2871X100 

—:8---  =9.98  per  cent. 


Stronger  ammonia  water  and  spirit  of  ammonia  may  be  estimated 
in  the  same  manner. 

The  following  are  the  alkalies  which  are  official  in  the  U.  S.  P.: 


ESTIMATION   OF   ALKALI   CARBONATES 


77 


N 
which  — 


Potassii  hydroxidi  .......................  85  per  cent. 

Sodii  hydroxidi  .........................  90  '  ' 

Liquor  potassii  hydroxidi  ................     5  '  ' 

Liquor  sodii  hydroxidi  ...................     5  " 

Aqua  ammoniae  .........................   10  " 

Aqua  ammoniae  fortior  ............  ......   28  " 

Spiritus  ammoniae  .......................   10  '  ' 

N 
In  the  titration  of  all  of  these  —  sulphuric  acid  is  used  and  methyl 

orange  is  the  indicator,  except  in  the  case  of  spiritus  ammoniae  in 
C>4  and  litmus  are  recommended. 

Estimation  of  Alkali  Carbonates.  When  carbonates  are  treated 
with  acids  carbonic  -acid  gas  is  liberated.  This  gas  shows  an  acid 
reaction  with  most  indicators,  and 
the  reaction  will  seem  to  be  com- 
pleted before  the  alkali  is  entirely 
neutralized. 

To  avoid  this,  the  titration  may 
be  conducted  at  the  boiling  tempera- 
ture (hot  way]  in  order  to  drive  off 
the  carbon  dioxid.  The  standard 
acid  being  added  until  two  minutes' 
boiling  fails  to  restore  the  color  in- 
dicating alkalinity.  If  the  titration 
is  conducted  at  a  boiling  tempera- 
ture, it  is  advisable  to  attach  to  the 
lower  end  of  the  burette  a  long  rub- 
ber tube  with  a  pinch-cock  fixed 
about  midway  on  the  tube. 

The  boiling  can  then  be  done  at 
a  little  distance  from  the  burette 
and  the  expansion  of  the  standard  solution  therein  thus  prevented. 

Another  method  is  to  add  to  the  carbonate  a  measured  excess  of 
the  standard  acid,  and  then  after  boiling  to  drive  off  the  carbon  dioxid, 
an  indicator  is  added,  and  the  excess  of  standard  acid  determined 
by  titration  with  a  standard  alkali  (residual  titration  way).  The  quantity 
of  the  latter  deducted  from  the  quantity  of  the  standard  acid  taken, 
gives  the  quantity  of  the  latter  which  reacted  with  the  carbonate. 


FIG.  47. 


80  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

Two  grams  of  the  salt  are  taken  dissolved  in  about  50  cc.  of  water 

N 
and  titrated  with  —  H2SO4  V.  S.     The  reaction  is  as  follows: 


6)312.02  6)292.05 

-  •  ^" 

52.003  gms.        48.675  gms.=  iooo  cc.  —  acid  V.  S. 


N 
Each  cc.  of  —  acid  V.  S.  represents  0.052  gm.  of  N3HuC2O5  or 


0.01693  gm.  of 

If  in  this  titration  37.3  cc.  of  the  standard  acid  are  required  then 
the  two  grams  of  ammonium  carbonate  contained  0.052  gm.X37»3 
=  1.939  gms.  of  the  salt. 

1.939X100 

—  =  96.9  per  cent. 

If  rosolic  acid  is  used  as  indicator  heat  must  be  applied  to  expel 
carbon  dioxid.  The  estimation  of  the  carbonic  acid  may  be  effected 
by  precipitating  a  definite  weight  of  the  salt  with  barium  chlorid, 
collecting  the  precipitated  barium  carbonate,  dissolving  it  in  a  measured 
excess  of  normal  hydrochloric  acid  and  retitrating  with  normal  alkali, 
as  described  in  Chapter  XVII. 

The  method  usually  employed  by  skilled  analysts  (the  residual 
titration  method),  is  to  add  a  measured  excess  of  the  standard  acid 
solution,  and  thus  convert  the  ammonium  carbonate  into  the  less 
volatile  ammonium  sulphate;  then  gently  boil  to  get  rid  of  CO2,  and 
titrate  back  with  a  standard  alkali  V.  S.  (using  litmus  as  an  indi- 
cator) until  the  excess  of  acid  is  neutralized.  The  quantity  of  free 
acid  thus  found,  when  deducted  from  the  amount  of  acid  first  added, 
gives  the  quantity  which  was  required  to  neutralize  the  ammonium 
carbonate. 

Thus,  2  gms.  in  solution  of  ammonium  carbonate  are  treated  with 

50  cc.  of  —  H2SO4  V.  S.,  which  is  more  than  sufficient  to  neutralize  it; 

the  solution  is  then  gently  boiled  to  drive  off   CO2,  a  few  drops  of 

N 
litmus  tincture  added,  and  then  titrated  with  —  KOH  V.  S.  until  the 

litmus  no  longer  shows  an  acid  reaction  and  the  solution  is  neutral. 


MIXED   ALKALI  HYDROXID   AND   CARBONATE  81 

N 
Let  us  assume  that  12.7  cc.  of  the  —  KOH  V.  S.  were  used.     By 

N 
deducting  the  12.7  cc.  from  the  50  cc.  of  —  acid  first  added,  we  find 

37.3  cc.  of  the  acid  went  into  combination  with  the  ammonium  salt, 
the  calculation  is  then  made  as  described  above. 


MIXED   ALKALI  HYDROXID    AND   CARBONATE 

If  it  is  desired  to  ascertain  the  proportion  in  which  these  exist  in 
a  mixture,  we  proceed  as  follows; 

First  determine  the  total  alkalinity  by  means  of  normal  hydro- 
chloric acid,  using  methyl  orange  as  indicator.  Then  dissolve  a  like 
quantity  of  the  mixture  in  150  cc.  of  water  and  add  sufficient  barium 
chlorid  to  precipitate  all  of  the  carbonate  as  barium  carbonate,  and 
then  add  water  to  make  200  cc.  and  set  aside  to  settle.  When  the 
supernatant  liquid  is  clear  take  one  fourth  (50  cc.)  of  it,  and  titrate 
with  normal  hydrochloric  acid,  using  phenolphthalein  as  indicator.* 
The  number  of  cc.  multiplied  by  4  will  be  the  quantity  of  normal 
acid  required  by  the  caustic  alkali.  The  difference  between  this  and 
the  number  of  cc.  representing  the  total  alkalinity  is  calculated  as 
carbonate. 

Example.  Assuming  that  we  are  analyzing  a  mixture  of  sodium 
hydroxid  and  carbonate. 

Two  grams  of  the  substance  are  dissolved  in  water  and  titrated  with 
normal  acid  solution.  43.2  cc.  of  the  latter  are  required.  Another 
2  grams  is  dissolved,  treated  with  barium  chlorid  as  directed,  and 
one  fourth  of  the  clear  solution  titrated  with  normal  acid.  5.6  cc. 
are  required;  then  5.6X4=  22.4  cc.,  representing  the  sodium  hydroxid. 

43.2  cc.  =  total  alkalinity; 

—  22.4X0.0397  =  0.889  grams  sodium  hydroxid; 
20.8X0.0526=1.094  grams  sodium  carbonate. 

Another  way  is  to  filter  the  mixture  after  barium  chlorid  has  been 
added,  titrate  the  filtrate  with  normal  acid  to  find  the  quantity  of 

*  The  slight  error  which  occurs  in  this  method  because  the  volume  of  the 
precipitate  is  included  in  the  measured  liquid,  may  be  overcome  by  using  the 
entire  quantity  of  liquid,  including  the  precipitate  (instead  of  taking  one-fourth 
of  it),  and  titrating  with  oxalic  acid  V.  S.  in  the  presence  of  phenolphthalein. 
Oxalic  acid  in  very  dilute  solutions  does  not  react  with  alkali  earth  carbonates. 


82  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

hydroxid,  then  dissolve  the  precipitated  barium  carbonate  in  normal 
hydrochloric  acid  in  excess,  and  retiterate  with  normal  alkali,  thus 
ascertaining  the  amount  of  carbonate. 

When  the  alkaline  carbonate  is  present  in  very  small  quantities  the 
method  of  Lunge  may  be  employed. 

A  few  drops  of  phenacetolin  solution  are  added  to  impart  a  scarcely 
perceptible  yellow  to  the  liquid.  Normal  acid  solution  is  then  run  in 
until  a  pale  rose  tint  appears,  indicating  that  all  the  alkali  hydroxid 
is  neutralized;  the  volume  of  acid  is  noted,  and  the  titration  continued; 
the  red  color  is  intensified,  and  when  the  carbonate  is  entirely  de- 
composed a  golden-yellow  color  results. 

Considerable  practice  is  required  with  solutions  of  known  compo- 
sition to  accustom  the  eye  to  the  changes  of  color. 


ESTIMATION     OF     ALKALI     BICARBONATES     WHEN     MIXED    WITH     CAR- 
BONATES 

Thompson's  Method.  Take  2  grams  of  the  salt  and  dissolve  in 
100  cc.  of  water.  Divide  the  solution  into  two  equal  parts  and  titrate 
one  portion  with  normal  acid  solution,  using  methyl  orange  as  indi- 
cator, and  note  the  quantity  required.  We  will  assume  13  cc. 

Then  treat  the  second  portion  with  a  measured  excess  (say  25  cc.) 
of  normal  sodium  hydroxid  solution  free  from  CC>2.  This  converts 
the  bicarbonate  into  carbonate.  Now  add  an  excess  of  pure  neutral 
barium  chlorid  solution  in  order  to  precipitate  all  the  carbonate  as 
barium  carbonate,  and  then  titrate  with  normal  acid,  using  phenol- 
phthalein  as  indicator,  to  determine  the  excess  of  sodium  hydroxid. 
15  cc.  are  required.  Thus 

25—       15=  10  cc.,  the  equivalent  of  bicarbonate, 
and  13—       io==   3cc.,  the  equivalent  of  carbonate; 

ioX. 0834=. 834  gm.,  sodium  bicarbonate; 
3 X. 0526=. 1578  gm.,  sodium  carbonate. 


ESTIMATION   OF    ALKALIES   IN    THE    PRESENCE   OF   SULPHITES 

This  is  accomplished  by  adding  hydrogen  peroxid  to  the  solution 
in  order  to  convert  the  sulphite  into  sulphate,  and  then  titrating  in  the 
usual  way  with  normal  acid. 


ESTIMATION  OF  ORGANIC  SALTS   OF   THE  ALKALIES     83 
MIXED    POTASSIUM  AND   SODIUM   HYDROXIDS 

These  are  estimated  by  treatment  with  tartaric  acid  solution,  which 
converts  them  into  bitartrates.  The  bitartrate  of  potassium  is  almost 
insoluble  in  solution  of  sodium  bitartrate  and  hence  may  be  separated 
by  filtering.  The  sodium  bitartrate  is  estimated  in  the  nitrate  by 
titration  with  normal  sodium  hydroxid  solution.  The  potassium  is 
found  by  difference. 

Estimation  of  Organic  Salts  of  the  Alkalies.  The  tartrates, 
citrates,  and  acetates  of  the  alkali  metals  are  converted  by  ignition 
into  carbonates,  the  whole  of  the  base  remaining  in  the  form  of 
carbonate. 

Each  molecular  weight  of  a  normal  tartrate  gives  when  ignited 
one  molecular  weight  of  carbonate: 


Every  two  molecular  weights  of  an  acetate  or  an  acid  tartrate  give 
one  molecular  weight  of  carbonate: 


2KHC4H4O6=K2CO3. 

Every  two  molecular  weights  of  a  normal  citrate  give  three  molec- 
ular weights  of  carbonate: 


These  reactions  are  taken  advantage  of  in  volumetric  analysis, 
and  the  tartrates,  citrates,  and  acetates  of  the  alkalies  are  indirectly 
estimated  by  calculating  upon  the  quantity  of  carbonate  formed  by 
burning  them,  the  quantity  of  carbonate  being  found  by  titration  in  the 
usual  manner. 

The  Process.  Before  igniting,  the  salt  to  be  examined  should  be 
thoroughly  dried  in  a  desiccator  over  calcium  chlorid  or  in  a  drying 
oven,  the  latter  only  for  such  salts  as  have  no  water  of  crystallization 
in  their  composition.  If  the  weight  is  taken  before  and  after,  the 
amount  of  moisture  present  is  determined.  One  or  two  grams  of  the 
dried  salt  is  weighed  accurately,  placed  in  a  porcelain  crucible,  and  heat 
applied  gradually,  until  dull  redness  is  reached  and  white  fumes  cease 
to  be  given  off.  Upon  applying  heat  to  the  salt,  the  latter  swells, 


82  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

hydroxid,  then  dissolve  the  precipitated  barium  carbonate  in  normal 
hydrochloric  acid  in  excess,  and  retiterate  with  normal  alkali,  thus 
ascertaining  the  amount  of  carbonate. 

When  the  alkaline  carbonate  is  present  in  very  small  quantities  the 
method  of  Lunge  may  be  employed. 

A  few  drops  of  phenacetolin  solution  are  added  to  impart  a  scarcely 
perceptible  yellow  to  the  liquid.  Normal  acid  solution  is  then  run  in 
until  a  pale  rose  tint  appears,  indicating  that  all  the  alkali  hydroxid 
is  neutralized;  the  volume  of  acid  is  noted,  and  the  titration  continued; 
the  red  color  is  intensified,  and  when  the  carbonate  is  entirely  de- 
composed a  golden-yellow  color  results. 

Considerable  practice  is  required  with  solutions  of  known  compo- 
sition to  accustom  the  eye  to  the  changes  of  color. 


ESTIMATION     OF     ALKALI      BICARBONATES     WHEN     MIXED    WITH     CAR- 
BONATES 

Thompson's  Method.  Take  2  grams  of  the  salt  and  dissolve  in 
100  cc.  of  water.  Divide  the  solution  into  two  equal  parts  and  titrate 
one  portion  with  normal  acid  solution,  using  methyl  orange  as  indi- 
cator, and  note  the  quantity  required.  We  will  assume  13  cc. 

Then  treat  the  second  portion  with  a  measured  excess  (say  25  cc.) 
of  normal  sodium  hydroxid  solution  free  from  CC>2.  This  converts 
the  bicarbonate  into  carbonate.  Now  add  an  excess  of  pure  neutral 
barium  chlorid  solution  in  order  to  precipitate  all  the  carbonate  as 
barium  carbonate,  and  then  titrate  with  normal  acid,  using  phenol- 
phthalein  as  indicator,  to  determine  the  excess  of  sodium  hydroxid. 
15  cc.  are  required.  Thus 

25—       15  =  10  cc.,  the  equivalent  of  bicarbonate, 
and  13—       10=  3  cc.,  the  equivalent  of  carbonate; 

ioX. 0834=. 834  gm.,  sodium  bicarbonate; 
3X. 0526=. 1578  gm.,  sodium  carbonate. 


ESTIMATION   OF    ALKALIES   IN   THE    PRESENCE   OF   SULPHITES 

This  is  accomplished  by  adding  hydrogen  peroxid  to  the  solution 
in  order  to  convert  the  sulphite  into  sulphate,  and  then  titrating  in  the 
usual  way  with  normal  acid. 


ESTIMATION  OF  ORGANIC  SALTS   OF   THE  ALKALIES     83 
MIXED    POTASSIUM   AND    SODIUM    HYDROXIDS 

These  are  estimated  by  treatment  with  tartaric  acid  solution,  which 
converts  them  into  bitartrates.  The  bitartrate  of  potassium  is  almost 
insoluble  in  solution  of  sodium  bitartrate  and  hence  may  be  separated 
by  filtering.  The  sodium  bitartrate  is  estimated  in  the  nitrate  by 
titration  with  normal  sodium  hydroxid  solution.  The  potassium  is 
found  by  difference. 

Estimation  of  Organic  Salts  of  the  Alkalies.  The  tartrates, 
citrates,  and  acetates  of  the  alkali  metals  are  converted  by  ignition 
into  carbonates,  the  whole  of  the  base  remaining  in  the  form  of 
carbonate. 

Each  molecular  weight  of  a  normal  tartrate  gives  when  ignited 
one  molecular  weight  of  carbonate: 


Every  two  molecular  weights  of  an  acetate  or  an  acid  tartrate  give 
one  molecular  weight  of  carbonate: 


Every  two  molecular  weights  of  a  normal  citrate  give  three  molec- 
ular weights  of  carbonate: 


These  reactions  are  taken  advantage  of  in  volumetric  analysis, 
and  the  tartrates,  citrates,  and  acetates  of  the  alkalies  are  indirectly 
estimated  by  calculating  upon  the  quantity  of  carbonate  formed  by 
burning  them,  the  quantity  of  carbonate  being  found  by  titration  in  the 
usual  manner. 

The  Process.  Before  igniting,  the  salt  to  be  examined  should  be 
thoroughly  dried  in  a  desiccator  over  calcium  chlorid  or  in  a  drying 
oven,  the  latter  only  for  such  salts  as  have  no  water  of  crystallization 
in  their  composition.  If  the  weight  is  taken  before  and  after,  the 
amount  of  moisture  present  is  determined.  One  or  two  grams  of  the 
dried  salt  is  weighed  accurately,  placed  in  a  porcelain  crucible,  and  heat 
applied  gradually,  until  dull  redness  is  reached  and  white  fumes  cease 
to  be  given  off.  Upon  applying  heat  to  the  salt,  the  latter  swells, 


84  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

fuses,  and  then  boils,  and  if  the  heat  is  applied  too  rapidly  at  this 
point,  there  is  apt  to  be  a  considerable  loss  of  material  through  sput- 
tering. The  completion  of  the  ignition  is  known  to  be  reached  when 
the  black  contents  of  the  crucible  is  dry  and  crisp.  The  crucible  is 
then  allowed  to  cool,  and  its  contents  treated  with  boiling  water  to 
dissolve  out  the  alkali  carbonate,  and  the  solution  filtered  through 
a  small,  wetted  filter  into  a  flask  or  beaker.  The  filtrate  should  be 
perfectly  colorless.  If  it  has  a  yellow  or  brownish  color  it  indicates 
incomplete  ignition  and  should  be  rejected,  and  a  fresh  quantity  of 
the  salt  subjected  to  ignition.  The  contents  of  the  crucible  and  the 
filter  should  be  washed  with  several  small  portions  of  water  until  the 
washings  no  longer  show  an  alkaline  reaction.  The  filtrate  mixed 
with  the  wash  water  is  now  titrated  with  standard  sulphuric  or  hydro- 
chloric acid,  using  methyl  orange  as  the  indicator.  From  the  quantity 
of  carbonate  found  in  the  filtrate,  the  equivalent  amount  of  the  organic 
salt  may  be  calculated.  The  quantity  of  standard  acid  employed  is 
multiplied  direct  by  the  factor  for  the  original  salt. 

In  the  case  of  salts  of  the  alkali  earths,*  residual  titration  should 
be  resorted  to.  The  residue  in  the  crucible  being  dissolved  in  standard 
hydrochloric  acid,  and  retitrated  with  standard  alkali. 

Lithium  salts,  because  of  the  sparing  solubility  of  the  carbonate 
in  water,  should  also  be  titrated  by  the  residual  method. 

Potassium  Tartrate  (2K2C4H4O6+H2O  =  476.16).  Two  grams 
of  the  salt  are  placed  in  a  platinum  or  porcelain  crucible  and  heated 
to  redness  in  contact  with  the  air  until  completely  charred;  that  is 
to  say,  until  .nothing  is  left  in  the  crucible  but  carbonate  and  free 
carbon. 

The  crucible  is  now  cooled,  and  its  contents  treated  with  boiling 
water,  which  dissolves  the  potassium  carbonate,  the  carbon  being 
separated  by  filtration.  In  order  to  obtain  every  trace  of  carbonate 
it  is  well  to  wash  the  crucible  with  several  small  portions  of  hot 
water,  and  add  the  washings  to  the  rest  of  the  filtrate  through  the 
filter. 

If  the  salt  is  completely  carbonized  the  filtrate  will  be  colorless, 
but  if  the  carbonization  is  not  complete  the  solution  will  be  more  or 
less  colored  and  should  be  rejected,  and  a  fresh  quantity  of  the  salt 
subjected  to  ignition. 


*  Organic  salts  of  the  alkali  earths  subjected  to  ignition  as  above  are  reduced 
partly  to  oxids. 


POTASSIUM   AND   SODIUM   TARTRATE  85 

To  the  filtrate,  which  contains  potassium  carbonate,  add  a  few 

N 
drops  of  methyl  -orange,  and  titrate  with  —  sulphuric  acid  V.  S.  until 

a  light  orange-red  color  appears  and  the  carbonate  is  neutralized. 
The  following  equations  will  explain  the  reactions  : 


476.16  274.54 

then 

2K2CO3  +  2H2SO4=  2K2SO4+  2H2O  +  2CO2; 
27*54          194.7 

therefore 


4)476.16  4)274.54      4)194-7 

119.15  gms.=  68.6  gms.=  48.675  gms.=  1000  C.  —  V.  S., 

i 

N 
and  each  cc.  of  —  H2SO4  represents  0.119  gm.  of  potassium  tartrate. 

Example.     Two  gms.  of  potassium  tartrate  treated  as  described 

N 
above    require  16.3   cc.  of   —  H2SO4  V.  S.      It  therefore  contains 

0.119X16.3  =  1.9397  gms. 

1.9397X100 

-  =96.98  percent. 


Potassium  and  Sodium  Tartrate  (KNaC4H4O6.4H2O=  280.18) 
(Rochelle  Salt).  This  salt  is  treated  in  exactly  the  same  way  as  de- 
scribed for  potassium  tartrate. 

When  ignited  the  double  tartrate  is  converted  into  a  double  car- 
bonate of  potassium  and  sodium  : 


560.36  242.58 

*  Since  some  carbon  is  always  left  behind,  the    reaction  is  probably  more 
accurately  written  thus: 


f  The  reaction  is  probably  more  accurately  written  thus: 


86  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

then  2KNaCO3+2H2SO4=2KNaSO4+2CO2+2H2O; 

24.258  194.7 

therefore 

2KNaC4H4064  .  H2O  =  2KNaCO3  =  2H2SO4, 

4)56036  4)242.56        4)194-7  N 

140.04  60.6  48.675=1000  cc.  —  •  V.  S. 

i 

N 
and  each  cc.  of  —  H2SO4  represents  0.140  gm.  of  KNaC4H4O6.4H2O. 

Example.  If  one  gram  of  Rochelle  salt  treated  as  above  described, 
requires  7  cc.  of  —  sulphuric  acid,  it  contains  0.140X7  =  0.980  gm.  =  98 

per  cent. 

Potassium  Bitartrate  (Cream  of  Tartar)  (KHC4H4O6=  186.78). 
The  estimation  of  this  salt  is  affected  in  the  same  way  as  the  tartrate. 

The  bitartrate  having  but  one  atom  of  potassium  in  its  molecule, 
it  takes  two  molecules  to  form  one  molecule  of  carbonate. 


373-56  137-27 

then 

K2C03+H2S04=K2S04+H20+CO2; 

I37-27        97-35 

therefore 

2KHC4H406=K2C03=H2S04. 

2)373-56          2)137.27      2)97.35  N 

186.78  68.635       48.675=100000.  of  —  V.  S. 


and  each  cc.  of  2  H2SO4  V.  S.= 0.18678  gm.  of  KHC4H4O6. 

Another  way  of  estimating  bitartrate   is   to   dissolve    a    weighed 

N 
quantity  in  hot  water  and  titrate  with  --   potassium  hydroxid   until 

neutral,  and  thus  the  amount  of  tartaric  acid  existing  as  bitartrate  is 
found.  The  bitartrate  is  acid  in  reaction.  In  detail  the  method  is 
as  follows: 

*  The  reaction  may  also  be  written  thus: 

2KHC4H4O6=  K2CO3+  50+  2CO2+  5H2O. 


POTASSIUM   ACETATE  87 

Two  gms.  of  the  bitartrate  are  dissolved  in  100  cc.  of  hot  water, 
a  few  drops  of  phenolphthalein  T.  S.  added,  and  then  titrated  with 

N 

—  KOH  V.  S.  until  a  faint  pink  color  indicates  that  all  of  the  acid 

has  been  neutralized.     Not  less  than    10.6  cc.  of   the   normal   alkali 
should  be  required,  corresponding  to  98.9  per  cent  of  pure  salt. 
The  following  equation  will  show  the  reaction: 

KHC4H4O6+KOH=K2C4H4O6+H2O. 

N 
186.78  55.74  =  1000  cc.  of  —  KOH  V.  S. 

Each  cc.  of  -  KOH  V.  S.  represents  0.18678  gm.  of  KHC4H4O6. 

If  10.6  cc.  are  required  for  neutralization,  then  10.6X0.18678= 
1.979  gms.: 

1.979X100 

—  =98.9  per  cent. 

Potassium  Citrate  (K3C6H5O7.H2O=322.o8). 


644.16  411.71 

then 


411.71         292.05 
therefore 


6)644.16  6)411.71      6)292.05 

107.36  gms.  68.635          48.675  gms.=  1000  cc.  —  acid. 

N 
Thus  each  cc.  of  —  acid  represents  0.10736  gm.  of  pure  potassium 

citrate. 

Potassium  Acetate  (KC2H3O2=  97.44).  In  estimating  potassium 
acetate  the  salt  is  ignited  and  the  residue  treated  in  exactly  the  same 
manner  as  in  the  estimation  of  the  citrates  and  tartrates  before  men- 
tioned. 

t  2KC2H302+402=K2C03+3H20+3C02; 
_  194-88  _  137-27  _ 

*  The  reaction  may  also  be  written  thus: 
2(K3C6H507.H20)  +  40  =  3K 
t  The  reaction  may  also  be  written  thus: 


88  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

then  K2CO3  +  H2SO4=K2SO4+H2O  +  CO2; 

i37-27        97-35 
therefore 

2KC2H302  =  K2C03  =  H2S04. 
2)194.88         2)137.27          2)97.35 


N 


97.44  gms.       68.635  gms.    48.675  gms.=  1000  cc.  -  H2SO4. 

N 
Each  cc.  therefore  of  —  H2SO4  V.  S.  corresponds  to  0.09744  gm. 

of  potassium  acetate. 

If  10  cc.  are  required  to  neutralize  the  residue  from  i  gm.  of  potas- 
sium acetate,  the  salt  contains  10X0.09744=0.9744  gm.,  or  97.44  per 
cent. 

Sodium  Acetate  (NaC2H3O2.3H2O=  135.10). 

2(NaC2H3O2.3H2O)-J-4O2==Na2CO3,  etc. 

260.2  105.3  . 

N 
Each  cc.  of  —  H2SO4  V.  S.  represents   0.1301   gm.  of  crystallized 

sodium  acetate. 

Sodium  Benzoate  (NaC7H5O2=  143.01). 


,  etc 
286.02  105.3 

N 
Each  cc.  of  —  H2SO4  V.    S.  represents   0.14301    gm.  of  sodium 

benzoate. 

Sodium  Salicylate  (NaC7H5O3=  158.89). 


2NaC7H5O3+i4O2  =  Na2CO3,  etc. 

3J7-78  105.3 

N 
Each  cc.  of  —  H2SO4  V.  S.  represents  0.15889  gm.  of  sodium  sali- 

cylate. 

Lithium  Citrate  (Li3C6H5O7=  208.56).  As  stated  before,  the 
organic  salts  of  lithium  and  those  of  the  alkali  earth  metals  are  best 
titrated  by  the  residual  method,  after  ignition,  because  the  carbonates 
formed  are  insoluble  in  water.  It  is  likewise  best  to  use  standard 
hydrochloric  instead  of  standard  sulphuric  acid.  The  process  for 
lithium  citrate  here  given  exemplifies  the  method. 


ESTIMATION  OF  ALKALI   METALS  IN   THEIR  SALTS 


One  gm.  of  the  salt  is  thoroughly  ignited  in  a  porcelain  crucible 
as  described  for  potassium  tartrate.     The  residue  of  lithium  carbonate 

N 
is  then  dissolved  out  of  the  crucible  by  adding  20  cc.  of  —  hydrochloric 

V.  S.  and  filtering.  The  crucible  and  filter  are  washed  with  several 
small  quantities  of  water  and  the  washings  added  to  the  acid  filtrate. 
Three  drops  of  methyl  orange  are  now  added,  and  the  solution  titrated 

N 
with  --    sodium    hydroxid    V.    S.    until    the    yellow    color    appears. 

Assuming  that  5.8  cc.  of  the  standard  alkali  were  required,  then 
20—5.8=14.2  cc.,  the  quantity  of  normal  hydrochloric  acid  which 
reacted  with  the  lithium  carbonate.  This  quantity  multiplied  by  the 
normal  factor  for  lithium  citrate  0.06952  gives  the  weight  of  pure 
salt  in  the  i  gm.  taken. 

0.06952X14.2  =  0.987  gm.  or  98.7  per  cent. 

The  other  lithium  organic  salts  of  the  U.  S.  P.  are  assayed  gravi- 
metrically  by  conversion  to  sulphate. 

TABLE  SHOWING  THE  NORMAL  FACTORS,  ETC.,  OF  THE  ORGANIC  SALTS  OF  THE 

ALKALI  METALS 


Substance. 

Formula. 

Molecular 
Weight. 

Equivalent 
Weight  in 
Carbonate. 

Normal 
Factor. 

LiC7H5O, 

127.11 

76.7^? 

o.  12711 

'  '        citrate                   .... 

Li3C6H5O7 

208  .  56 

110.265 

o  .  060  5  2 

'  '        salicylate  

LiC7H5O3 

142.99 

36.755 

0.14299 

Sodium  acetate                 .    . 

NaC2H3O2.3H2O 

175.10 

52.  6? 

O.  1^1 

'  '       benzoate  

NaC7H5O2 

143.01 

52.65 

0.14701 

'  c       salicylate 

NaC7H5O3 

158.89 

52.65 

O.I  <;88Q 

Potassium  acetate   ...       .... 

KC2H3O2 

97-44 

68.685 

0.00744 

'  '          bitartrate 

KHC4H4O8 

186.78 

68.685 

0.18678 

'  '          citrate           

K3C6H5O7.H2O 

322.08 

205  .  95 

o.  107^6 

'  '          tartrate 

2K2C4H4O62H2O 

476.16 

274.  54 

O  .  I  I  OO4 

"          and    sodium    tar- 
trate   

KNaC4H4O6.4H2O 

280.18 

121.29 

o.  14009 

Estimation  of  Alkali  Metals  in  their  Salts.  This  may  be  done 
by  first  converting  the  salt  into  a  sulphate,  and  then  by  means  of 
barium  hydroxid,  forming  an  alkali  hydroxid  which  is  finally  con- 
verted into  an  alkali  carbonate  by  means  of  carbon  dioxid. 

(a)  K2SO4+Ba(OH)2  =  BaSO44-2KOH; 

(b)  2KOH+CO2         =  K2CO3  +  H2O. 


90  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

The  conversion  of  the  original  salt  into  a  sulphate  may  be  done 
in  several  ways,  depending  upon  whether  the  acid  in  combination  is 
a  volatile  or  a  non-volatile  one,  as  described  below. 

Alkalies  Combined  with  Volatile  Acids.  A  definite  quantity  of  the 
salt  in  solution  is  treated  with  an  excess  of  sulphuric  acid  and  evap- 
orated to  dryness,  and  then  further  heated  to  drive  off  some  of  the 
excess  of  sulphuric  acid.  The  residue  which  consists  of  the  alkali 
as  a  sulphate  is  dissolved  in  water  and  treated  with  a  slight  excess  of 
barium  hydroxid  solution.  The  mixture  now  contains  the  alkali 
in  solution  as  hydroxid,  and  a  precipitate  of  barium  sulphate  (see 
equation  (a)),  also  the  excess  of  barium  hydroxid  in  solution.  A 
stream  of  carbon  dioxid  (CO2)  is  now  passed  through  the  mixture; 
this  converts  the  alkali  hydroxid  into  carbomate  and  at  the  same 
time,  removes  the  barium  hydroxid  by  precipitating  it  as  barium 
carbonate  (see  equation  (b)).  When  this  conversion  into  carbonate  is 
complete,  the  free  carbon  dioxid  must  be  driven  off  by  boiling,  because 
barium  carbonate  is  converted  into  the  soluble  barium  bicarbonate, 
in  the  presence  of  free  carbon  dioxid.  The  mixture  now  contains 
the  alkali  in  solution  as  a  carbonate,  and  a  sediment  consisting  of 
barium  sulphate  and  barium  carbonate.  This  mixture  is  now  made 
up  to  a  definite  volume  and  the  alkali  carbonate  titrated  in  the  usual 
manner  in  an  aliquot  portion,  which  may  be  removed  by  filtration, 
or  by  means  of  a  pipette,  if  the  precipitate  settles  rapidly  and  leaves 
a  clear  supernatant  liquid. 

Alkalies  Combined  with  Non-Volatile  Acids.  In  the  case  of  alkali 
salts  of  non-volatile  acids,  as  phosphoric,  boric,  chromic,  arsenic, 
molybdic,  etc.,  the  acid  is  removed  by  means  of  lead  acetate,  and  the 
resulting  alkali  acetate  converted  into  sulphate,  by  means  of  sulphuric 
acid. 

(c)  K2CrO4+Pb(C2H3O2)2=PbCrO4+  2KC2H3O2; 

(d)  2KC2H3O2+H2SO4       =  K2SO4+2HC2H3O2; 

(e)  Pb(C2H3O2)2+H2SO4  =PbSO4+2HC2H3O2. 


To  the  solution  of  the  salt  an  excess  of  lead  acetate  solution  is 
added;  this  causes  a  precipitation  of  the  acid  as  a  lead  salt  and  con- 
verts the  alkali  into  an  acetate  which  remains  in  solution.  (See  equa- 
tion (c)).  The  excess  of  lead  acetate  is  also  in  solution.  The  mixture 
is  filtered  and  the  filtrate  treated  with  a  slight  excess  of  sulphuric 
acid.  This  converts  the  alkali  acetate  into  a  sulphate  (see  equation  (</)) 
and  removes  the  lead  acetate  by  precipitation  in  the  form  of  lead  sul- 


ESTIMATION  OF  THE  SALTS  OF  THE  ALKALI  EARTHS      91 

phate  (see  equation  (e))  which  is  filtered  out,  and  the  solution  of  alkali 
sulphate  treated  as  above  discribed. 


ESTIMATION   OF   THE   SALTS   OF   THE   ALKALI  EARTHS  , 

Standard  solution  of  hydrochloric  or  of  nitric  acid  is  preferred 
by  many  operators  for  the  titration  of  hydroxids  or  carbonates  of 
the  alkali  earths. 

These  acids  possess  the  advantage  over  most  other  acids  of  forming 
soluble  salts.  The  hydroxids  may  be  estimated  by  any  of  the  indi- 
cators, but  as  they  readily  absorb  CO2  out  of  the  air,  they  are  generally 
contaminated  with  more  or  less  carbonate,  and  the  residual  method 
should  be  used,  i.e.,  a  known  excess  of  standard  acid  should  be  added, 
the  mixture  boiled  to  expel  any  trace  of  CO2,  and  retitrated  with 
standard  alkali. 

The  carbonates  are  of  course  estimated  in  the  same  way,  as  are 
also  the  organic  salts  of  the  alkali  earths,  after  ignition.  As  an 
example : 

One  gram  of  calcium  carbonate  is  mixed  with  5  cc.  of  water.  A 
measured  excess  of  normal  hydrochloric  acid  V.  S.  is  now  added, 
and  the  solution  boiled  to  drive  off  the  CO2.  Then  add  a  few  drops 

N 
of  phenolphthalein,  and  titrate  with  —  alkali  V.  S.  until  a  faint  pink 

color  is  obtained. 

N 
Note  the  quantity  of  —  alkali  used,   and  deduct  this  from  the 

N 
quantity  of  --  acid  first  added,  and  the  remainder  will  represent  the 

amount  of  acid  which  combined  with  the  calcium. 

N 
Each  cc.  of  —  acid  V.  S.  represents  0.049675  gm.  of  CaCO3. 

CaCO3   +    2HC1   =    CaCl2    +    H2O    +    CO2. 


2)99-35        2) 

49.675  gms.  36.18  gms.  or  1000  cc.  —  acid  V.  S. 


N 


N 
Assuming  that  30  cc.  of  —  HC1  V.  S.  were  added  to  the  i  gm.  of  CaCO3, 

and  that  n  cc.  of  —  KOH  V.  S.  were  required  to  bring  the  mixture 


92  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

N 
back  to  neutrality,  then  19  cc.  of  —  HC1  were  actually  required  to 

saturate  the  CaCO3. 

Therefore  0.049675X19=0.9438  or  94.38  per  cent. 

The  hydroxids  and  carbonates  may  also  be  estimated  by  direct 
titration  with  standard  hydrochloric  acid  (in  the  cold)  using  methyl 
orange  as  indicator.  A  better  plan,  however,  would  be  to  add 
the  standard  acid  in  slight  excess,  and  then  standard  alkali  until  a 
distinct  yellow  color  appears;  the  slight  excess  of  alkali  is  then  deter- 
mined by  adding  standard  hydrochloric  acid  until  the  red  color  re- 
appears. A  much  more  distinct  color  reaction  is  thereby  obtained. 
The  quantity  of  the  standard  alkali  used  is  deducted  from  the  total 
quantity  of  standard  acid  added. 

Soluble  salts  of  calcium,  barium,  and  strontium,  such  as  chlorids, 
nitrates,  etc.,  may  be  readily  estimated  as  follows: 

A  weighed  quantity  of  the  salt  is  dissolved  in  water,  cautiously 
neutralized  if  it  is  acid  or  alkaline,  phenolphthalein  is  added,  the 
mixture  heated  to  boiling,  and  standard  solution  of  sodium  carbonate 
delivered  in  from  time  to  time,  with  constant  boiling  until  the  red 
color  is  permanent. 

This  process  depends  upon  the  fact  that  sodium  carbonate  forms 
with  soluble  salts  of  these  bases  insoluble  neutral  carbonates. 


+  Na2C03=  CaC03+  2NaCl. 
Ba(NO3)2+Na2CO3= 


Magnesium  salts  cannot  be  estimated  in  this  way,  as  magnesium 
carbonate  affects  the  indicator. 

The  alkali  earth  salts  may  also  be  estimated  by  dissolving  them 
in  water,  precipitating  the  base  as  carbonate,  with  an  excess  of  ammo- 
nium carbonate  and  some  free  ammonia.  The  mixture  is  then  heated 
for  a  few  minutes,  and  the  carbonate  separated  by  nitration,  thoroughly 
washed  with  hot  water  till  all  soluble  matters  are  removed,  and 
then  titrated  with  normal  acdi  V.  S.  as  carbonate. 

Normal  Sodium  Carbonate  V.  S.  (Na2CO3=  105.31)  contains 
52.65  gms.  in  i  liter.  This  solution  is  made  by  dissolving  52.65  gms. 
of  pure  sodium  carbonate  (anhydrous)  previously  ignited  and  cooled, 
in  distilled  water,  and  diluting  to  i  liter  at  15°  C.  (59°  F.). 

If  a  pure  salt  is  not  at  hand  the  solution  may  be  made  as  follows. 


HYDROXIDS  AND  CARBONATES  OF  ALKALINE  EARTHS     93 

About  85  gms.  of  pure  sodium  bicarbonate,  free  from  thiosulphate, 
chlorid,  etc.,  are  heated  to  dull  redness  (not  to  fusion)  for  about 
fifteen  minutes  to  expel  one  half  of  the  CO2;  it  is  then  cooled  under 
a  desiccator.  When  cool,  weigh  off  52.65  gms.  and  dissolve  it  in 
distilled  water  to  make  i  liter  at  15°  C.  (59°  F.).  This  solution  should 

N 

neutralize  —  acid  V.  S.  volume  for  volume. 
i 

As  an  example  of  the  process:  Take  of  calcium  chlorid  one  gm., 
dissolve  it  in  a  small  quantity  of  water,  neutralize  the  solution  if  it  is 
acid  or  alkaline,  heat  to  boiling,  add  a  few  drops  of  phenolphthalein, 

N 
and  titrate  with  —  sodium  carbonate,  delivered  cautiously  whilst  boiling 

until  the  red  color  is  permanent. 

CaCl2  +  Na2CO3  =   CaCO3  +    2NaCl 
2)110.16        2)105.31  N 

55.o8gms.       52.65  gms.  or  1000  cc.  —  V.  S. 

N 
Each  cc.  of  —  Na2CC>3  V.S.  represents  0.05508  gm.  of  CaC^. 

If  18  cc.  are  used  the  salt  contains  0.05508  gm.X  18  =  0.991  gm.  or 
99.1  per  cent. 

In  the  other  method  in  which  an  excess  of  ammonium  carbonate  is 
added  together  with  some  free  ammonia,  the  calcium  is  precipitated 
as  carbonate,  this  is  then  separated  by  filtration,  thoroughly  washed 
with  hot  water  to  remove  all  soluble  matters,  and  then  titrated  as 
directed  for  carbonate. 


CaBr2  =  CaCO3  =  H2SO4. 
2)198.52      2)99^5         2)98  N 

99.26  gms.  49.675gms.   49  gms.  or  1000  cc.  —  V.  S. 

N 
Each  cc.  of  —  acid  thus  represents  0.09926  gm.  of  CaBr2- 

The  Estimation  of  Mixed  Hydroxids  and  Carbonates  of 
Alkali  Earths.  This  may  be  done  as  described  under  estima- 
tion of  mixed  alkali  hydroxids  and  carbonates,  page  81,  except 
that  it  is  in  this  case  unnecessary  to  precipitate  the  carbonate 
by  barium  chlorid  in  that  the  alkali  earth  carbonates  are  already 
insoluble. 


94  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

TABLE  SHOWING  NORMAL  FACTORS,  ETC.,  OF  THE  ALKALI  EARTH  METALS. 


Name  of  Salt. 

Formula. 

Molecu- 
lar 
Weight. 

Standard  Solution 
Used. 

—  Factor. 

i 

Barium  carbonate  

BaC03 
BaCl2 
Ba(OH)2 
Ba(N03)2 
CaBr2 
CaC03 
CaCl2 
Ca(OH)2 
CaO 

MgC03 

(MgC03)4Mg(OH)2 
+  5H20 

MgO 

SrBr2 
SrC03 
SrI2 
Sr(C3H503)2 
SrO 
Sr(C7H5O3)2+2H2O 

195-95 
206.76 
170.16 

259-54 
198.52 

99-35 
110.16 

73-56 
55-68 

83-73 
482.26 

40.06 

245.66 
146.49 
338-74 
263  .  68 
102.82 
394.72 

HC1 

Na2C03 
HC1 
Na2C03 
Na2CO3 
HC1+KOH 
NaC03 
HC1 
HC1 
HC1  or  H2SO4 
+  KOH 
HC1  or  H2S04 
+  KOH 
HC1  or  H2SO4 
+  KOH 
Na2CO3 
HC1+KOH 
Na2C03 
HC1  after  ignition 
HC1+KOH 
HC1  after  ignition 

0.09797 
0.10338 
0.08506 
0.12977 
0.09926 
0.049675 
0.05508 
0.03678 
0.02784 

0.041865 
0.048266 

0.02003 

0.12283 
0.073245 
0.16937 
0.13184 
0.05141 
0.19736 

<  <        chlorid 

'  '        hydroxid  

'  '        nitrate  

Calcium  bromid  

'  '        carbonate 

<  *         chlorid 

'  '         hydroxid  ..... 

'  «        oxid         

Magnesium  carbonate  \ 
(true)                          1 

Magnesium  carbonate  \ 
(U  S.  P.)  / 

Magnesium  oxid 

'         carbonate  .  .  . 
iodid      

'        lactate  

'         oxid              .  . 

'         salicylate  

N 
The  haloid  salts  may  be  estimated  by  titration  with  —  AgNO3  V.  S. 


ACIDIMETRY 

The  Estimation  of  Acids  by  Neutralization. — In  the  previous 
pages  it  has  been  shown  how  alkalies  are  estimated  by  the  use  of  acid 
solutions  of  known  neutralizing  power.  In  the  estimation  of  acids, 
which  will  now  be  described,  the  order  is  reversed;  alkaline  solutions 
of  known  power  being  used  in  determining  the  strength  of  acids 
and  of  acid  salts.  Thus  the  procedure  is  analagous  to  that  of  the 
alkalimetric  methods.  The  choice  of  the  indicator,  whether  litmus, 
phenolphthalein  or  methyl  orange,  depends  upon  the  particular  acid 
to  be  estimated.  Phenolphthalein  is  employed  for  the  organic  acids 
and  boric  acid  and  is  preferred  for  phosphoric  acid;  while  methyl 
orange  and  litmus  are  usually  employed  in  the  titration  of  the  inor- 
ganic acids. 

The  standard  alkali  used  may  be  either  an  hydroxid  or  a  car- 
bonate, the  former  is,  however,  usually  preferred,  because  the  car- 


THE  ESTIMATION  OF  ACIDS   BY   NEUTRALIZATION      95 

bonate  when  brought  in  contact  with  an  acid  gives  off  carbonic  acid 
gas  (CO2)  which  interferes  to  a  great  extent  with  most  indicators. 
On  the  other  hand,  it  must  be  remembered  that  the  alkali  hy- 
droxids  are  very  prone  to  absorb  carbon  dioxid  from  the  atmos- 
phere, therefore  their  solutions  often  contain  some  carbonate,  the 
presence  of  which  even  in  small  quantities  will  occasion  errors 
when  used  with  most  indicators,  especially  with  litmus  and  phenol- 
phthalein.  It  is  therefore  advisable,  when  using  these  indicators  or 
others  which  are  affected  by  carbon  dioxid,  to  employ  gentle  heat 


FIG.  48.  Fig.  49. 

toward  the  close  of  each  titration,  in  order  to  drive  off  the  liberated 
gas.  Methyl  orange  is  not  affected  by  this  gas,  and  therefore  heating 
is  not  necessary  when  this  indicator  is  used.  In  fact,  it  is  imperative 
that  heat  should  not  be  employed  with  this  indicator. 

In  acidimetrical  operations  when  methyl  orange  is  used  as  indi- 
cator, residual  titrations  may  be  advantageously  done,  because  the 
change  of  color  from  yellow  to  red  which  is  brought  about  by  the 
acid  is  much  more  readily  seen  than  that  from  red  to  yellow. 

In  the  U.  S.  P.  standard  solutions  of  both  potassium  and  sodium 
hydroxid  are  official.  The  former,  however,  is  preferable,  because 
it  attacks  glass  more  slowly,  and  less  energetically,  and  also  foams 


96  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

much  less  than  does  the  sodium  hydroxid  solution.  The  neutralizing 
power  of  each  is,  however,  the  same.  Standard  solutions  of  alkali 
hydroxid  should  be  preserved  in  small  vials,  provided  with  well- 
fitting  rubber  stoppers,  or  better  still  these  should  be  provided  with 
tubes  filled  with  a  mixture  of  soda  and  lime,  which  absorbs  CO2  and 
prevents  its  access  to  the  solution.  A  vessel  of  this  description  is 
pictured  in  Fig.  48. 

An  improvement  upon  this  is  shown  in  Fig.  49,  since  it  allows 
of  the  burette  being  filled  without  removing  the  stopper,  and  conse- 
quently without  any  access  of  CO2  whatever. 

Where  a  series  of  titrations  of  the  same  kind  have  to  be  made  with 
the  same  alkali  standard  solution,  the  arrangement  shown  in  Fig.  13 
may  be  used,  both  the  reservoir  and  the  burette  in  this  case  being 
provided  with  soda  -lime  tubes. 

PREPARATION  OF  STANDARD  ALKALI.  SOLUTIONS 

Normal   Potassium   Hydroxid   (KOH  =  55.y4;     —  V.  S.  =  S5^4 


gms.  in  1000  cc.). 

Potassium  hydroxid  being  prone  to  absorb  carbon  dioxid  out  of 
the  air  the  pure  article  is  not  readily  obtained  in  commerce.  If 
pure  potassium  hydroxid  were  easily  obtained  it  would  only  be  neces- 
sary to  dissolve  55.74  gms.  in  sufficient  water  to  make  1000  cc.  But 
since  it  always  contains  some  CO2  and  H2O,  it  is  necessary  to  take  a 
slight  excess  and  dilute  the  solution  to  the  proper  volume  after  having 
determined  its  strength. 

The  standardization  may  be  effected  by  means  of  any  of  the  stand- 
ard acid  solutions. 

A  satisfactory  method  for  the  preparation  and  standardization  of 
this  solution  is  as  follows  : 

Dissolve  75  gms.  of  potassium  hydroxid  in  sufficient  water  to 
make  about  1050  cc.  at  15°  C.  (59°  F.),  and  fill  a  burette  with  a  portion 
of  this  solution. 

Dissolve  0.6255  gm.  of  pure  oxalic  acid  in  about  10  cc.  of  water 
in  a  beaker  or  flask,  add  a  few  drops  of  phenolphthalein  T.  S.,  and 
then  carefully  add  from  the  burette  the  potassium  hydroxid  solution, 
agitating  frequently  and  regulating  the  flow  to  drops  towards  the 
end  of  the  operation  until  a  permanent  pale  pink  color  is  obtained. 
Note  the  number  of  cc.  of  the  alkali  solution  consumed,  and  then  dilute 
the  remainder  so  that  exactly  10  cc.  of  the  diluted  liquid  will  be  re- 
quired to  neutralize  0.6255  gm.  of  oxalic  acid.  Instead  of  weighing 


STANDARDIZATION   BY  POTASSIUM  BITARTRATE         97 

off  0.6255  gm-  °f  the  acid,   10  cc.  of  its  normal  solution   may  be 
used.. 

Example.  Assuming  that  8  cc.  of  the  stronger  potassium  hydroxid 
solution  had  been  consumed  in  the  trial,  then  each  8  cc.  must  be  diluted 
to  10  cc.,  or  the  whole  of  the  remaining  solution  in  the  same  propor- 
tion. Thus  if  8  cc.  must  be  diluted  to  10  cc.,  1000  cc.  must  be  diluted 
to  1250  cc. 

8  :  10  : :  1000  :  x        #=1250  cc. 

It  is  always  advisable  to  make  another  trial  after  diluting.  10  cc. 
should  then  neutralize  0.6255  gm.  of  pure  oxalic  acid. 

Standardization  by  Means  of  Potassium  Bitartrate.  The 
U.  S.  P.,  8th  Dec.  Revis.,  recommends  this  method.  It  is  based  upon 
the  reaction 

KHC4H406+KOH=K2C4H406+H2O. 

N 
186.78  gms.     =  55. 74  gms.     =1000  cc.  —  V.  S.; 

N 
18.678  gms.  =  5.574  gms.  =  100  cc.  —  V.  S.; 

N 
1.8678  gms.=  0.5574  gm.  =  10  cc.  —  V.  S.; 

N 
3.72  gms.  =  •  1.1148  gms.=  20  cc.  —  V.  S. 

A  solution  of  potassium  hydroxid,  75  gms.  in  1050  cc.,  is  prepared 
and  titrated  against  pure  potassium  bitartrate,  using  phenolphthalain 
as  indicator. 

3.72  gms.  of  purified  dry  potassium  bitartrate  *  are  dissolved  in 

*  Purified  potassium  bitartrate  for  standardizing  caustic  alkali  volumetric 
solutions,  is  obtained  according  to  the  U.  S.  P.  as  follows:  "To  100  gms.  of  the 
salt  contained  in  a  beaker  is  added  a  mixture  of  85  cc.  of  water  and  25  cc.  of 
diluted  hydrochloric  acid;  the  covered  beaker  is  then  placed  upon  a  bath  of 
boiling  water  and  the  mixture  digested  with  occasional  stirring  for  three  hours. 
After  quickly  cooling,  the  solution  is  drained  off  from  the  precipitate,  which  is 
washed  by  effusion  and  decantation  with  two  successive  portions  of  100  cc.  each 
of  water;  after  collecting  the  precipitate  upon  a  plain  filter,  the  washing  with 
cold  water  is  continued  until  the  filtrate,  after  adding  a  few  drops  of  nitric  acid, 
ceases  to  become  opalescent  upon  the  addition  of  silver  nitrate  T.  S.  The  pre- 
cipitate of  potassium  bitartrate  is  then  dissolved  in  the  smallest  possible  volume 
of  boiling  water  (about  1500  cc.),  filtered,  and  the  filtrate  while  being  rapidly 
cooled  is  constantly  stirred.  When  the  mixture  is  cold,  the  crystalline  pre- 
cipitate is  collected  upon  a  plain  filter,  washed  with  300  cc.  of  cold  water,  and, 
after  thoroughly  draining,  dried  at  120°  C.  (248°  F.)  until  of  constant  weight. 
It  should  be  kept  in  dry,  securely  stoppered  bottles." 


98  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

60  cc.  of  boiling  distilled  water,  a  few  drops  (3)  of  phenolphlhnlein 
are  added,  and  the  solution  of  potassium  hydroxid  run  into  it  (the 
solution  being  frequently  boiled)  until  a  pale  pink  color  appears. 
Exactly  20  cc.  will  be  required  if  the  alkali  solution  is  normal.  If 
only  18  cc.  are  consumed,  then  each  18  cc.  must  be  diluted  to  20  cc. 
or  the  wThole  of  the  remaining  solution  in  the  same  proportion. 

Standardization  by  Means  of  Potassium  Bi-iodate.*  Potassium 
bi-iodate  is  an  acid  salt  and  may  be  directly  titrated  with  potassium 
hydroxid  using  phenolphthalein  as  indicator. 

One  molecule  of  the  bi-iodate  is  equivalent  to  one  molecule  of 
potassium  hydroxid  as  shown  by  the  equation 


386.94          55.74 

To  standardize  a  potassium-hydroxid  solution,  weigh  accurately 
3.8694  gms.  of  potassium  bi-iodate,  dissolve  it  in  about  25  cc.  of  water, 
add  a  few  drops  of  phenolphthalein,  and  then  run  into  this,  from  a 
burette,  the  hydroxid  solution  which  is  to  be  standardized,  until  a 
pale  pink  color  appears.  Note  the  number  of  cc.  used  and  dilute 
the  solution  so  that  exactly  10  cc.  of  it  will  neutralize  3.8694  gms. 
of  the  bi-iodate. 

Example.  Assuming  that  8.2  cc.  had  been  consumed,  then  each 
8.2  cc.  must  be  diluted  to  10  cc.,  or  the  whole  of  the  remaining  solu- 
tion in  the  same  proportion. 

The  advantages  of  this  salt  as  an  ultimate  standard  are  (i)  that 
it  may  be  procured  in  the  market  in  a  state  of  absolute  purity;  f  (2)  that 
it  is  permanent,  being  neither  deliquescent  nor  efflorescent;  (3)  that 
it  can  be  dried  at  110°  C.  without  decomposition;  (4)  that  the  results 
obtained  with  it  are  quite  accurate,  and  (5)  that  it  may  be  employed 
for  standardizing  most  of  the  volumetric  solutions  commonly  found 
in  the  laboratory. 


*  See  Meinecke,  Chem.  Ztg.,  XIX.,  2.     Also  Caspari,  Proc.  A.  Ph.  A.,  1904, 

389- 

f  According  to  Caspari,  the  salt  may  be  readily  prepared  as  follows:  See 
A.  Ph.  A.,  1904,  390.  Potassium  bicarbonate  is  mixed  in  solution  with  an 
equivalent  amount  of  iodic  acid,  and  to  the  neutral  solution  is  added  an  amount 
of  iodic  acid  equal  to  the  quantity  first  used.  The  solution  is  then  evaporated 
until  crystallization  begins,  and  the  first  crop  of  crystals  rejected.  Those  which 
separate  after  the  solution  has  cooled  to  50°  C.  are  almost  pure  and  will  be 
rendered  absolutely  pure  if  recrystallized. 


ESTIMATION   OF   THE   INORGANIC   ACIDS  99 

N 
Normal  Sodium  Hydroxid  (NaOH=39.76;  —  V.  S.  =  39-76  gms. 

in  1000  cc.). 

Dissolve  54  gms.  of  sodium  hydroxid  in  enough  water  to  make 
about  1050  cc.  of  solution,  fill  a  burette  with  a  portion  of  this,  and 
check  it  with  normal  acid,  or  a  weighed  quantity  of  oxalic  acid  or 
potassium  bitartrate,  in  exactly  the  same  manner  as  described  for 
normal  potassium  hydroxid. 

The  standard  alkali  volumetric  solutions  which  are  official  in  the 
U.  S.  P.  are: 

Normal  potassium  hydroxid; 
Tenth  normal  potassium  hydroxid; 
Fiftieth  normal  potassium  hydroxid; 
Hundredth  normal  potassium  hydroxid; 
Half  normal  potassium  hydroxid  (alcoholic) ; 
Normal  sodium  hydroxid; 
Double  normal  sodium  hydroxid. 

Other  standard  ajkali  solutions  in  frequent  use  are  normal  sodium 
carbonate,  normal  and  other  strengths  of  ammonia,  and  decinormal 
barium  hydroxid. 

Estimation  of  the  Inorganic  Acids.  To  weigh  off  directly  a 
definite  quantity  of  a  fluid  acid,  is  not  a  very  easy  matter.  It  is  always 
a  better  plan  to  measure  a  small  quantity  of  the  acid  and  weigh  it 
accurately  in  a  tared  and  stoppered  weighing  flask  (Fig.  50),  then 
to  add  water  and  titrate  with  the  standard  alkali  in  the 
presence  of  a  suitable  indicator.  If  the  specific  gravity  of 
the  acid  is  known  or  can  be  easily  taken,  it  is  sufficient 
to  measure  a  certain  quantity  of  it  by  means  of  a  pipette, 
and  then  determine  its  weight  by  multiplying  the  volume  in 
cubic  centimeters  by  the  specific  gravity.  It  must  be 
remembered,  however,  that  the  liquid  must  be  measured  FlG-  5°- 
at  the  same  temperature  at  which  the  specific  gravity  was  taken. 
This  method  is  applicable  to  the  diluted  acids  as  well  as  to  the  con- 
centrated acids  of  commerce,  as  hydrochloric,  nitric,  and  sulphuric 
acids. 

In  the  case  of  very  volatile  acids,  i.e.,  such  as  evolve  acid  vapors 
at  ordinary  temperatures,  the  determination  of  the  weight  by  means 
of  the  specific  gravity  is  inadmissable.  Such  acids  should  be  weighed 


100 


A    MANUAL   OF    VOLUMETRIC   ANALYSIS 


in  a  Lunge  pipette,  Fig.  51,  or  in  a  simple  bulb  pipette  provided  \\ith 
a  glass  stop-cock,  Fig.  52,  or  in  a  Grethan's  pipette,  Fig.  53. 

The  Lunge  pipette  is  used  by  producing  a  vacuum  in  the  bulb  (a), 
the  air-tight  glass  mantle  (c)  is  then  removed,  and  the  tip  of  the 
tube  (d)  sunk  into  the  acid  which  is  drawn  up  into  the  bulb,  upon 
opening  the  cock  (5);  when  sufficient  of  the  acid  has  been  drawn  into 
the  apparatus  the  cock  is  closed,  the  tip  of  the  pipette  wiped,  the 
glass  mantle  put  in  place,  and  the  whole  weighed.  The  weight  of 
the  empty  pipette  deducted  gives  the  weight  of  the  acid  taken  up. 
The  pipettes  shown  in  Figs.  52  and  53  are  filled  by  applying  direct 


FIG.  51. 


FIG.  52. 


FIG.  53. 


suction  with  the  lips,  the  operator  protecting  himself  against  inhala- 
tion of  harmful  vapors  by  attaching  an  absorption  tube  containing 
soda-lime,  caustic  soda,  or  similar  substance. 

The  quantity  of  acid  to  be  taken  (in  most  cases)  should  be  such 
as  will  require  for  neutralization  from  20  to  50  cc.  of  the  standard 
alkali.  In  the  case  of  concentrated  inorganic  acids,  2  or  3  gms.  may 
be  taken,  while  in  the  case  of  the  dilute  acids,  from  6  to  8  gms. 

Any  of  the  indicators  may  be  employed  for  the  inorganic  acids, 
but  because  of  the  usual  presence  of  carbonate  in  the  standard  alkali, 
methyl  orange  is  preferred. 

Hydrochloric  Acid  (H 01  =  36.18).  About  2  cc.  of  hydrochloric 
acid  (sp.gr.  1.048)  are  introduced  into  a  tared  weighing  flask  and 
its  weight  accurately  taken.  (The  weight  is  found  to  be  2.098  gms.). 


PHOSPHORIC   ACID  101 

20  cc.  of  water  are  now  added,  followed  by  tvo  d-'ops;  of.  insth^l 
orange,  and  the  solution  carefully  titrated  with  normal  potassium' 
hydroxid  until  the  reddish  color  of  the  solution  is  changed  to  yellow. 

Assuming  that  18.4  cc.  were  required,  then  18.4  cc.Xo.o36i8  gm. 
=  0.6657  gm-  °f  absolute  hydrochloric  acid  in  the  2.098  gms.  taken. 

To  find  the  per  cent  apply  the  proportion 

2.098  gms.  :  0.6657  §m-  : :  IO°  :  x          #=31.68  per  cent. 
The  equation  is: 

HC1     +     KOH     =     KC1     +     H20 

36.18  gms.      =55.74  gms.      =  1000  cc.  —  V.  S. 

N 
.03618  gm.  =      .05574  gm.=  i  cc.  —  V.  S. 

Sulphuric  Acid  (H2SO4= 97.35).  About  i  cc.  of  the  concentrated 
acid  is  weighed  in  a  tared  weighing  flask  and  found  to  weigh  1.8  gm. 
20  cc.  of  water  are  added  and  then  2  drops  of  methyl  orange,  and 
the  titration  with  normal  potassium  hydroxid  begun,  and  cautiously 
continued  until  the  yellowish  color  of  the  solution  indicates  the  com- 
pletion of  the  operation;  note  the  number  of  cc.  of  alkali  solution  used 
and  apply  the  equation 

H2SO4    +     2KOH   =    K2SO4    +    2H2O 

2)97-35  2)111.48  N 

48.675  gms.    =    55. 74  gms.    =1000  cc.  —  V.  S. 

N 
0.048675  gm.  =   0.05574  gm.=  i  cc.       V.  S. 

Thus  each  cc.  of  normal  KOH  V.  S.  represents  0.048675  gm.  of 
pure  H2SO4. 

Assuming  that  in  the  above  assay  34.1  cc.  were  required,  then 
34.1X0.048675  gm.  =  1.659  gm. 

Then 

1.659X100 

— sZ-g =92.1  per  cent. 

I.O 

Phosphoric  Acid  (H3PO4=97.29).  In  the  assay  of  phosphoric 
acid  by  direct  neutralization  with  standard  alkali,  the  acid  is  con- 
verted into  first  KH2PO4  then  K2HPO4,  and  finally  into  the  normal 


,1  -MANUAL  OF   VOLUMETRIC  ANALYSIS 

."We  'have  no.  indicator  which  reliably  shows  the  completion 
of  the  neutralization,  i.e.,  the  formation  of  the  tribasic  K3PO4.  Litmus 
cannot  be  used  as  indicator  here  for  the  dipotassic  or  disodichydric 
phosphate  (K2HPO4  or  Na2HPO4)  which  is  formed  is  slightly  alka- 
line towards  litmus;  the  same  is  true  of  most  other  indicators. 

It  is  recommended,  therefore,  in  order  to  estimate  phosphoric  acid 
alkalimetrically,  to  prevent  the  formation  of  soluble  phosphate  of  the 
alkali,  and  to  bring  the  acid  into  a  definite  compound  with  an  alkali 
earth,  as  follows: 

The  free  acid  in  a  diluted  state  is  placed  in  a  flask  and  a  known 
volume  of  normal  alkali  in  excess  added  in  order  to  convert  the  whole 
of  the  acid  in  a  basic  salt.  A  few  drops  of  rosolic  acid  are  now  added, 
and  sufficient  neutral  BaCl2  solution  poured  in  to  combine  with  the 
phosphoric  acid.  The  mixture  is  heated  to  boiling,  and  while  hot 

N 

the  excess  of  alkali  is  titrated  with  —  acid. 

i 

The  suspended  basic  phosphate,  together  with  the  liquid,  possesses 
a  rose-red  color  until  the  last  drop  or  two  of  acid,  after  continuous 
heating  and  agitation,  gives  a  permanent  white  or  slightly  yellowish 
milky  appearance,  when  the  process  is  ended. 

The  volume  of  normal  alkali,  less  the  volume  of  normal  acid, 
represents  the  amount  of  alkali  required  to  convert  the  phosphoric 
acid  into  a  normal  trisodic  or  tripotassic  phosphate. 

H3P04  +  3KOH  =  K3P04  +  3H20. 


32.43  gms.     55.74  gms.=  1000  cc.  of  —  KOH  V.  S. 


N 
Thus  i  cc.  of  —  alkali  =0.03  2  43  gm.  of 

Thompson,  however,  has  demonstrated  that  this  acid  may  be 
accurately  titrated  by  standard  alkali  when  using  either  methyl  orange 
or  phenolphthalein,  or  both,  successively. 

If  methyl  orange  is  used  the  color  changes  upon  the  completion 
of  the  formation  of  monobasic  phosphate,  KH2PO4,  as  per  the  follow- 
ing equation  : 

H3PO4+  KOH=  KH2P04+H2O. 

If  phenolphthalein  is  used,  the  color  changes  upon  the  completion 
of  the  formation  of  the  dibasic  phosphate  K2HPO4. 

H3PO4+  2KOH=  K2HPO4+  2H2O. 


NITRIC  ACID  103 

In  the  method  of  Thompson,  a  weighed  quantity  of  the  acid, 
diluted  with  a  little  water,  is  titrated  with  normal  potassium  hydroxid, 
using  methyl  orange  as  an  indicator  until  the  red  color  of  the  solu- 
tion changes  to  yellow,  indicating  the  formation  of  the  monobasic 
salt.  The  number  of  cc.  of  alkali  solution  required  is  multiplied  by 
the  factor  0.09729.  A  few  drops  of  phenolphthalein  are  now  added 
and  the  titration  continued  until  a  pale  red  color  appears,  indicating 
the  formation  of  the  dibasic  salt.  The  total  number  of  cc.  of  alkali 
added  is  multiplied  by  0.04865.  In  either  case,  the  quantity  of  abso- 
lute H3PO4  present  in  the  sample  is  obtained.  The  U.  S.  P.  recom- 
mends the  use  of  phenolphthalein  alone.  For  other  methods  for  the 
assay  of  phosphoric  acid,  see  Part  II. 

Hypophosphorous  Acid  (HPH2O2=65.53). 

HPH2O2 + KOH  =  KPH2O2 + H2O 

N 
65-53  gms-  =  55-74  gms.  =  1000  cc.  —  V.  S. 

N 
Thus  each  cc.  of  —  alkali  represents  0.06553  gm.  of  HPH2O2. 

Take  about  5  gms.  of  the  acid,  dilute  it  with  a  small  quantity  of 

N 
water,  add  a  few  drops  of  methyl  orange,  and  titrate  with  —  KOH 

until  the  solution  assumes  a  yellow  color. 
Nitric  Acid  (HNO3= 62.57). 

HNO3  +  KOH  =  KNO3  +  H2O 

N 
62.57gms.=  55.74  gms.=  1000  cc.  —  V.  S. 

N 
Each  cc.  of  —  KOH=o.o6257  gm.  of  nitric  acid.     Take  3  cc.  of 

the  acid  for  analysis,  weigh  it  accurately  in  a  stoppered  weighing 
bottle,  dilute  it  with  about  50  cc.  of  distilled  water,  and  then  titrate 

N 
with  —  KOH  V.  S.,  using  methyl  orange  as  indicator.     Multiply  the 

number  of  cc.  of  standard  alkali  used  by  0.06257  gm.  then  multiply 
this  product  by  100  and  divide  by  the  weight  of  the  acid  taken,  the 
quotient  will  represent  the  percentage  of  absolute  acid  present  in  the 
sample.  If  the  official  nitric  acid  is  being  analyzed  the  specific  gravity 
of  which  is  1.403,  the  weight  of  the  3  cc.  will  be  3X1.403  =  4.209  gms. 
Hydrobromic  and  hydriodic  acids  may  be  estimated  in  the  same 
way  as  the  foregoing,  but  it  is  usually  preferred  to  estimate  them  by 


104  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

precipitation  analysis,  as  described  on  pages  120  and  121  respectively. 
Sulphurous  acid  is  best  assayed  by  oxidation  with  iodin  as  described 
on  page  198. 

Boric  Acid  (H3B 03=61.54).     This   acid  is  estimated  by  neutra- 

N 

lization  with  —  NaOH  in  the  presence  of  a  large  quantity  of  glyc- 
erin. (Thompson's  Method,  J.  S.  C.  I.,  XII,  432).  The  addition 
of  sufficient  glycerin  to  a  boric  acid  solution,  so  that  no  less  than 
30  per  cent  be  present  throughout  the  titration,  develops  the  acidity 
of  boric  acid  with  regard  to  phenolphthalein  to  a  great  degree,  and 
enables  one  to  titrate  direct  with  standard  soda  solution,  i  gm.  of 
boric  acid  is  dissolved  in  50  cc.  of  water  to  this  is  added  an  equal 
volume  of  glycerin,  then  a  few  drops  of  phenolphthalein  T.  S.,  and 
the  titration  with  normal  sodium  hydroxid  begun  and  continued  until 
a  pink  color  appears. 

N 
Each  cc.  of  —  NaOH=o.o6i54  gm.  of  H3BO3. 

H3BO3+NaOH=NaH2BO3  +  H2O 

N 
61.54    39-76  gms.  in  1000  cc.  —  V.  S. 

N 
0.06154  gm.  i  cc.  —  V.  S, 

Organic  Acids.  As  the  individual  organic  acids  require  different 
indicators,  the  table  on  page  31  should  be  consulted  in  the  selection 
of  an  indicator  for  a  particular  organic  acid.  Phenolphthalein  is, 
however,  the  most  suitable  for  organic  acids  generally. 

Acetic  Acid  (HC2H3O2  =  59.58).  Mix  3  gms.  of  the  acid  with  a 
small  quantity  of  water,  add  a  few  drops  of  phenolphthalein  T.  S., 
and  titrate  with  normal  potassium  hydroxid  V.  S.  until  a  permanent 
pale-pink  color  is  obtained,  and  apply  the  following  equation : 

HC2H3O2+ KOH=  KC2H303 + H2O. 

59-58         55.74 

N 
Thus  55.74  gms.  or  1000  cc.  of  —  KOH  V.  S.  will  neutralize  59.58 

N 
gms.  of  acetic  acid;    therefore  each  cc.  of  —  KOH  V.  S.  represents 

0.05958  gm.  of  acetic  acid. 


OXALIC   ACID  105 

If  18  cc.  are  required  to  neutralize  3  gms.  of  the  acid,  it  contains 
18X0.05958=1.0724  gms.  of  absolute  acetic  acid. 

1.0724X100 

— =35.74  per  cent. 

o 

Tartaric  Acid  (H2C4H4O6=i48.92).     Dissolve  2  gms.  of  tartaric 
acid  in  sufficient  water  to  make  a  solution,  add  a  few  drops  of  phenol- 

N 
phthalein  and  then  pass  into  the  solution  from  a  burette  —  potassium 

hydroxid  V.  S.  until  a  faint  pink  tint  is  acquired  by  the  solution,  and 
apply  the  equation 

H2C4H406+  2KOH= K2C4H4O6+ 2H20 

2)148.92        2)111.48 

74.46  gms.      '55-74  gms.  =  1000  cc.  —  V.  S. 

Thus  each  cc.  required  for  the  neutralization  of  the  acid  represents 
0.07446  gm.     If  26.6  cc.  are  required  then  26.6X0.07446=1.98  gms. 
Then 

1.98X100 

—  =  99  per  cent. 

The  other  organic  acids  are  assayed  in  exactly  the  same  manner 
as  that  described  for  the  foregoing. 

Citric  Acid  (H3C6H5O7-H2O  =  208.50). 

H3C6H507.H20+3KOH=K3C6H507+4H20. 

3)208.50  3)^7^  2  N 

69.5  55-74  gms.=  iooo  cc.  —  V.  S. 

i 

N 
Each  cc.  of  —  KOH  represents  0.0695  gm-  °f  crystallized  citric 

acid. 

Oxalic  Acid  (H2C2O4.2H2O=  125.10). 

H2C204.2H20+2KOH=K2C2O4+4H2O. 

2)125.1  2)111.48 

62.55  55.74  gms.=  rooo  cc.  —  V.  S. 

N 
Each  cc.  of  —  KOH  represents  0.06255  gm.  of  crystallized  oxalic 

acid. 


106  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Lactic  Acid  (HC3H5O3  =  89.37). 

HC3H5O3  +  KOH  =  KC3H5O3  +  H2O. 


89-37 


55.74  gms.  =  iooo  cc.  —  V.  S. 


N 


Each  cc.  of  —  KOH  represents  0.08937  gm.  of  lactic  acid. 


TABLE  OF  ACIDS  WHICH  MAY  BE  ESTIMATED  BY  NEUTRALIZATION. 


Molec- 
ular 
Weight. 

Standard 
Solution. 

Indicator. 

Factor.  * 

Acetic  HC2H3O2               

5Q.58 

*KOH 

Phenolphthalein 

0.05958 

Boric  H2BO3       

61.54 

i 
-NaOH 

"   with  glycerin 

0.06154 

Citric  H3C6H5O7  H2O            ..    . 

208.50 

»KOH 

Phenolphthalein 

0.06950 

Hydriodic   HI                               .    . 

1  26  .  90 

I 

Methyl  orange 

o  .  i  2690 

Hydrobromic   HBr   ..       

80.36 

<  < 

0.08036 

Hydrochloric   HC1 

36.18 

(  < 

o  03618 

Hypophosphorous   HPH2O2.  .  .  . 

65.  ^3 

(  4 

o  06^53 

Lactic    HC3H5O3 

80.^7 

Phenolphthalein 

0.08037 

Nitric  HNO3 

62.57 

Methyl  orange 

0.06257 

Oxalic   H2C2O4-  2H2O  

125.10 

Phenolphthalein 

0.06255 

Phosphoric   H3PO4 

97.29 

To  formation  of  KH2PO4  

Methyl  orange 

o  00720 

**        "         "   K2HPO 

Phenolphthalein 

O   Od.864.  s 

"         "         "  K,P(X 

Rosolic    acid 

O   O3243 

Asammonio-magnesium  phos-  \ 

*HC1 

M^ethyl  orange 

phate  .                                 / 

Sulphuric,  H2SO4  

07    3^ 

*KOH 

<  < 

o  04867^ 

Tartaric,  HC4H4O6  

14.8   02 

I 

t  < 

Phenolphthalein 

74 
o  o    46 

Trichloracetic,  HC,C13O2  

162    12 

N 
-  NaOH 

<  « 

i 

Certain  of  these  acids  are  best  assayed  by  other  methods  than  neutralization. 
For  example,  hydriodic,  hydrobromic,  and  hydrocyanic  are  usually  assayed  by 

N  N 

precipitation  with  —  silver  nitrate;  sulphurous  acid  is  assayed  by  —  iodin. 


*  This  is  the  coefficient  by  which  the  number  of  cc.  of  standard  solution  is  to  be  mul- 
tiplied in  order  to  obtain  the  quantity  of  pure  acid  in  the  sample  analyzed. 


DIRECT  PERCENTAGE  ESTIMATION   TABLE 


107 


TABLE  SHOWING  QUANTITY  OF    SUBSTANCE  TO  BE    TAKEN  FOR  ANALYSIS  IN 
DIRECT  PERCENTAGE  ESTIMATIONS 


Substance. 

Molecular 
Weight. 

Quantity  of 
Substance  to 
be  taken,  so 
that  each  cc. 
of  the  V.  S. 
will  Represent 
i%. 
Grams. 

Percentage 
Strength  of 
the  Official 
Substances. 

Per  Cent. 

Acetic  acid.  ..  . 

rn    eg 

e    org 

76 

"     glacial  

CO.  eg 

e    ocg 

ou 

Ammonium  carbonate.  . 

1  56  01 

52 

yy 

Ammonia  water  

16  93  NH, 

I     60  3 

y/ 

"      stronger.  .  .  . 

16  03  NH, 

08 

Boric  acid  

61    <4 

6     I  ?4 

Citric  acid  (  +  H2O). 

208  50 

yy-  £ 

Hydrochloric  acid  

16  18 

-j  6ig 

99-5 

Hypophosphorous  acid 

6c   c? 

3J-9 

Lactic  acid  

80    37 

^-  jj6 
8    O77 

3° 

Lithium  carbonate 

7-7       CT 

75 

no    - 

"        citrate.  .  . 

208      36 

-  (JJ 

6    OC2 

9°  -5 

ftQ     c 

Lime  water  (Ca(OH)2)  

7-2  t6 

**•?;)" 

o  6?g 

9°  -5 

Nitric  acid 

62    <7 

68 

Oxalic  acid.  . 

1  25  10 

**•  ^J/ 

6    2^C. 

Phosphoric  acid  

07.  2O 

'    with  methyl  orange  
"       "    phenolphthalein.  .. 
Potassium  acetate 

97.29 
97.29 
O7   4.4. 

9.729 
4.8645 

85 
8s 

fto 

bicarbonate  

IQQ     82 

9O4.I 

90 

'  '          bitartrate 

1  86  78 

18  67g 

yy 

carbonate       

137    27 

6  g635 

99 
Oa 

'  '          citrate  

322.08 

IO    7^6 

90 

'  '          hydroxid  

cc.74 

c    1:74 

yy 
8< 

'  '         liquor  

trr   74 

f      C>JA 

°j 

sodium  tartrate 

280  08 

Sodium  acetate  

13"?    IO 

13    ^1 

99 

'  '        benzoate 

14."?    OI 

99-5 

'  '        bicarbonate.  . 

1  66  86 

8    34.'? 

99 

'  '       carbonate  (+  ioH2O)  

284.11 

14    2 

yy 

Sulphuric  acid  

07    3"? 

4    867  £ 

Tartaric  acid 

14.8    O2 

92-5 

99-5 

108  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

Referring  to  the  table  it  will  be  seen  that  if  the  quantities  indicated 
are  taken  for  analysis,  the  amount  of  standard  solution  required  for 
substances  of  high  percentage  strength  will  be  very  large  (in  some 
cases  over  99  cc.),  while  for  substances  of  low  percentage  strength, 
as  for  instance  lime  water,  so  small  a  volume  of  standard  solution  is 
required  as  to  be  unreadable  (0.14  cc.).  It  is  therefore  advisable 
to  take  for  analysis  a  smaller  quantity  of  high  percentage  substances 
and  a  larger  quantity  of  such  substances  as  contain  a  low  percentage. 
It  is  usually  best  to  so  adjust  it,  that  no  less  than  10  nor  more  than 
30  cc.  of  the  standard  solution  be  required.  For  example :  In  the  case 
of  citric  acid,  instead  of  taking  for  analysis  6.95  gms.  it  will  be  better 
to  take  one-fourth  of  this  quantity,  then  each  cc.  of  the  standard  solu- 
tion used  will  represent  4  per  cent,  and  only  one  fourth  as  much  will 
be  required,  i.e.,  24.9  cc.  instead  of  99.5  cc.  Again  in  the  case  of 
lime  water,  if  36.78  gms.  are  taken  instead  of  3.768  gms.,  1.4  cc.  of 

N 
the  standard  solution  —will  be  required,  which  is  better  than  0.14  cc., 

(N\ 
—  )  solution, 
10 1  ^ 

then  36.78  gms.  of  lime  water  would  require  for  neutralization  just 

N 
14  cc.  of  the  —  acid  V.  S.     If  half  the  quantity  indicated  in  the  table 

is  taken,  then  each  cc.  of  the  standard  solution  will  represent  2  per 
cent.  If  one-tenth  the  quantity  is  taken  each  cc.  will  represent  10  per 
cent.  If  double  the  quantity  is  taken  each  cc.  will  represent  0.5  per 
cent,  etc. 


ESTIMATION  OF  ACIDS  IN  COMBINATION  IN  NEUTRAL   SALTS 

This  may  be  done  in  the  case  of  a  large  number  of  salts,  by  adding 
to  the  solution  of  the  salt  a  measured  excess  of  alkali  or  alkali  carbonate 
in  the  form  of  normal  solution,  and  then  ascertaining  the  excess  by 
retitration  with  normal  acid.  Thus  the  amount  of  alkali  which  went 
into  combination  with  the  acid  is  obtained.  Most  bases  are  pre- 
cipitated by  the  hydroxid;  some,  however,  require  the  addition  of 
carbonate  to  effect  their  precipitation. 

The  carbonate  is  required  for  alkali  earth  salts,  magnesium 
salts,  alum,  zinc  salts,  bismuth  salts,  nickel,  cobalt,  and  lead  salts. 

Example.  2  gms.  of  barium  chlorid  are  dissolved  in  water  and 
sufficient  normal  sodium  carbonate  added  to  make  the  liquid  decidedly 


ACIDS   IN   COMBINATION  IN   NEUTRAL  SALTS          109 

alkaline  (say  20  cc.),  and  the  whole  diluted  to  300  cc.  and  set  aside 
to  settle.  100  cc.  of  the  clear  supernatant  liquid  are  then  removed 
with  a  pipette  and  titrated  for  excess  of  alkali  with  normal  nitric  acid 
or  normal  hydrochloric  acid,  of  which  say  1.2  cc.  are  required,  making 
it  3.6  cc.  for  the  whole  quantity;  therefore  20—3.6=16.4  cc.  is  the 
measure  of  the  alkali  which  combined  with  the  acid  of  the  original 
salt.  This  multiplied  by  0.03518,  the  factor  for  chlorin,  gives  0.57695 
gm.  of  chlorin. 


CHAPTER  XI 

ANALYSIS  BY  PRECIPITATION 

THE  general  principle  of  this  method  is  that  the  determination 
of  the  quantity  of  a  given  substance  is  effected  by  the  formation  of  a 
precipitate  upon  the  addition  of  the  standard  solution  to  the  substance 
under  examination.  There  are  three  ways  of  determining  the  end 
reaction  in  precipitation  analyses. 

1.  By  adding  the  standard  solution  until  it  ceases   to  produce  any 
more  precipitate,  as  in  the  estimation  of  silver  by  standard  sodium 
chlorid,  and  the  estimation   of   haloid  salts   and  acids  by  means  of 
standard  silver   nitrate.     The   application   of   this   ending   is   almost 
limited  to  the  above  estimations,  because  in  these  only  can  accurate 
results  be  obtained.      The  silver  halids    formed  are  not  only  quite 
insoluble,  but  they,  have  a  tendency  to  curdle  closely  upon  shaking 
(especially  in  acid  solutions),  and  thus  leave  a  clear  supernatant  liquid 
in  which  any  further  precipitation  can  be  readily  seen.     Most  of  the 
other  precipitates,   such  as   barium   sulphate,   calcium   oxalate,   etc., 
although  heavy  and  insoluble,  are  so  finely  divided  and  powdery  thai 
they  do  not  readily  subside. 

2.  By  the  use  of  an  indicator,  as  in  the  estimation  of  haloid  salts 
by  means  of  standard  silver  nitrate  solution,  using  neutral  potassium 
chromate  as  the  indicator.     The  latter  is  added  to  the  haloid  solu- 
tion, and  the  silver  nitrate  V.  S.  delivered  into  the  mixture  until  a 
permanent   red  color    (silver  chromate)   is   produced.     Silver  nitrate 
reacts  by  preference  with  the  halogen,  and  does  not  react  with  the 
chromate  until  the  halogen   has   been   entirely  precipitated.     Hence 
the  production  of  a  permanent  red  color  marks  the  completion  of  the 
precipitation  of  the  halogen. 

Another  illustration  is  in  the  estimation  of  silver  by  sulphocyanate 
solution,  using  ferric  alum  as  indicator.  The  sulphocyanate  produces 
with  the  silver  a  white  precipitate  of  silver  sulphocyanate,  but  when 
the  precipitation  of  silver  is  complete  the  sulphocyanate  reacts  with 
the  ferric  alum  present  and  a  red  ferric  sulphocyanate  appears  and 
marks  the  end  point.  On  the  other  hand  the  indicator  may  be  used 


PRECISION  IN  DETERMINING  END-REACTIONS         111 

externally,  i.e.,  alongside  of  the  liquid  being  analyzed.  A  drop  of 
the  latter  being  brought  in  contact  with  a  drop  of  the  indicator  at 
frequent  intervals  in  the  course  of  the  titration.  As  in  the  estima- 
tion of  phosphoric  acid  by  means  of  uranium  nitrate  solution,  in  which 
potassium  ferrocyanid  is  used  as  indicator. 

3.  By  adding  the  standard  solution  until  the  first  appearance  of  a 
precipitate,  as  in  the  estimation  of  cyanogen  by  silver  nitrate  solution, 
and  the  estimation  of  chlorin  by  mercuric  nitrate  V.  S.  In  these 
estimations  the  standard  solution  is  added  to  the  solution  of  the  sub- 
stance under  analysis  until  a  precipitate  appears. 

PRECISION  IN   DETERMINING   END-REACTIONS 

In  most  volumetric  precipitation  processes  no  direct  reading  of 
the  end-point  is  possible;  nitration  and  trial  with  small  quantities  of 
the  clear  filtrate  being  usually  necessary.  P.  N.  Rakow  (Chem.  Zeit.) 
has  found  that  many  precipitates  which  remain  obstinately  suspended 
under  ordinary  conditions,  and  cause,  in  the  liquid  being  titrated, 
an  unmanageable  turbidUy,  can  be  induced  to  collect  and  subside 
by  the  addition  of  some  immiscible  liquid  heavier  than  water;  for 
example,  carbon  disulphid  or  chloroform.  Such  liquids,  although 
exerting  no  solvent  action  on  the  precipitate,  mix  intimately  with  it 
and  carry  it  down,  leaving  the  supernatant  liquid  sufficiently  clear 
for  the  observation  of  any  turbidity  produced  by  the  addition  of  a 
further  quantity  of  the  precipitating  solution.  Carbon  disulphid  and 
chloroform  are  usually  but  not  invariably  effective.  Thus  the  former 
carries  down  silver  chlorid  rapidly  and  completely,  but  has  no  influ- 
ence on  the  precipitation  of  barium  sulphate.  The  author  has  found 
the  method  to  work  fairly  well  in  the  few  cases  he  tried. 

In  the  titration  of  chlorids  by  means  of  silver  nitrate  with  neutral 
chromate  as  indicator,  the  end-reaction  is  more  distinctly  seen  by 
gaslight  than  by  daylight;  and  if  very  dilute  solutions  of  chlorid  are 
estimated  the  titration  is  best  performed  by  gaslight,  and  even  then 
the  change  of  color  from  yellow  to  red  is  not  easily  perceived. 

In  order  to  overcome  this  difficulty  Dupre  suggests  the  following 
simple  method: 

The  chlorid  solution  is  placed  in  a  white  porcelain  dish,  a  small 
quantity  of  neutral  chromate  added  (sufficient  to  make  the  liquid 
yellow).  Then  the  titration  is  begun  and  watched  by  looking  through 
a  flat  glass  cell  containing  some  of  the  neutral  chromate. 


112  A   MANUAL   OF   VOLUMERTIC  ANALYSIS 

If  the  solution  in  the  cell  corresponds  fairly  with  the  tint  of  the 
liquid  in  the  porcelain  dish,  the  latter  will  appear  to  be  perfectly  color- 
less, like  pure  water,  and  the  first  faint  appearance  of  red  becomes 
strikingly  manifest,  and  no  mistake  can  be  made. 

The  same  plan  may  be  followed  in  other  titrations,  where  the  end- 
reaction  depends  upon  the  perception  of  color  changes. 

Preparation  of  Decinormal  ( —  j  Silver  Nitrate  (AgNO3=  168.69; 

N 
-  V.  S.  =  16.869  gms-  in  I00°  cc)-     Dissolve    16.869    gms.  of  pure 

silver  nitrate  *  in  sufficient  water  to  make,  at  or  near  15°  C.  (59°  F.), 
exactly  1000  cc.  i  liter  of  this  solution  thus  contains  -fa  of  the  molec- 
ular weight  in  grams  of  silver  nitrate.  It  is  therefore  a  decinormal 
solution. 

If  pure  crystals  of  silver  nitrate  are  not  readily  obtainable,  and 
pure  sodium  chlorid  is  at  hand,  a  solution  of  the  silver  nitrate  may 
be  made  of  approximate  strength,  a  little  stronger  than  necessary, 
and  then  standardized  by  means  of  the  sodium  chlorid,  as  follows: 
o.i  1612  gm.  of  sodium  chlorid  is  dissolved  in  water,  and  a  burette 
is  filled  with  the  solution  of  silver  nitrate  to  be  standardized.  The 
silver  solution  is  now  slowly  added  from  the  burette  to  the  sodium- 
chlorid  solution  contained  in  a  beaker  until  no  more  precipitate  of 
silver  chlorid  is  produced. 

'If  neutral  potassium  -chromate  is  used  as  an  indicator,  the  end 
of  the  reaction  is  shown  by  the  appearance  of  yellowish-red  silver 
chromate.  This  indication  is  extremely  delicate.  The  silver  nitrate 
does  not  act  upon  the  chromate  until  all  of  the  chlorid  is  converted 
into  silver  chlorid. 

In  the  above  reaction  20  cc.  of  silver  nitrate  should  be  required. 
But  since  the  silver-nitrate  solution  is  too  strong,  less  of  it  will  com- 
plete the  reaction,  and  the  solution  must  be  diluted  so  that  exactly 
20  cc.  will  be  required  to  precipitate  the  chlorin  in  0.11612  gm.  of 
NaCl. 

Thus  if  17  cc.  are  used,  each  17  cc.  must  be  diluted  to  20  cc.,  or 
each  170  cc.  to  200  cc.,  or  the  entire  remaining  solution  in  the  same 
proportion. 

After  dilution  a  fresh  trial  should  always  be  made. 

*  The  U.  S.  P.  directs  that  the  silver  nitrate  should  previous  to  weighing, 
be  pulverized  and  dried  in  a  covered  porcelain  crucible  in  an  air-bath  at  130°  C. 
(266°  F.)  for  one  hour. 


DECINORMAL   N/io   SODIUM   CHLORID  113 

Silver  nitrate  solution  should  be  kept  in  dark  amber-colored,  glass- 
stoppered  bottles,  carefully  protected  from  dust. 

Titration  by  decinormal  silver  nitrate  V.  S.  may  be  managed  in 
various  ways,  adapted  to  the  special  preparation  to  be  tested. 

1.  In  most  cases  it  is  directed  by  the  U.  S.  P.  to  be  used  in  the 
presence  of  a  small  quantity  of  potassium  chromate  T.  S. 

2.  In  some  cases  it  is  added  until  the  first  appearance  of  a  per- 
manent precipitate,  as  in  potassium  cyanid  and  hydrocyanic  acid  assays. 

3.  It  may  be  used  in  all  cases  without  an  indicator  by  observing 
the  exact  point  when  no  further  precipitate  occurs.     But  since  this 
consumes  too  much  time  in  waiting  for  the  precipitate  to  subside, 
so  as  to  render  the  supernatant  liquid  sufficiently  clear  to  recognize 
whether  a  further  precipitate  is  produced  by  the  addition  of  the  silver 
solution,  it  is  impracticable. 

4.  It  may  be  added  in  definite  amount,  known  to  be  in  excess  of 
the  quantity  required,   and   the  excess   measured  back  by  titration 
with  decinormal  potassium  sulphocyanate  V.  S.,  or  even  with  decinormal 
sodium  chlorid  V.  S.  (residual  titration). 

Decinormal  —  Sodium  Chlorid   (NaCl=58.o6;  —  V.  S.  =  5.8o6* 

gms.  in  1000  cc).  Dissolve  5.806  gms.  of  pure  sodium  chlorid  in 
enough  water  to  make  exactly  1000  cc.  at  the  ordinary  temperature 
of  the  atmosphere. 

Check  this  solution  with  decinormal  silver  nitrate.  The  two  solu- 
tions should  correspond,  volume  for  volume. 

Pure  Sodium  Chlorid  may  be  prepared  by  passing  into  a  saturated 
aqueous  solution  of  the  purest  commercial  sodium  chlorid  a  cur- 
rent of  dry  hydrochloric  acid  gas.  The  crystalline  precipitate  is  then 
separated  and  dried  at  a  temperature  sufficiently  high  to  expel  all 
traces  of  free  acid. 

N 
The  method  of  standardizing  —  NaCl  solution  is  as  follows : 

0.33738  gm.  of  silver  nitrate  is  dissolved  in  10  cc.  of  distilled  water, 
and  the  solution  carefully  titrated  with  —  NaCl  V.  S.  until  precipita- 
tion ceases.  20  cc.  of  the  standard  solution  should  be  required. 

AgN03  +  NaCl  =  AgCl  +  NaNO3. 
10)168.69  10)58.06 

16.869  gms.  5.806 gms.,  or  1000  cc.  —  NaCl  V.  S. 

10 


114  A   MANUAL  OF    VOLUMETRIC  ANALYSIS 

Each  cc.  of  the  standard  solution  represents  0.016869  gm.  of  pure 

AgN03. 

0.016869X20=0.33738  gm. 

0.^738X100 

^'^         —  =  100  per  cent. 
0.33738 

TVT 

Decinormal   :  -   Potassium  Sulphocyanate  (Volhard's  Solution) 

N 
(KSCN  =  96.53   :  -  V.  S.  =  9.653  gms.  in  1000  cc.).     Dissolve  10  gins. 

of  pure  crystallized  potassium  sulphocyanate  (thiocyanate)  in  1000  cc. 
of  water. 

This  solution,  which  is  too  concentrated,  must  be  adjusted  so  as 
to  correspond  exactly  in  strength  with  decinormal  silver  nitrate  V.  S. 

N 
For  this  purpose  introduce  into  a  flask  10  cc.  of  —  AgNO3  V.  S.,  5  cc. 

of  ammonioferric  sulphate  T.  S.,  and  5  cc.  of  diluted  nitric  acid. 

Run  into  this  mixture  from  a  burette  the  sulphocyanate  solution. 

At  first  a  white  precipitate  of  silver  sulphocyanate  is  produced, 
giving  the  fluid  a  milky  appearance,  and  then  as  each  drop  of  sulpho- 
cyanate falls  in  it  is  surrounded  by  a  deep  brownish-red  cloud  of 
ferric  sulphocyanate,  which  quickly  disappears  on  shaking,  as  long 
as  any  of  the  silver  nitrate  remains  unchanged. 

When  the  point  of  saturation  is  reached  and  the  silver  has  all 
been  precipitated,  a  single  drop  of  the  sulphocyanate  solution  produces 
a  faint  brownish-red  color,  which  does  not  disappear  on  shaking. 

Note  the  number  of  cc.  of  the  sulphocyanate  solution  used,  and 
dilute  the  whole  of  the  remaining  solution  so  that  equal  volumes  of 
this  and  of  the  decinormal  silver  nitrate  will  be  required  to  pro- 
duce the  permanent  brownish-red  tint.  (The  same  tint  of  brown  or 
red  to  which  the  volumetric  solution  is  adjusted  must  be  attained 
when  the  solution  is  used  in  volumetric  testing.) 

Assuming  that  9.5  cc.  of  the  sulphocyanate  solution  were  required 
to  produce  the  reaction,  then  each  9.5  cc.  must  be  diluted  to  make 
10  cc.,  or  the  whole  of  the  remaining  solution  in  the  same  proportion. 

Always  make  a  new  trial  after  the  dilution  to  see  if  the  solutions 

N 
correspond,  e.g.,   50  cc.  of  -  -  silver  nitrate  are  taken,  and  5  cc.  of 

ammonioferric  sulphate,  5  cc.  of  pure  nitric  acid  and  200  cc.  of 
water  are  added  and  there  should  be  required  exactly  50  cc.  of  the 
potassium  sulphocyanate  solution.  The  same  depth  of  reddish-brown 


MOHRS   METHOD   WITH  CHROMATR   INDICATOR        115 

tint  should  be  obtained  in  all  assays  by  this  method,  as  is  obtained 
in  standardizing  the  solution. 

ESTIMATION  OF   SOLUBLE    HALOID  SALTS 

The  estimation  of  these  salts  is  based  upon  the  powerful  affinity 
existing  between  the  halogens  and  silver,  and  the  ready  precipitation 
of  the  resulting  chlorid,  bromid,  and  iodid.  Standard  solution  of 
silver  nitrate  is  used  for  this  purpose,  and  for  the  sake  of  exactness 
and  convenience  is  made  of  decinormal  strength.  In  some  cases  it 
is  advisable  to  use  centinormal  solutions. 

Mohr's  Method  with  Chromate  Indicator.  This  method  is  the 
best  to  use,  if  the  haloid  salts  are  in  neutral  solution,  and  salts  of  lead, 
bismuth,  barium,  or  iron  are  absent.  If  the  solution  is  acid  the  indi- 
cator is  inadmissable,  in  that  acids  have  a  solvent  action  upon  silver 
chromate  and  thus  prevent  the  end-reaction  from  being  clearly  and 
accurately  observed.  If  the  above-mentioned  metals  are  present,  the 
indicator  is  likewise  useless  as  these  bases  form  insoluble,  highly 
colored  compounds  with  the  chromate.  The  neutral  potassium 
chromate  (yellow  chromate)  which  is  used  as  the  indicator  must  be 
free  from  chlorid  *  and  should  be  used  in  the  form  of  a  10  per  cent 
solution. 

In  the  volumetric  analysis  of  soluble  haloid  salts  (chlorids,  bro- 
mids,  and  iodids)  0.5  gm.  of  the  well-dried  salt  is  dissolved  in  40  cc. 
of  water  in  a  beaker.  This  is  placed  upon  a  white  surface  and  a 
few  drops  of  the  chromate  indicator  (or  sufficient  to  give  the  solution 

N 
a  pale  yellow  tint),  added.     The  decinormal  —  silver  nitrate  solution 

is  then  added  cautiously  from  a  burette,  stirring  constantly  until  a 
permanent  red  tint  is  produced.     The  red  tint  is  due  to  the  forma- 
tion of  silver  chromate,  which  does  not  appear  permanent  until  the 
last  trace  of  halogen  has  been  precipitated. 
The  reactions  are  as  follows: 

NaCl  +  AgNO3  =  AgCl  +  NaNO3 
and 


*  The  presence  of  chlorid  in  the  chromate  solution  may  be  determined  by 
adding  a  small  quantity  of  silver  nitrate  solution,  and  then  some  nitric  acid.  If 
the  red  precipitate  dissolves  completely  and  leaves  a  clear  solution,  chlorid  is 
absent.  If  it  is  found  to  be  present  it  may  be  removed  by  the  addition  of  a  few 
drops  of  silver  nitrate  solution,  and  filtering,  without  using  any  nitric  acid. 


116  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

If  the  solution  to  be  estimated  is  acid  it  should  be  accurately  neu- 
tralized with  ammonia,  or  sodium  or  calcium  carbonate.  If  it  is 
alkaline  in  reaction  it  should  likewise  be  neutralized,  using  acetic  acid 
for  this  purpose. 

In  the  estimation  of  bromids  and  iodids  it  must  not  be  forgotten 
to  take  into  account  the  invariable  presence  of  chlorids  as  an 
impurity. 

The  method  in  detail  is  exemplified  in  the  following  assays: 

Estimation  of  Sodium  Chlorid.  i  gm.  of  the  well-dried  sodium 
chlorid  is  dissolved  in  sufficient  distilled  water  to  measure  100  cc. 
Of  this  solution  10  cc.  (representing  o.i  gm.  of  the  salt)  is  taken,  a 
few  drops  of  neutral  potassium  chromate  solution  added,  and  then 

N 

the    —  silver  solution  delivered  from  a  burette  with  constant  stirring 
10 

or  shaking  until  the  chlorid  is  entirely  precipitated  as  evidenced  by 
the  formation  of  a  permanent  red  color  (silver  chromate).  The  equa- 
tion is 

NaCl  +  AgNO3  =  AgCl  +  NaNO3. 
10)58.06    10)168.69 

5.806  gms.        16.869  gms.  =  1000  cc.  —  V.  S. 

10 

N 
Thus  each  cc.  of  the  —  V.  S.  represents  0.005806  gm.  of  NaCl. 

If  in  the  above  assay  17  cc.  of  the  silver  solution  were  required, 
then  17X0.005803  gm.  =  0.098651  gm.  or  .98. 651  per  cent. 


0.098651X100 

—  =  98.651  per  cent. 


o.i 

Estimation  of  Ammonium  Bromid.  3  gms.  of  the  salt  are  dried 
at  ioo°  C.  (212°  F.)  (to  remove  moisture,  which  the  salt  readily  absorbs 
out  of  the  air),  and  dissolved  in  sufficient  water  to  measure  100  cc. 
10  cc.  of  this  solution  (representing  0.3  gm.  of  the  salt)  are  placed  in  a 
beaker,  a  few  drops  of  potassium  chromate  T.  S.  added,  and  then  the 

N 

-  silver  nitrate  V.  S.  carefully  delivered  from  a  burette,  until  a  per- 
10 

manent  red  coloration  is  produced.     Apply  the  equation 

NH4Br  +  AgNO3  =  AgBr  +  NH4NO3 
10)97.29      10)168.69 

9.7^9  gms.    16.869  gms.  or  1000  cc.  — AgNO3  V.  S. 

10 


ESTIMATION   OF  POTASSIUM   IODID  117 

N 
Thus  each  cc.  of  the   —  V.  S.  represents  0.009729  gm.  of  NH4Br. 

N 
The  U.  S.  P.  requirement  is  that  not  more  than  31.600.  of—  AgNOs 

be  required  for  0.3  gm.  of  ammonium  bromid.  If  the  salt  is  abso- 
lutely pure  only  30.84  cc.  would  be  required  for  0.3  gm.  The  excess 
s  due  to  the  presence  in  the  commercial  salt  of  a  certain  amount  of 
ammonium  chlorid  which  is  precipitated  by  the  silver  nitrate  as  well 
as  the  bromid,  and  which,  having  a  lower  molecular  weight,  requires 
proportionately  more  silver  nitrate  to  precipitate  it  than  the  bromid 
does.  The  presence  of  chlorids  must  always  be  taken  into  account 
in  the  valuation  of  bromids,  because  the  latter  usually  contain  more 
or  less  of  the  former  as  an  impurity. 

The  Determination  of  the  Amount  of  Chlorid  Present  is  Calculated 
as  Follows:    The  amount  of  the  salt  examined  equivalent  to  1000  cc. 

N 

of  -—  silver  nitrate  solution  is  first  found  thus: 
10 

31.6  :  0.3  : :  1000  cc.  :  x  #=9.493  gms. 

This  is  then  deducted  from  the  quantity  of  pure  ammonium  bromid 

N 
(9.729  gm.)  which  is  equivalent  to  1000  cc.  of  —  silver  nitrate  solution. 

9.729-Q.493  =  ;y.  ^=0.236  gm. 

N 
y  represents  the  excess  of  -  -   silver  nitrate  solution    used  up  by 

the  ammonium  chlorid,  reckoned  in  terms  of  ammonium  bromid, 
and  since  5.311  gms.  of  NH4C1  is  equivalent  to  9.729  gms.  of  NH4Br, 
the  excess  which  NH4C1  can  consume  is  represented  by 

9.729- 5.311  =  4.418  gms. 
therefore, 

4.418  :  5.311  : :  0.236  :  z.  2=0.283  gm- 

z  represents  the  amount  of  NH4C1  present  in  9.493  gms.  of  the 
sample.     Lastly,  calculate  the  percentage 

9.493  :  0.283  : :  100  :  p.         ^=2.98  per  cent  of  NH4C1. 

Thus  the  salt  examined  contained  97.02  per  cent  of  NH4Br. 
Estimation  of  Potassium  lodid.     This  is  conducted  in  exactly 
the  same  manner  as  the  preceding  salts.     The  presence   of  chlorid 


118  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

(KC1)  as  an  impurity  must  likewise  be  taken  into  account,  and  the 
calculations  made  to  determine  its  quantity,  in  the  same  manner  as 
described  under  estimation  of  ammonium  bromid. 

0.5  gm.  of  the  well-dried  salt  is  dissolved  in  10  cc.  of  water,  2  drops 

N 
of  neutral  potassium  chromate  T.  S.  are  added,  and  then  the  —  AgNO3 

V.  S.  slowly  added  from  a  burette  until  a  permanent  red  color  of  silver 
chromate  is  produced.  Not  more  than  30.5  cc.  nor  less  than  30  cc. 
of  decinormal  silver  nitrate  V.  S.  should  be  required.  This  quantity 
corresponds  to  100  per  cent  of  the  pure  salt. 

KI     +     AgN03     =     Agl     +     KN03. 

10)164.76       10)168.69 

16.476  gms.      16.869  gms.  or  1000  cc.  —  AgNO3  V.  S. 

10 

N 
Each  cc.  of   —  AgNOs  V.  S.  thus  corresponds  to  0.016476  gm.  of 

potassium  iodid. 
Thus, 

0.016476X30.5  =  0.5025  gm. 

To  determine  the  quantity  of  chlorid  present  as  an  impurity  we 
calculate  as  follows: 

The  amount  of  the  salt  under  examination  equivalent  to  1000  cc. 
is  first  found. 

30.5  cc.  :  0.5  gm.  : :  1000  cc.=#.          #=16.393  §ms- 

This  is  deducted  from  16.476  gms.,  the  quantity  of  pure  KI  equiva- 

N 
lent  to  1000  cc.  of  —  AgNO3  V.  S. 

16.476 
16.393 


.083  gm. 

This  represents  the  excess  of  standard  silver  solution  used  up 
by  the  KC1,  reckoned  in  terms  of  KI. 

Since  7.404  gms.  of  KC1  is  equivalent  to  16.476  gms.  of  KBr  the 
excess  which  KC1  can  consume  is, 

16.476-7.404=9.072  gms.; 
therefore, 

9.072  :  7.404  : :  0.083  :  x-        #=0.615  Sm- 


ZINC  BROMID  119 

0.615  gm.  is  the  amount  of  KC1  present  in  16.393  gms.  of  the  salt 
examined. 

The  percentage  is  now  calculated 

16.393  :  0.615  : :  100  :  x.        #=3-75  per  cent  of  KC1, 

which  leaves  96.25  per  cent  of  pure  KI. 

The  same  method  of  assay  is  applied  to  the  following  salts: 
Lithium  Bromid. 

LiBr  +  AgNO3  =   AgBr  4-  LiNO3. 
10)86.34      10)168.69 

8.634  gms.    16.869  gms.=  iooo  cc.  —  V.  S. 

10 

Each  cc.  of  the  standard  solution  represents  0.008634  gm.  of  LiBr. 
Potassium  Bromid. 

KBr  +   AgNO3  =  AgBr  +   KNO3. 
10)118.22      10)168.69 

.N" 

11.822  gms.   16.869  gms.  =  1000  cc.  —  V.  S. 

10 

Each  cc.  of  the  silver  solution  represents  0.011822  gm.  of  KBr. 
Sodium  Bromid. 

NaBr  +   AgNO3  =  AgBr  +  NaNO3. 
10)102.24      10)168.69 

10.224  gms.    16.869  gms.=  1000  cc.  —  V.  S. 

10 

Each  cc.  of  the  standard  silver  solution  represents  0.010224  gm.  of 
NaBr. 

Strontium  Bromid  (SrBr2). 

SrBr2  +  2AgNO3  =  2AgBr  +   Sr(NO3)2. 
20)  245.66     20)337.38 

-1—  ^^" 

12.283  gms.  16.869  gms.  =  1000  cc.  —  V.  S. 

10 

Each  cc.  of  the  standard  silver  solution  represents  0.012283  g01-  °f 
SrBr2. 

Zinc  Bromid. 

ZnBr2  -f   2AgNO3  =   2 AgBr  +  Zn(NO3)2. 
20)223.62       20)337.38 

11.181  gms.     16.869  gms.=  iooo  cc.  —  V.  S. 

10 


120  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Each  cc.  of  the  standard  silver  solution  represents  0.011181  gnu 
of  ZnBr2. 

Sodium  lodid. 

Nal    +    AgNO3   =   Agl   +   NaNO3. 

10)148.78      10)168.69 

14.878  gms.    16.869  gms.=  1000  cc.  —  V.  S. 

10 

Each  cc.  of  the  standard  silver  solution  represents  0.014878  gm.  of 
Nal. 

Ammonium  Chlorid. 

NH4C1  +  AgNO3  =   AgCl  +  NH4NO3. 
10)53.11       10)168.69 

5.311  gms.    16.869  gms.=  iooo  cc.  —  V.  S. 

10 

Each  cc.  of  the  standard  silver  solution  represents  0.005311  gm.  of 
NH4C1. 

Titration  without  an  Indicator— Gay-Lussac's  Method.  In 
this  method  no  indicator  is  used.  The  standard  solution  being  added 
until  it  ceases  to  produce  any  further  precipitation.  This  method  is 
applicable  to  acid  solution  of  the  haloid  salts,  and  to  the  haloid  acids 
hydrochloric,  hydrobromic,  and  hydriodic.  Also  to  the  estimation 
of  silver  by  standard  solution  of  sodium  chlorid.  The  method  is 
carried  out  in  hot  solutions,  slightly  acidulated  with  nitric  acid,  in 
order  to  facilitate  the  precipitation  of  the  silver  halid.  The  haloid 
acids  are  neutralized  with  an  alkali  and  then  slightly  acidulated  with 
nitric  acid  before  the  titration  is  begun.  The  calculations  are  pre- 
cisely like  those  in  the  foregoing  assays. 

ESTIMATION   OF  HALOID  ACIDS 

These  acids,  namely,  hydrochloric,  hydrobromic,  and  hydriodic, 
may  be  estimated  by  Gay-Lussac's  method  above  described,  or  they 
may  be  estimated  by  Mohr's  Method,  using  neutral  potassium  chromate 
as  an  indicator.  In  this  case  it  is  necessary  to  carefully  neutralize 

N 

the  acid  with  ammonia  and  then  titrate  with  —  silver  nitrate  solution, 

10 

using  a  few  drops  of  chromate  as  indicator  in  the  manner  described 
in  the  foregoing  assays.  They  may  also  be  estimated  by  Volhard's 
Method,  in  which  an  excess  of  the  standard  silver  nitrate  solution 


ASSAY   OF  HYDROBROMIC   ACID  121 

is  used,  in  the  presence  of  nitric  acid,  and  the  amount  of  the  excess 
determined  by  residual  titration  with  potassium  sulphocyanate,  using 
ferric  alum  as  the  indicator.  This  method  is  especially  useful  for 
iodids  and  hydriodic  acid,  in  that  the  nitric  acid  need  not  be  added 
until  after  an  excess  of  silver  nitrate  solution  is  used,  and  thus  libera- 
tion of  iodin  by  the  nitric  acid  avoided.  This  method  is  more  fully 
described  further  on. 

The  estimation  of  the  haloid  acids  may  also  be  effected  by  neu- 
tralization with  standard  alkali,  in  the  same  way  as  other  acids,  but 
since  hydrobromic  and  hydriodic  acids  are  now  frequently  prepared 
by  the  method  of  Fothergill,  in  which  potassium  bromid  or  potassium 
iodid  (according  to  the  acid  to  be  made)  is  brought  in  contact  with 
tartaric  acid  (as  shown  in  the  equation),  an  excess  of  the  latter  acid 
is  unavoidably  present,  and  hence  the  neutralization  method  is  in- 
applicable. 

KI  +  H2C4H406  =  KHC4H406  +  HI. 

Potassium      Tartaric  acid.  Potassium  Hydriodic 

Iodid.         .  Bitartrate.  Acid. 

KBr  "  "  HBr 

Assay  of  Hydrobromic  Acid,  Using  Chromate  as  Indicator. 

10  gms.  of  hydrobromic  acid  are  diluted  with  sufficient  distilled  water 
to  make  100  cc.  10  cc.  of  this  solution,  representing  i  gm.  of  the 
acid,  is  exactly  neutralized  with  diluted  ammonia  water  (using  litmus 
solution  as  indicator);  3  drops  of  neutral  potassium  chromate  are 

N 
added,  and  then  the  —  silver  nitrate  run  in  from  a  burette  until  the 

JO 

solution  acquires  a  permanent  red  tint.  The  following  equation  is 
then  applied: 

HBr      +      AgN03     =     AgBr     +     HNO3. 

10)80.36  gms.    io)r68.6Q  gms. 

8.036  gms.=     16.869  gms.=  1000  cc.  of  —  V.  S. 

10 

N 
Thus  each  cc.  of  the  —  V.  S.  represents  0.008036  gm.  of  HBr. 

Assuming  that  12.44  cc.  of  the  silver  solution  were  consumed, 
then 

0.008036X12.44=0.0999  gm. 

which  is  9.99  per  cent. 


122  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

If  the  assay  is  to  be  made  by  the  direct  percentage  method,  8.036  cc. 
(8.04  cc.)  of  the  solution  (10  gms.  in  100  cc.)  (representing  0.8036 
gms.  of  the  acid)  should  be  taken,  in  which  case  each  cc.  of  the 
standard  silver  solution  consumed  will  at  once  indicate  i  per 
cent. 

Volhard's  or  Sulphocyanate  Method.  This  method  depends 
upon  entirely  precipitating  the  halogen  in  the  presence  of  nitric  acid, 
by  a  measured  excess  of  standard  silver  nitrate  solution,  and  then 
estimating  the  excess  of  silver  by  retitrating  with  standard  sulpho- 
cyanate  solution,  using  ferric  alum  as  an  indicator. 

The  sulphocyanate  has  a  greater  affinity  for  silver  than  it  has  for 
iron,  and  therefore,  so  long  as  any  silver  is  in  solution,  the  sulpho- 
cyanate will  combine  with  it  and  form  a  precipitate  of  silver  sulpho- 
cyanate. 

As  soon  as  the  silver  is  all  taken  up,  the  sulphocyanate  will  com- 
bine with  the  ferric  alum  and  strike  a  brownish-red  color. 

The  sulphocyanate  solution  is  to  be  made  of  such  strength  that 
it  corresponds  with  the  silver  solution,  volume  for  volume. 

The  difference  between  the  volume  of  silver  solution  originally 
added  and  the  volume  of  sulphocyanate  solution  used,  will  give  the 
volume  of  silver  solution  equivalent  to  the  haloid  salt  present. 

This  method  has  the  advantage  over  the  direct  method  for  haloids 
with  chromate  indicator,  in  that  it  may  be  used  in  the  presence  of 
nitric  acid.  It  thus  enables  one  to  estimate  the  haloids  in  the  presence 
of  phosphates  or  other  salts  which  precipitate  silver  in  neutral  but 
not  in  acid  solutions,  and  also  in  that  the  presence  of  barium,  bis- 
muth, lead,  iron,  and  other  metals  do  not  interfere,  as  they  do  with 
the  chromate  in  Mohr's  method.  The  presence  of  mercury,  however, 
exerts  a  disturbing  influence  upon  the  end  reaction.  The  nitric  acid 
acidulates  the  solution  and  thus  facilitates  the  precipitation  of  silver 
by  the  halogens,  and  prevents  its  precipitation  by  other  substances. 
The  quantity  of  nitric  acid  employed  is  of  no  great  importance,  except 
in  the  case  of  iodids  (because  silver  iodid  is  slightly  soluble  in  nitric 
acid).  Usually  sufficient  of  the  acid  is  added  to  just  remove  the 
color  produced  0by  the  indicator.  A  very  large  excess  of  the  acid 
would,  however,  interfere  with  the  proper  determination  of  the  end 
reaction,  in  that  it  to  a  slight  extent  prevents  the  formation  of  ferric 
sulphocyanate.  In  the  estimation  of  iodids  by  this  method,  the  nitric 
acid  should  be  added  after  the  standard  silver  solution,  while  in  the 
case  of  the  other  haloid  salts  the  acid  may  be  added  before. 


ASSAY   OF  HYDRIODIC   ACID  123 

The  solutions  required  for  this  method  are: 
(I)  Decinormal  Silver  Nitrate  (Page  112); 
(II)  Decinormal  Potassium  Sulphocyanate  (Page  114); 

(III)  Ferric  Alum  Solution.     (The  indicator.) 

This  is  a  10  per  cent  aqueous  solution  of  ferric -ammonium  sulphate 
Fe2(S04)3  •  (NH4)2S04-f  24H20. 

(IV)  Nitric  Acid  (C.  P.).     This  must  be  free  from  nitrous  acid. 
If  it  or  any  of  the  lower  oxids  of  nitrogen  are  present  they  may  be 
removed  by  diluting  with  one  fourth  part  of  water,  and  boiling  until 
colorless. 

The  process  is  exemplified  in  the  following  assays: 
Assay  of  Hydriodic  Acid  by  the  Sulphocyanate  Method. 
Introduce  into  a  200  cc.  stoppered  flask  2  gms.  of  the  acid,  add  50 
cc.  of  distilled  water  and  25  cc.  (accurately  measured)  of  decinormal 
silver  nitrate,  shake  thoroughly,  and  then  add  5  cc.  of  the  ferric 
alum  solution  and  3  cc.  of  nitric  acid  C.  P.  The  flask  is  stoppered  and 
again  thoroughly  shaken,  and  finally,  the  decinormal  potassium  sulpho- 
cyanate  run  in  slowly  from  a  burette,  until  a  permanent  reddish- 
brown  tint  is  produced.  Note  the  number  of  cc.  of  Sulphocyanate 
solution  employed^ 

Deduct  this  from  the  25  cc.  of  silver  solution  added,  and  multiply 
the  remainder  by  the  factor  for  HI,  which  is  0.01269. 

I.  HI     +     AgNO3     =     Agl     +     HNO3. 

10)126.9^      10)168.69 

12.69  gms.   16.869  gms.=  1000  cc.  —  V.  S. 

N     I0 

0.01269  gm.  of  HI=  i  cc.  —  V.  S. 
10 

II.  AgN03  +  KSCN=AgSCN+KN03. 

N 

16.869  gms.  =  9.653  gms.=  iooo  cc.  — V.  S. 

10 

III.  Fe2(NH4)2(SO4)4+6KSCN=Fe2(SCN)6+  (NH4)2SO4+3K2SO4. 

The  reddish-brown  color  which  marks  the  end  reaction  is  due 
to  the  formation  of  Fe2(SCN)6  ferric  Sulphocyanate. 

Assuming  that  in  the  above  titration  9.3  cc.  of  decinormal  Sulpho- 
cyanate were  employed,  then  25  cc.  —  9.3  cc.  =  15.7  cc. 

0.01269  X 15. 7  =  0.199233  gm. 


124  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

The  U.  S.  P.  acidum  hydriodicum  dilutum  is  10  per  cent  in  strength. 
Syrupus  acidi  hydriodici  is  i  per  cent. 

Assay  of  Syrup  of  Hydriodic  Acid.  6  gms.  of  the  syrup  are 
weighed  off  carefully  in  a  200  cc.  stoppered  flask,  20  cc.  of  distilled 
water  are  added,  followed  by  10  cc.  of  decinormal  silver  nitrate 
and  the  mixture  thoroughly  shaken.  5  cc.  of  diluted  nitric  acid  and 
3  cc.  of  the  ferric  alum  solution  are  now  added,  and  after  again  shaking 
the  mixture,  it  is  titrated  with  decinormal  potassium  sulphocyanate 
until  a  permanent  reddish-brown  tint  appears.  If  5.1  cc.  of  the 
sulphocyanate  solution  are  used  this  quantity  is  deducted  from 

N 

the  10  cc.  of  —  silver  nitrate  solution  added,  which  leaves  4.9  cc., 
10 

the  quantity  of  the  latter  which  reacted  with  the  syrup.    Then 
0.01269  X4.9=o.c62i8i  gm. 
0.062181X100 


Assay  of  Syrup  of  Ferrous  lodid  by  the  Sulphocyanate  Method. 

The  U.  S.  P.  directs  to  take  10  gms.  of  the  syrup,  dilute  it  with 
distilled  water  to  measure  100  cc.  Of  this  solution  15.36  cc.  are 
mixed  with  15  cc.  of  water,  6  cc.  of  decinormal  silver  nitrate  and 
2  cc.  each  of  diluted  nitric  acid  and  ferric  alum  solution,  and  then 
after  thoroughly  shaking  the  mixture  is  titrated  with  decinormal 
sulphocyanate  until  a  permanent  reddish-brown  tint  appears.  Not 
more  than  i  cc.  of  the  latter  should  be  used.  This  i  cc.  deducted 
from  the  6  cc.  of  decinormal  silver  nitrate  leaves  5  cc.,  the  quantity 
of  the  latter  which  reacted  with  the  ferrous  iodid.  Each  cc.  rep- 
resents i  per  cent.  The  equation  is 

FeI2  +  2AgNO3  =  2AgI  +  Fe(NO3)2. 

2)307-3°        2)337.38 
10)153.65      10)168.69 

15.365  gms.    16.869  gms.  =  1000  cc.  —  V.  S. 

10 

N 
.015365  gm.  of  FeI2      =        i  cc.  —  V.  S. 


10 


N 
If  each  cc.  of      -  AgNO3  V.  S.  represent  0.015365  gm.  of 


cent. 


then  5  cc.  =  5X0.015365  gm.  =  o 

0.076825X100 


ESTIMATION  OF    CYANOGEN  125 

In  this  (direct  percentage)  method,  a  quantity  of  the  syrup  is 
taken  which  equals  the  weight  of  pure  FeI2  represented  by  100  cc. 
of  the  decinormal  silver  nitrate  solution. 

Strontium  lodid  (SrI2)  and  Zinc  lodid  (ZnI2)  are  assayed  in 
the  U.  S.  P.  by  sulphocyanate  method  above  described. 

The  sulphocyanate  method  may  be  used  for  the  estimation  of 
chlorids  and  bromids,  as  well  as  iodids. 

When  used  for  the  estimation  of  chlorids,  however,  the  precipitated 
silver  chlorid  must  be  removed  by  filtration,  because  of  the  action  of 
ferric  sulphocyanate  upon  silver  chlorid  which  causes  the  results  of 
the  analysis  to  be  too  high.  In  the  case  of  silver  bromid  no  such 
reaction  takes  place,  or  if  it  does,  the  reaction  is  so  slow  as  not  to 
interfere  in  the  least  with  the  getting  of  accurate  results.  Therefore, 
when  this  method  is  used  for  the  determination  of  bromids  or  iodids, 
there  is  no  need  for  filtering  to  remove  the  precipitate.  This  matter 
is  dealt  with  more  fully  on  page  261. 

ESTIMATION   OF  CYANOGEN 

Titration  with  Standard  Silver  Solution  to  Fkst  Appearance 
of  a  Precipitate — Liebig's  Method.  This  gives  fairly  accurate 
results.  The  cyanogen  must  be  in  the  form  of  an  alkali  salt  and 
in  an  alkaline  solution.  If  hydrocyanic  acid  is  to  be  estimated,  it 
must  be  made  alkaline  by  the  addition  of  potassium  or  sodium  hy- 
droxid.  The  standard  silver  solution  is  then  added  cautiously  and 
with  constant  stirring  until  a  permanent  precipitate  of  silver  cyanid 
is  produced.  When  silver  nitrate  is  added  to  an  alkaline  solution 
of  a  cyanid,  the  precipitate  which  at  first  forms  redissolves  on  stir- 
ring and  a  soluble  double  cyanid  (AgCN,KCN  or  AgCN,NaCN,  de- 
pending upon  the  alkali  used)  is  formed,  and  when  all  of  the  cyanid 
has  been  taken  up,  the  further  addition  of  silver  nitrate  causes  a  decom- 
position of  this  soluble  double  salt  and  the  formation  of  a  permanent 
precipitate  of  silver  cyanid.  Therefore,  the  first  appearance  of  this 
precipitate  affords  a  delicate  proof  of  the  completion  of  the  reaction. 

These  equations  illustrate  the  reactions. 

2NaCN+AgN03=AgCN,NaCN-fNaN03. 

Double  cyanid 
of  silver  and  sodium. 

AgCN,NaCN+AgNO3=2AgCN+NaNO3. 

Silver  cyanid. 


126  A    MANUAL   OF    VOLUMETRIC    ANALYSIS 

According  to  these  equations  it  is  seen  that  the  end-reaction  is 
reached  when  two  molecules  of  the  alkali  cyan  id  have  reacted  with 
one  molecule  of  silver  nitrate.  The  slightest  excess  of  silver  nitrate 
above  this  quantity  brings  about  a  decomposition  of  the  double  salt 
and  a  precipitation  of  the  silver  cyanid,  as  above  stated. 

This  double  combination  is  so  firm  that  if  the  estimation  is  done 
in  the  presence  of  a  halogen,  no  permanent  precipitate  of  silver  halid 
is  formed  until  after  all  of  the  cyanogen  present  has  been  converted 
into  a  double  salt.  This  fact  is  taken  advantage  of  in  the  U.  S.  P. 
processes  for  hydrocyanic  acid  and  potassium  cyanid  in  which  potas- 
sium iodid  is  employed  as  indicator,  in  the  presence  of  ammonia 
water.  The  latter  prevents  the  precipitation  of  silver  cyanids  and 
thus  allows  the  silver  iodid  to  precipitate  alone. 

i  cc.  of  -  AgNO3  V.  S.  =  0.005268  gm.  CN; 

0.005368  gm.  HCN; 
0.009744  gm.  NaCN; 
0.01294    gm.  KCN. 

Assay  of  Hydrocyanic  Acid  (HCN =26.84).  Dilute  hydrocyanic 
acid  may  be  estimated  by  weighing  out  about  5  gms.,  and  adding  it 
without  delay  (to  avoid  evaporation)  to  sufficient  sodium  or  potassium 
hydroxid  solution  to  convert  the  acid  into  sodium  or  potassium  cyanid 
(NaCN  or  KCN)  and  leave  the  solution  strongly  alkaline.  The 
mixture  is  then  largely  diluted  with  water  (50  to  100  cc.);  this  is  to 
enable  one  to  more  clearly  observe  the  end-point. 

The  decinormal  silver  nitrate  solution  is  then  delivered  in,  until 
a  permanent  turbidity  occurs. 

The  difficulty  experienced  in  this  process  is  in  the  conversion  of 
the  acid  into  the  cyanid.  Sodium  cyanid  has  a  strong  alkaline  reaction, 
turning  litmus  blue,  when  only  a  small  proportion  of  the  acid  has 
been  neutralized.  If  the  titration  is  conducted  before  the  acid  is 
completely  neutralized  that  which  is  free  will  not  be  acted  upon. 
Indeed,  cyanid  of  sodium  may  be  estimated  in  the  presence  of  hydro- 
cyanic acid  in  this  way. 

According  to  Senier  the  following  procedure  will  answer  well : 

To  the  dilute  hydrocyanic  acid  add  sodium  hydroxid  to  strong 
alkaline  reaction,  determined  by  litmus  tincture.* 

*  Poirrier  Blue  C4B  is  better,  in  that  it  is  not  affected  by  alkali  cyanids,  but 
gives  a  very  sharp  indication  in  the  presence  of  the  slightest  excess  of  alkali 


ASSAY    OF  HYDROCYANIC   ACID  127 

N 

Then  titrate  with    —  silver  nitrate,  drop  by  drop.    If  the  liquid 
10 

becomes  acid,  add  a  little  more  soda  solution  to  bring  it  back  to 
alkalinity,  and  continue  the  titration  until  the  turbidity  indicates 
the  end  of  the  reaction.  The  liquid  must  be  kept  alkaline  throughout 
the  process.  It  is  not  well  to  add  too  much  alkali  at  the  beginning 
as  this  will  use  up  too  much  of  the  silver  solution  and  make  the  read- 
ing a  trifle  too  high.  The  following  equations,  etc.,  explain  the 
reactions: 

2HCN+  2NaOH=  2NaCN+  2H2O. 

10)53.68  10)97.44 

5.368  gms.  9.744  gms. 

2NaCN  +  AgNO3  =  AgCN,NaCN  +  NaNO3. 
io)97-44       10)168.69  N 

9.744  gms.     16.869  gms.  or  1000  cc.  —  V.  S. 

It  is  seen  that  5.368  gms.  of  real  HCN  are  equivalent  to  9.744  gms. 
of  sodium  cyanid,  and  represent  16.869  gms-  °f  silver  nitrate  or  1000  cc. 

N  N 

of  the  —  V.  S.    That  is,  1000  cc.  of  the  —  AgNO3  V.  S.  may  be  added 
10  10 

to  a  solution  containing  9.744  gms.  of  sodium  cyanid  and  no  precipitate 
will  be  produced,  but  if  one  or  two  drops  more  of  the  standard  solu- 
tion be  added,  a  precipitate  is  at  once  formed,  the  double  salt  being 
broken  up  and  silver  cyanid  produced. 

AgCN,NaCN+AgN03  =  2AgCN+NaN03. 

N 

Each  cc.  of  the  -     silver  solution  which  fails  to  produce  a  pre- 
10 

cipitate  represents  0.009744  gm.  of  NaCN,  which  is  equivalent  to 
0.005368  gm.  of  HCN. 

Titration  with  Standard  Silver  Solution,  Using  Chromate 
Indicator — Vielhaber's  Method.  This  method  is  especially  recom- 
mended for  the  assay  of  weak  solutions  containing  hydrocyanic  acid, 
as  bitter  almond  oil,  bitter  almond  water,  cherry  laurel  water,  etc., 
but  it  may  also  be  employed  for  alkaline  cyanids. 

hydroxid.  The  amount  of  alkali  used  should  be  as  near  as  possible  that  which 
is  required  to  just  convert  the  acid  into  the  alkali  cyanid,  too  much  or  too  little 
alike  affect  the  accuracy  of  the  result. 


128  A  MANUAL  OF   VOLUMETRIC  ANALYSIS 

A  sufficient  quantity  of  an  aqueous  suspension  of  magnesium 
hydroxid  *  to  make  the  solution  opaque  and  distinctly  alkaline  is 
added;  this  is  followed  by  a  few  drops  of  potassium  chromate  indi- 

N 

cator  and  then  the  —  silver  nitrate  delivered  into  the  mixture  from 
10 

a  burette  until  a  permanent  red  tint  appears,  as  in  the  titration  of 
haloid  salts.  The  method  is  a  very  satisfactory  one,  if  chlorids  are 
absent. 

The  reactions  in  this  method  are  the  same  as  in  the  foregoing, 
but  the  end-reaction  (the  production  of  silver  chromate)  does  not 
occur  until  the  double  cyanid  is  completely  decomposed,  at  which 
point  the  addition  of  another  drop  of  silver  solution  reacts  with  the 
chromate  and  produces  the  red  precipitate  (silver  chromate). 

The  equations  are  as  follows:  Sodium  is  used  in  the  equations 
instead  of  magnesium  in  order  to  make  the  explanation  clearer. 

(a)  2NaCN + AgNO3  =  AgCN,NaCN + NaNO3  =  (2  HCN) ; 

(b)  AgCN,NaCN+AgNO3=2AgCN+NaNO3. 

These  equations  show  that  it  requires  two  molecules  of  silver 
nitrate  to  completely  precipitate  two  molecules  of  cyanid.  168.69 
gms.  of  AgNO3  is  equivalent  to  26.84  gms.  of  HCN,  while  by  Liebig's 
method,  168.16  gms.  of  AgNO3  is  equivalent  to  53.68  gms.  of  HCN. 

i  cc.  —  AgN03  V.S.  =  0.002584  gm.  CN; 
10 

0.002684  gm.  HCN; 
0.004872  gm.  NaCN; 
0.006470  gm.  KCN. 

Example.  1.35  gms.  of  the  diluted  acid  is  mixed  with  enough 
water  and  magnesia  to  make  an  opaque  mixture  of  about  10  cc.  Add 
to  this  2  or  3  drops  of  potassium  chromate  T.  S.,  and  then  from  a 
burette  deliver  the  decinormal  silver  nitrate  V.  S.  until  a  red  tint  is 
produced  which  does  not  disappear  by  shaking. 

Titration  with  Standard  Silver  Solution,  Using  Potassium 
lodid  and  Ammonia  as  Indicator.  This  method  is  recommended 
by  W.  J.  Sharwood,  J.  A.  C.  S.,  1897,  400-434,  and  is  a  modification 

*  Calcined  magnesia  triturated  with  water. 


ESTIMATION   OF  POTASSIUM   CYANID  129 

of  the  method  proposed  by  M.  Georges  Deniges,  Ann.  chim.  phys., 
(7)  6.381. 

In  this  method  5  gms.  of  hydrocyanic  acid  are  diluted  with  dis- 
tilled water  to  measure  50  cc.  Then  25  cc.  of  this  solution,  after  the 
addition  of  5  cc.  of  ammonia  water  and  3  drops  of  a  20  per  cent  potas- 
sium iodid  solution,  are  titrated  with  tenth-normal  silver  nitrate,  until 
a  slight  permanent  precipitate  occurs.  The  ammonia  water  and 
potassium  iodid  in  this  process  act  as  indicator. 

The  reactions  may  be  expressed  thus: 

(1)  HCN+NH4OH=NH4CN+H20; 

(2)  2NH4CN+AgN03=NH4Ag(CN)2+NH4NO3; 

(3)  NH4Ag(CN)2+AgN03=NH4 

(4)  KI+AgN03=KN03 


The  silver  nitrate  forms  with  the  cyanid  a  double  salt  which  is 
soluble,  no  precipitate  occurring  until  after  all  of  the  cyanid  has 
entered  into  combination  as  the  double  salt;  then  the  further  addition 
of  silver  nitrate  decomposes  the  double  salt,  and  a  precipitate  of  silver 
cyanid  occurs.  In  the  presence  of  ammonia  water,  however,  as  in 
the  above  assay,  the  precipitation  of  silver  cyanid  is  prevented,  but 
the  iodid  is  now  (not  before)  acted  upon  by  the  silver  solution  and  a 
precipitate  of  silver  iodid  occurs,  which  very  delicately  indicates  the 
end  reaction. 

Each  cc.  of  the  standard  silver  nitrate  solution  used  represent 
0.005368  gm.  of  absolute  HCN.  If  26.84  cc-  °f  the  above  solution 
are  taken  instead  of  25  cc.,  each  cc.  will  represent  at  once  0.2  per 
cent.  The  U.  S.  P.  dilute  hydrocyanic  acid  should  require  when  treated 
this  way  exactly  10  cc.  of  silver  solution,  indicating  2  per  cent  of  HCN. 

Potassium  cyanid  is  assayed  in  the  same  way. 

Estimation  of  Potassium  Cyanid  (KCN=  64.70).  i  gm.  of 
potassium  cyanid  is  dissolved  in  sufficient  distilled  water  to  make 
100  cc.,  then  64.7  cc.  of  this  solution  mixed  with  5  cc.  of  ammonia 

N 
water  and  3  drops  of  potassium  iodid  T.  S.  are  titrated  with  —  AgNO3 

V.  S.  until  the  appearance  of  a  permanent  precipitate.    47.5  cc.  should 
be  required.     Each  cc.  indicates  2  per  cent. 

i  cc.  of  —  AgNO3  V.  S.=  0.01294  gm.  KCN. 


130  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

ESTIMATION  OF   SILVER   SALTS 

Soluble  silver  salts  are  estimated  by  direct  titration  with  standard 
sodium  chlorid,  the  process  being  exactly  the  converse  of  the  precipita- 
tion methods  for  halogens.  The  standard  sodium  chlorid  solution  is 
added  to  the  solution  of  the  silver  salt  until  precipitation  ceases :  Or  the 
titration  may  be  done  in  the  presence  of  chromate  indicator,  the  end- 
point  being  then  known  to  be  reached  when  the  red  color  of  silver 
chromate  disappears.  The  first  of  these  methods  is  impracticable.  Too 
much  time  being  consumed  in  waiting  for  the  precipitate  to  settle  so  as  to 
render  the  supernatant  liquid  sufficiently  clear  to  recognize  whether  a  pre- 
cipitate is  produced  in  it  by  the  further  addition  of  the  standard  solution. 

If  chromate  indicator  is  used,  the  end-point  is  easily  over-stepped, 
because  of  the  slow  decomposition  of  the  silver  chromate  by  the  chlorid. 
It  is  best  to  add  an  excess  of  sodium  chlorid  solution  and  then  re- 
titrate  with  standard  silver  nitrate  solution  until  the  red  color  appears. 

Silver  salts  may  also  be  titrated  by  means  of  standard  sulphocyanate 
solution,  using  ferric  alum  as  indicator. 

Assay  of  a  Solution  of  Silver  Nitrate  by  Means  of  —  Sodium 

Chlorid  10  gins,  of  the  solution  are  introduced  into  a  beaker  and 
diluted  with  10  cc.  of  distilled  water.  Two  drops  of  yellow  potassium 
chromate  solution  are  added  as  indicator,  and  then  a  measured  excess 

N 

of  —    sodium  chlorid    (sufficient   of  this   must  be   added    to  com- 
10 

pletely  destroy  the  red  color),  added  slowly  from  a  burette  and  with 
constant  stirring.  Assuming  that  20  cc.  were  used,  then  the  excess 

N 
may  be  ascertained  by  titrating  back  with  —  silver  nitrate  V.  S.  until 

N 
a  permanent  red  tint  is  produced.     Whatever  number  of  cc.  of  — 

silver    nitrate     are  used,   that    number    represents    the     quantity   of 

N 

—    sodium   chlorid    which   was   added   in  excess,  and    must  be  de- 

10 

ducted  from  the  20  cc.  of  the  sodium  chlorid  solution  employed. 
Assuming  this  number  to  be  3  cc.,  then  3  from  20  leaves  17  cc.,  which 

N 
is  the  exact  quantity  of  —  NaCl  V.  S.  which  reacted  with  the  silver 

in  the  solution  examined. 

N 
The  —  factor  for  silver  nitrate  multiplied  by  17  will  then  give 


ASSAY   OF  SILVER   NITRATE  131 

the  exact  weight  of  silver  nitrate  in  the  10  gms.  of  solution  taken. 
The  following  equation  illustrates  the  reaction  which  occurs: 

AgN03     +     Nad     =     AgCl     +     NaNO3. 
io)_i68.69_          10)58.06 

16.869  gms.         5.806  gms.=  1000  cc.  —  V.  S. 

10 

0.01689  gm.  of  silver  nitrate  is  thus  represented  by  each  cc.  of 

—  sodium  chlorid. 
10 

In    the    above    assay    17    cc.  of    the    sodium    chlorid    solution 

were    used,    therefore    the    silver  solution    under   analysis    contains 

17X0.016869  gm.  =  0.286773  gm.  of  pure  AgNOs.  The  percentage 
of  the  solution  is  then  found, 

0.286773X100 

—  =2.86773  per  cent. 

In  the  assay  of  silver  nitrate  crystals,  0.2  gm.  is  taken,  dissolved 
in  10  cc.  of  distilled  water,  and  then  treated  as  in  the  foregoing  assay. 
In  the  case  of  molded  silver  nitrate  about  the  same  quantity  is  taken 
for  assay.  Of  mitigated  silver  nitrate,  i  gm.  may  be  taken. 

Silver  oxid  (Ag2O)  may  be  converted  into  nitrate  by  solution  in 
nitric  acid  and  then  tested  as  above.  Free  nitric  acid  is  apt  to  be 
present  in  this  case  and  therefore  the  solution  should  be  neutralized, 
before  it  is  assayed,  if  the  above  described  method  is  to  be  employed. 
The  presence  of  free  acid  does  not  interfere,  however,  if  the  method 
of  Gay-Lussac  or  the  sulphocyanate  method  be  employed. 

Assay  of    Silver    Nitrate    by  Means    of      -  Sulphocyanate. 

This  method,  as  applied  to  the  assay  of  halogen  compounds,  is 
described  in  the  preceding  pages.  The  great  advantage  which  this 
method  presents  over  the  others,  is  that  the  presence  of  most  other 
metals  does  not  interfere.  The  only  metal  which  does  materially 
interfere  with  the  determination  of  silver  is  mercury. 

Example.  A  weighed  quantity  (0.2  to  0.5  gm.)  of  the  silver  salt 
is  dissolved  in  water,  some  diluted  nitric  acid  and  ammonium  ferric 

N 
sulphate  solution  are  added,  and  the  mixture  then  titrated  with  — 

potassium  sulphocyanate  until  a  permanent  reddish -brown  color 
of  ferric  sulphocyanate  is  produced. 


132  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

The  following  equation  explains  the  reactions: 

AgN03    +    KSCN   =    AgSCN    +    KNO3. 
10)168.69         10)96.53 

16.869  gms.        9.653  gms.  or  1000  cc.  standard  V.  S. 

Thus  each  cc.  of  the  standard  V.  S.  represents  0.016869  gm.  of 
pure  silver  nitrate,  or  0.010712  gm.  of  metallic  silver. 

Estimation  of  Metallic  Silver  and  Silver  Alloys.  A  quantity 
of  the  metal,  weighing  about  0.5  gm.,  is  dissolved  in  10  cc.  of  nitric 
acid,  and  after  complete  solution  is  attained  it  is  heated  sufficiently 
to  drive  off  all  traces  of  nitrous  acid.  The  solution  is  then  diluted 
with  about  100  cc.  of  distilled  water  and  assayed  by  one  of  the  methods 
described  under  the  assay  of  solution  of  silver  nitrate.  The  sulpho- 
cyanate  method  is  the  preferred  one. 

THE   ANALYSIS   OF    CERTAIN  NEUTRAL  SALTS  BY    CONVERSION   INTO 

CHLORIDS. 

Many  neutral  salts  may  be  indirectly  estimated  with  great  accu- 
racy, by  converting  them  into  chlorids,  salts  of  acids  weaker  than 
hydrochloric;  and  salts  of  volatile  acids,  of  nearly  the  same  chemical 
strength  as  hydrochloric  acid,  may  be  readily  converted  into  chlorids 
by  frequent  evaporation  to  dryness  with  an  excess  of  hydrochloric 
acid. 

Sulphates  are  converted  into  chlorids  by  precipitation  with  barium 
chlorid,  or  better  with  barium  hydroxid;  the  excess  of  the  latter  is 
then  removed  by  passing  CO2  through  the  solution,  and  then  after 
the  addition  of  an  excess  of  hydrochloric  acid  evaporating  to  dryness. 

Borates  and  Phosphates  are  precipitated  by  lead  acetate  solution, 
the  excess  of  lead  removed  by  means  of  sulphuric  acid,  and  the  result- 
ing sulphate  then  treated  as  a,bove  described. 

Carbonic  Acid  in  Combination.  The  compound  is  slightly 
supersaturated  with  pure  hydrochloric  acid,  the  solution  evaporated 
to  dryness  on  a  water-bath,  and  then  heated  for  a  short  time  in  an 
air-bath.  The  residue,  which  consists  of  chlorid,  is  dissolved  in  water 

N 
and  titrated  in  the  usual  way  with  —  silver  nitrate,  V.  S.,  using  neutral 

chromate  as  indicator. 

Or  the  carbonate  is  treated  with  barium  chlorid  T.  S.,  the  precipi- 
tate well  washed,  and  then  dissolved  on  the  filter  with  hydrochloric 
acid  (covering  it  with  a  watch  glass  to  prevent  loss)  the  solution  is 
then  repeatedly  evaporated  to  dryness,  till  all  free  hydrochloric  acid  is 


AMMONIA,   AMMONIUM   SALTS,   AND   NITROGEN        133 

driven  off.  The  barium  is  then  precipitated  by  the  addition  of  sodium 
sulphate  solution,  which  is  added  in  slight  excess,  and  the  mixture 
titrated  with  silver  nitrate  V.  S.  in  the  presence  of  chromate  indicator. 
There  is  no  necessity  for  filtering,  as  the  precipitated  barium  sulphate 
does  not  interfere. 

One  molecule  of  sodium  carbonate  is  equivalent  to  two  molecules 
of  sodium  chlorid. 

Na2CO3     =     2NaCl     +     2AgNO3    =    2AgCl    +    2NaNO3. 
2)105.31  2)116.12  2)337-38 

10)52.655  gms,     10)58.06  gms.    10)168.69  gms. 

5.2655  gms.    5.806  gms.  =  16.869  gms.  =  1000  cc.  —  V.  S. 

10 

N 
Thus  i  cc.  of  —  AgNO3  V.  S.  represents  0.005806  gm.  of  NaCl, 

which  is  equivalent  to  0.0052655  gm.  of  Na2CO3. 

Free  Carbonic  Acid  Gas.  The  gas  is  collected  by  means  of 
ammonia  water  and  barium  chlorid  solution,  and  treated  as  described 
in  the  case  of  carbonic  acid  in  combination. 

N 
i  cc.  of  —  AgNO3=o.oo2i835  gm.  of  CC>2. 

Organic  Salts  of  the  Alkalies  and  Alkali  Earths.  These  salts 
are  ignited  to  convert  them  into  carbonates,  then  treated  with  hydro- 
chloric acid,  evaporated,  and  titrated  as  above  described. 

Chlorates  are  converted  by  ignition  into  chlorids. 

Nitrates  are  converted  into  chlorids  by  evaporating  with  concen- 
trated hydrochloric  acid. 

Ammonia,  Ammonium  Salts,  and  Nitrogen.  Ammonia  is 
conducted  into  diluted  hydrochloric  acid,  the  liquid  then  cautiously 
evaporated  to  dryness,  the  residue  (NH4C1)  dissolved  in  water  and 
titrated. 

Ammonium  salts  and  nitrogenous  compounds,  when  burned  with 
soda  lime,  evolve  ammonia  (NH3)  which  is  conducted  into  diluted 
hydrochloric  acid,  and  treated  as  above. 

i  cc.  — AgNO3  V.  S.  =  0.001693  gm-  NH3. 

Besides  the  substances  mentioned  in  the  foregoing  pages,  many 
others  may  be  accurately  estimated  by  precipitation,  as  for  instance, 
copper,  manganese,  lead,  sulphocyanic  acid,  hydrosulphuric  acid, 


134  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

phosphoric  acid,  glucose,  etc.,  the  methods  for  which  are  described 
under  other  headings. 

Estimation  of  Alkali   lodids   by  Precipitation  with  Mercuric 
Chlorid    Solution   (Personne).     Alkali  iodids  may  also  be  estimated 

N 
by  titration  with  -  ~  mercuric  chlorid  V.  S.,  the  termination  of  the 

10 

operation  being  indicated  by  the  formation  of  a  red  precipitate. 

4KI  +  HgCl2=2KCl  +  HgI2.2KI  (soluble).     .     .     .     (i) 

(2) 


This  process  originated  with  M.  Personne,  and  is  founded  on  the 
fact  that  if  a  solution  of  mercuric  chlorid  be  added  to  one  of  potas- 
sium iodid,  in  the  proportion  of  one  equivalent  of  mercuric  chlorid 
to  four  of  potassium  iodid,  red  mercuric  iodid  is  formed,  which  dis- 
solves at  once  to  a  colorless  solution.  The  slightest  excess  of  mercuric 
chlorid  will  cause  a  brilliant  red  precipitate  (HgI2)  to  make  its  ap- 
pearance. 

4KI     +     HgCl2     =     2KC1     +     HgI2  .  2KI  (soluble). 

20)659.04        20)268.86 

32.952  gms.      13.443  gms.  of  jooo  cc.  of  standard  solution. 

Thus  each  cc.  of  standard  solution  of  the  above  strength  repre- 
sents 0.032952  gm.  of  potassium  iodid,  which  means  that  i  cc.  is 
the  largest  quantity  of  this  standard  solution  which  can  be  added 
to  0.032952  gm.  of  potassium  iodid  without  producing  a  permanent 
precipitate. 

N 

The  above  solution   of  mercuric   chlorid   is   strictlv  a   —   V.   S. 

10 

(Consult  Estimation  of  Mercuric  Salts  on  page  409.) 

The  author  of  this  process  states  that  neither  chlorids,  bromids, 
nor  carbonates  interfere  with  the  reaction. 


ESTIMATION  OF  ALKALI   IODIDS 
TABLE  OF  SUBSTANCES  ESTIMATED  BY  PRECIPITATION 


135 


Name. 

Formula. 

Molecular 
Weight. 

Standard 
Solution 
Used. 

Factor.* 

Acid  hydrobromic.  ..      

HBr 
HC1 
HCN 

HCN 
HCN 

HI 

CSNCSH5 
NH4Br 
NH4C1 
NH4I 
CaBr2 
CaCl2 
FeBr2 
FeI2 
LiBr 
KBr 
KC1 

KCN 
KCN 
KCN 

KI 
KSCN 

80.36 
36-18 
26.84 

26.84 
26.84 

126.9 

98.4 
97.29 

S3-" 
143-83 
198.52 
110.16 

214.  22 
307.30 
86.34 
Il8.22 
74.04 

64.70 
64.70 
64.70 

164.76 
96.53 

^AgN03 

N 
AffNO 

0.008036 
0.003618 
0.005368 

0.002684 
0.005368 

0.01269 

0.00492 
0.009729 
0.005311 
0.014383 
0.009926 
0.005508 
0.010711 
0.015365 
0.008634 
0.0-11822 
0.007404 

0.01294 
0.006470 
0.01294 

0.016476 
0.009653 

'  '     hydrochloric 

*  *     hydrocyanic.  .  .             ... 

10     g        3 
^AgN03 

without 
indicator 

N 
-AgNO, 

with  chromate 
indicator 

N 
-AgNO, 

with  iodid 
indicator 

^AgN03 

<  « 
<  C 
(I 
(  t 

(  t 
(( 

I  I 
I  I 
t  t 

N 
-AgNO, 

without 
indicator 

^AgN03 

with  chromate 
indicator 

N 
-AgN03 

with  iodid 
indicator 

^AgN03 

<  ( 

CC                             (I 

tt                 ft 

'  '     hydriodic  .        

Allyl-iso-thiocya.na.te  f  t        .... 

Anmionium  broroid 

'  '           chlorid 

1  '           iodid 

Calcium  bromid                  .    . 

'  '       chlorid   

Ferrous  bromid 

«  '       iodid       

Lithium  bromid 

Potassium  bromid.  ..... 

"          chlorid  

1C                     « 

11            tt 

"         iodid  

"         sulphocyanate  

*  This  is  the  coefficient  by  which  the  number  of  cc.  used  of  the  decinormal  solution 
is  to  be  multiplied  in  order  to  obtain  the  quantity  of  pure  substance  in  the  sample  analyzed. 
It  represents  the  weight  of  the  substance  precipitated  by  i  cc.  of  the  decinormal  solution. 


136  A    MANUAL    OF   VOLUMETRIC    ANALYSIS 

TABLE  OF  SUBSTANCES  ESTIMATED  BY  PRECIPITATION — Continued 


Name. 

Formula. 

Molecular 
Weight. 

Standard 
Solution 
Used. 

Factor.* 

Silver  (metallic)  

Ajr, 

2X  IO7    I  2 

N 
—  NaCl  or 

O    OIO7I2 

'  '      nitrate  

AgNO, 

1  68  69 

10 
—  KSCN 

10 

<  < 

o  01680 

'  '      oxid 

Ae,O 

23O    1  2 

<  < 

NaBr 

IO2.  24 

N 
—  AgNO, 

o  010224 

"       chlorid  

NaCl 

58.06 

10 

0.005806 

iodid  

Nal 

148.78 

«  « 

0.014878 

Strontium  bromid  

SrBr2 

245.66 

<  « 

O.OI2283 

iodid  

SrI2 

^8.74 

t  < 

o  .016037 

Zinc  bromid  

ZnBr2 

223.62 

n 

o  011181 

"     chlorid  

ZnCl2 

135.26 

« 

0.006763 

"     iodid  

Znl, 

3l6.7O 

«« 

o  oinS^ 

CHAPTER  XII 
ANALYSIS  BY  OXIDATION  AND  REDUCTION 

AN  extensive  series  of  analyses  are  made  by  these  methods  with 
extremely  accurate  results,  in  fact,  the  results  are  generally  more 
accurate  than  those  obtained  by  gravimetric  methods. 

The  principle  involved  is  exceedingly  simple.  An  oxidizing 
agent  is  employed  for  the  estimation  of  an  oxidizable  substance, 
and  likewise  a  reducing  agent  is  employed  for  the  estimation  of  a 
reducible  substance.  Oxidizing  agents  are  always  reducible  and 
reducing  agents  are  always  oxidizable.  An  oxidation  and  a  reduction 
take  place  at  the  same  time,  i.e.,  the  oxidizing  agent  is  itself 
reduced  in  the  operation  and  the  reducing  agent  is  at  the  same  time 
oxidized. 

Thus  substances  which  are  capable  of  absorbing  oxygen  or  are 
susceptible  of  an  equivalent  action  may  be  accurately  estimated  by 
subjecting  them  to  the  action  of  an  oxidizing  agent  of  known  power, 
and  from  the  quantity  of  the  latter  required  for  complete  oxidation, 
the  weight  of  the  oxidizable  substance  is  ascertained. 

Example.  Ferrous  oxid  (FeO),  an  oxidizable  substance,  is  ever 
ready  to  take  up  oxygen,  while  potassium  permanganate  and  potas- 
sium dichromate  are  always  ready  to  give  up  some  of  their  oxygen. 
When  potassium  permanganate  gives  up  its  oxygen  in  this  way  it  is 
reduced  and  decolorized,  while  the  ferrous  oxid  in  taking  up  oxygen 
is  oxidized  to  ferric  oxid  (Fe2O3).  The  decolorization  of  the  per- 
manganate here  spoken  of  is  taken  advantage  of  in  volumetric  analysis 
for  the  determination  of  the  completion  of  the  oxidation.  The  per- 
manganate in  the  form  of  a  standard  solution  being  slowly  delivered 
from  a  burette,  until  it  is  no  longer  decolorized,  the  iron  salt  is  known 
to  be  completely  oxidized,  when  the  permanganate  is  no  longer 
reduced.  The  reaction  is  as  follows : 

ioFeO+ 2KMnO4=  5Fe2O3-f  2MnO2+ K2O. 

Ferrous  oxid  Ferric  oxid 

137 


138  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

The  oxidation  of  ferrous  oxid  by  potassium  dichromate  is  shown 
by  the  followng  equation: 


As  before  stated,  an  oxidation  is  always  accompanied  by  a  reduc- 
tion, the  oxidizing  agent  being  itself  reduced  in  the  operation.  As 
shown  in  the  above  equations,  the  manganic  compound  is  reduced  to 
a  manganous,  and  the  chromic  to  a  chromous,  while  the  ferrous  salt 
is  oxidized  to  a  ferric  condition. 

In  the  same  way  any  substance  which  readily  yields  oxygen  in 
definite  quantity  or  is  susceptible  of  an  equivalent  action  which  in- 
volves its  reduction  to  a  lower  quantivalence,  may  be  estimated  by 
ascertaining  how  much  of  a  reducing  agent  of  known  power  is  required 
for  its  complete  reduction. 

Example.  The  available  chlorin  in  bleaching  powder  may  be 
accurately  ascertained  by  treating  it  with  a  standard  solution  of  ar- 
senous  oxid,  and  from  the  volume  of  the  solution  required  for  the 
complete  reduction  of  the  chlorin,  the  quantity  of  the  latter  present 
is  found,  or  in  other  words,  from  the  quantity  of  arsenous  oxid  (As2O3), 
oxidized  to  arsenic  oxid  (As2C>5)  the  weight  of  the  chlorin  present  is 
ascertained. 

The  principal  substances  which  are  used  as  oxidizing  agents-  in 
volumetric  analysis,  are  potassium  permanganate,  potassium  dichro- 
mate, and  iodin.  The  latter  contains  no  oxygen,  but  it  abstracts 
hydrogen  from  accompanying  water  and  liberates  the  oxygen  which 
does  the  oxidizing,  hence  iodin  is  known  as  an  indirect  oxidizing. 
agent.  The  other  two  contain  available  oxygen  which  they  readily 
give  up  when  brought  in  contact  with  an  oxidizable  substance. 

The  principal  reducing  agents  or  deoxidizers  which  are  used  in 
volumetric  analysis  are,  sodium  thiosulphate,  sulphurous  acid,  oxalic 
acid,  arsenous  oxid,  stannous  chlorid,  ferrous  oxid,  hydriodic  acid. 
hydrosulphuric  acid,  metallic  zinc,  and  magnesium. 

VOLUMETRIC    ANALYSES    BY    MEANS   OF    POTASSIUM    PERMANGANATE 

When  potassium  permanganate  solution  is  added  to  a  solution  of 
any  readily  oxidizable  substance  strongly  acidulated  with  sulphuric 
acid,  it  undergoes  reduction  as  shown  in  the  equation  below.  The 
molecule  (2KMnO4)  has  eight  atoms  of  oxygen  which  it  gives  up  in 
the  process  of  oxidation.  These  eight  atoms  of  oxygen  unite  with 
the  replaceable  hydrogen  of  an  accompanying  acid,  liberating  an 


VOLUMETRIC  ANALYSES  BY  POTASSIUM  PERMANGANATE    139 

equivalent  amount  of  acidulous  radical.  Three  of  these  atoms  of 
oxygen  liberate  sufficient  acidulous  residual  to  combine  with  the 
potassium  and  manganese  of  the  permanganate,  while  the  other  five 
atoms  are  available  for  direct  oxidation. 

2KMnO4+3H2SO4=K2SO4+2MnSO4+3H2O  +  5O, 

or,  for  combination  with  the  hydrogen  of  more  acid,  more  acidulous 
residual  being  set  free,  to  combine  with  the  salt  acted  upon. 

2KMnO48  +  H2SO4=  K2SO4+  2MnSO4+8H2O-f-  s(SO4). 

5(SO4)  when  combined  with  ioFeSO4  forms  Feio(SO4)15  or 
5Fe2(SO4)3,  ferric  sulphate.  Thus  it  is  seen  that  one  molecule  of 
potassium  permanganate  (2KMnO4)  has  the  power  of  converting 
10  molecules  of  a  ferrous  salt  to  the  ferric  state. 

The  equation  in  full  is 

ioFeSO4+2KMnO4+8H2SO4=K2SO4+2MnSO4+8H2O+5Fe2(SO4)3. 

We  have  seen  that  2KMnO4  has  5  atoms  of  oxygen  available  for 
oxidizing  purposes,  and  that  each  of  these  will  combine  with  2  atoms 
of  hydrogen.  2KMnO4  is  consequently  chemically  equivalent  to 
10  atoms  of  hydrogen,  and  a  normal  solution  of  this  salt  when  used 
as  an  oxidizing  agent  is  one  that  contains  in  one  liter  one-tenth  of  the 
weight  of  2KMnO4  expressed  in  grams,  and  a  decinormal  solution, 
one  which  contains  one-hundredth  of  this  weight. 

As  before  stated,  when  potassium  permanganate  is  brought  in 
contact  with  a  ferrous  salt  or  other  oxidizable  substance,  it  is  decom- 
posed and  decolorized.  Hence  when  titrating  with  a  standard  solu- 
tion of  this  salt  it  is  decolorized  so  long  as  an  oxidizable  substance  is 
present;  as  soon,  however,  as  the  oxidation  is  completed  the  standard 
solution  retains  its  color  when  added  to  the  substance,  and  the  first 
appearance  of  a  faint  red  color  is  the  end-reaction,  and  the  oxidation 
is  known  to  be  completed. 

In  titrating  with  potassium  permanganate  it  must  be  remembered 
that  excess  of  free  acid  (preferably  sulphuric)  should  always  be 
present  in  the  solution  titrated,  in  order  to  keep  the  resulting  manga- 
nous  and  manganic  oxids  in  solution;  these,  forming  a  dense  brown 
precipitate,  would  make  it  difficult  if  not  quite  impossible  to  recog- 
nize the  pinkish  color  of  the  end-reaction.  Sulphuric  acid  alone  if 
in  large  excess,  has  a  reducing  effect  upon  potassium  permanganate. 


140  A    MANUAL    OF   VOLUMETRIC    ANALYSIS 

Nitric  and  hydrochloric  acids  are  prejudicial  and  should  be  avoided; 
they  are,  however,  frequently  present  in  salts  which  are  to  be  analyzed, 
and  in  such  event  should  be  removed  by  converting  them  into  sulphate. 
By  adding  a  small  excess  of  sulphuric  acid  and  applying  heat,  until 
hydrochloric  acid  or  nitrous  vapors  are  no  longer  evolved,  the  chlorid 
or  the  nitrate  is  converted  into  sulphate,  and  the  deleterious  effect  of 
their  presence  overcome.  Hydrochloric  acid,  unless  present  in  very 
small  quantities,  and  the  titration  conducted  at  a  low  temperature, 
will  vitiate  the  analysis  through  its  action  upon  the  permanganate 
whereby  chlorin  is  liberated  *  thus : 

KMnO4+8HCl=KCl+MnCl2+4H2O+5Cl. 

A  very  convenient  way  of  obviating  the  irregularities  due  to  the 
presence  of  hydrochloric  acid  is  to  add  a  few  grams  of  manganous 
sulphatef  to  the  solution  before  titrating  it. 

Mercuric  sulphate  {  and  magnesium  sulphate  may  also  be  used 
with  satisfactory  results. 

Potassium  permanganate  being  so  readily  decomposed  by  contact 
with  organic  matter,  should  be  protected  from  such  contact.  It 
should  never  be  filtered  through  paper  (glass-wool  or  guncotton 
may  be  used),  nor  should  it  be  used  in  a  Mohr's  burette  or  in  any 
other  apparatus  in  which  it  is  in  contact  with  rubber  or  cork.  Fur- 
thermore, all  substances  of  an  oxidizing  or  reducing  nature,  aside 

*  This  decomposition  of  the  permanganate  by  hydrochloric  acid  is  due  to  the 
presence  of  ferric  salt,  which  latter  seems  to  act  catalytically,  for  oxalic  acid  may 
be  accurately  titrated  with  permanganate  even  in  the  presence  of  hydrochloric 
acid,  no  chlorin  being  given  off.  Thus  the  decomposition  of  the  permanganate 
is  not  due  to  the  hydrochloric  acid  alone. 

f  Kessler  and  Zimmermann  suggest  using  20  cc.  of  a  solution  of  manganous 
sulphate  (200  gms.  per  liter.) 

%  Cady  and  Ruediger  (J.  A.  C.  S.,  XIX-575)  concluded  from  the  following 
general  principles  that  it  is  possible  to  titrate  iron  with  permanganate  in  the  pres- 
ence of  hydrochloric  acid  if  an  excess  of  mercuric  sulphate  be  added  to  the  solution. 
Mercuric  halids  in  solution  ionize  to  an  extremely  slight  extent,  while  the  mer- 
curic salts  of  oxyacids  are  readily  ionized,  since  compounds  of  slight  ionization 
always  result  when  their  constituent  ions  meet,  mercuric  halids  are  always  pro- 
duced when  a  mercuric  salt  of  an  oxyacid  is  added  to  a  solution  containing  halogen 
ions.  Therefore  when  mercuric  sulphate  solution  and  hydrochloric  acid  are 
mixed,  ionization  of  both  occurs,  and  the  mercuric  ions  unite  with  the  chlorin 
ions  and.  produce  mercuric  chlorid  which  is  only  very  slightly  ionized.  In  the 
presence  of  a  large  excess  of  mercuric  sulphate,  the  mercuric  ions  resulting  from 
its  dissociation  diminish  the  ionization  of  the  mercuric  chlorid  until  it  is  prac- 
tically nil.  Thus  no  chlorin  ions  will  be  present  in  the  solution  to  induce  de- 
composition of  the  permanganate.  The  method  is  described  in  Chapter  XXXIV. 


THE  TITRATION  METHODS  141 

from  that  being  analyzed,  must  be  excluded  from  the  solution.  Among 
such  substances  may  be  mentioned  hydriodic  acid,  sulphureted  hydro- 
gen, nitrous  acid  and  the  lower  oxids  of  nitrogen,  phosphorous  and 
hypophosphorous  acids,  thiosulphuric,  sulphurous,  and  all  the  other 
acids  of  sulphur  except  sulphuric,  also  ous  salts  and  the  metallic  sub- 
oxids  and  peroxids. 

Burettes  and  other  apparatus  which  have  been  used  for  perman- 
ganate, should  be  emptied  and  rinsed  immediately  after  use,  and 
any  manganic  oxid  which  may  be  adhering  to  the  glass  should  be 
removed  by  means  of  hydrochloric  acid  and  boiled  water. 

Not  only  oxidizable  substances  but  reducible  substances  may 
be  estimated  by  means  of  potassium  permanganate. 

In  the  estimation  of  oxidizable  substances  the  standard  potassium 
permanganate  is  added  directly  to  the  acidulated  solution  of  the  sub- 
stance being  analyzed.  The  completion  of  the  oxidation  being  then 
known  by  the  appearance  of  a  faint  pinkish  tint.  This  is  the  direct 
method. 

In  the  estimation  of  reducible  substances  (i.e.,  oxidizing  substances) 
the  indirect  or  the  residual  method  is  employed. 

In  this  an  accurately  weighed  or  measured  quantity  of  the  sub- 
stance is  brought  together  with  an  excess  of  a  third  substance  having 
reducing  power,  and  which  is  similarly  effected  by  the  permanganate 
and  by  the  substance  analyzed.  After  completion  of  the  reaction 
the  excess  of  the  reducing  substance  is  found  by  titration  with  standard 
permanganate.  The  difference  between  the  quantity  so  found  and 
that  originally  added  gives  the  quantity  which  reacted  with  the  salt 
under  analysis,  and  from  this  the  calculation  is  made. 

The  Titration  Methods  in  which  permanganate  is  employed 
are  classified  in  this  Chapter  under  the  following  headings: 

A.  Direct  Titrations. 

a.  Estimation  of  ferrous  salts; 

b.  Estimation  of  iron  in  ferrum  reductum; 

c.  Estimation  of  oxalic  acid  and  oxalates ; 

d.  Estimation  of  hydrogen  dioxid  and  barium  dioxid; 

e.  Estimation  of  ferric  salts  after  reduction; 
/.  Estimation  of  nitrous  acid  and  nitrites. 

B.  Residual  Titrations  and  Indirect  Methods. 

a.  Methods  involving  the  addition  of  an  excess  of  standard  per- 
manganate, and  retitration  with  standard  oxalic  acid,  as  estimation  of 
hypophosphites. 


142  A    MANUAL   OF    VOLUMETRIC    ANALYSIS 

b.  Methods  involving  a  precipitation  by  oxalic  acid  and  titration 
of  the  excess  of  the  latter  by  standard  permanganate,  as  estimation  of 
calcium,  gold,  and  lead. 

c.  Methods   involving  a  reduction  by  means  of  oxalic  acid  and 
titration  of  the  excess  of  the  latter  by  standard  permanganate,  as 
estimation  of  manganese  dioxid. 

d.  Methods  involving  a  reduction  by  means  of  a  ferrous  salt  and 
titration  of  the  remaining  unoxidized  ferrous  salt  by  standard  per- 
manganate, as  estimation  of  nitrates  (Peiouze)  and  chromates. 

e.  Methods  involving  the  oxidation  of  the  substance  analyzed  by 
means  of  a  ferric  salt  and  titration  of  the  resultant  ferrous  salt,  as 
estimation  of  tin  and  copper. 

Preparation  of  Decinormal  ( — )  Potassium    Permanganate 

N 
(2KMnO4=3i3-96;    :  -  V.  S.  =  3. 1396  gms.   in   i   liter).     Absolutely 

pure  potassium  permanganate  cannot  be  obtained,  therefore  the  prepa- 
ration of  a  decinormal  solution  of  this  salt  cannot  be  effected  by  simply 
dissolving  the  requisite  proportion  of  the  molecular  weight  in  the 
water.  The  presence  of  oxidizable  matter  in  the  water  used,  the 
contact  of  dust  and  exposure  to  light,  have  a  tendency  to  decompose 
the  salt  and  hence  weaken  the  standard  solution.  It  is  therefore 
advisable  to  use  boiling  distilled  water,  to  preserve  the  solution  in 
amber  glass  bottles,  provided  with  ground-glass  stoppers.  It  will 
then  retain  its  strength  for  several  weeks,  but  should  nevertheless  be 
checked  by  titration  immediately  before  using.  It  is  not  necessary, 
and  it  is  usually  undesirable,  to  make  the  solution  an  exact  deci- 
normal one.  It  is  preferable  to  fix  the  titer  of  the  solution  and  employ 
it  as  it  is. 

Place  3.5  gms.  of  pure  crystallized  potassium  permanganate  in  a 
flask,  add  1000  cc.  of  distilled  water,  and  boil  until  the  crystals  are 
dissolved;  put  a  plug  of  absorbent  cotton  in  the  mouth  of  the  flask 
and  set  it  aside  for  two  days  so  that  any  suspended  matters  may  de- 
posit. After  the  lapse  of  this  time  pour  off  the  clear  solution  into  a 
glass-stoppered  bottle,  and  when  wanted  for  use  standardize  by 
either  of  the  following  methods: 

Standardization  by  Means  of  Iron.  Thin  annealed  binding- 
wire,  free  from  rust,  is  one  of  the  purest  forms  of  iron.* 

*  This  contains  99.6  per  cent  of  iron. 


STANDARDIZATION  BY  MEANS  OF  IRON 


143 


o.i  gm.  of  such  iron  is  placed  in  a  flask  which  is  provided  with  a 
cork  through  which  a  piece  of  glass  tubing  passes,  to  the  top  of  which 
a  piece  of  rubber  tubing  is  attached,  which  has  a  vertical  slit  about 
one  inch  long  in  its  side,  and  which  is  closed  at  its  upper  end  by  a 
piece  of  glass  rod  (this  arrangement  is  known  as  the  "Bunsen  Valve"). 
(See  Fig.  54.)  Diluted  sulphuric  acid  is  added  and  gentle  heat  applied. 
The  iron  dissolves  and  the  steam  and  liberated  hydrogen  escape  through 
the  slit  under  slight  pressure.  The  air  is  thus  prevented  from  entering 
and  the  ferrous  solution  protected  from  oxidation. 

A  better  form  of  apparatus  in  which  to  dissolve  the  iron  and  avoid 
oxidation  through  admission  of  air  is  shown 
in  Fig.  55.  A  100  cc.  flask  is  fitted  with 

a  rubber  stopper    and    a  I  I  shaped 

glass  tube;  into  this  flask  is  placed  20  cc. 
of  diluted  sulphuric  acid  (1:5)  and  then 
2  or  3  crystals  of  pure  sodium  carbonate; 


FIG.  54. 


FIG.  55. 


this  causes  an  evolution  of  carbon  dioxid  which  expels  the  air  from 
'flask.  The  o.i  gm.  of  iron  wire  above  described  is  now  introduced, 
the  stopper  inserted,  and  a  beaker  containing  a  solution  of  pure 
sodium  carbonate  placed  in  position  so  that  the  tube  will  dip  into 
the  solution.  Gentle  heat  is  applied  until  the  iron  is  wholly  dissolved, 
and  only  a  few  minute  particles  of  carbon  remain  (which  must  not  be 
mistaken  for  iron).  When  the  flame  is  withdrawn,  the  cooling  of  the 
flask  and  contents  causes  a  drawing  up  of  the  sodium  carbonate 
solution,  but  the  first  drops  that  enter  the  flask  cause  an  effervescence 
with  evolution  of  carbon  dioxid,  which  drives  the  liquid  back  and  at 
the  same  time  fills  the  flask  with  the  gas;  this  is  repeated  until  the 
flask  and  contents  are  cold.  Another  useful  form  of  apparatus  for  this 
purpose  is  depicted  in  Fig.  56. 


144 


A    MANUAL  OF   VOLUMETRIC   ANALYSIS 


When  the  iron  is  completely  dissolved  a  small  quantity  of  cold, 
recently  boiled,  distilled  water  should  be  used  to  rinse  the  lower  end 

of  the  stopper  and  the  neck  of  the 
flask,  and  the  titration  with  potas- 
sium permanganate  at  once  begun 
and  continued  until  a  faint  permanent 
pink  color  is  produced.  If  the  solu- 
tion is  decinormal,  exactly  17.94  cc. 
will  be  required  to  produce  this 
result. 

The  iron  is  converted  by  the  sul- 
phuric acid  into  ferrous  sulphate, 
Fe2  +  2H2SO4=  2FeSO4  +  2H2.  This 
ferrous  sulphate  is  easily  oxidized 
by  the  air,  and  therefore  it  is  directed 
that  access  of  air  should  be  pre- 
vented, and  the  distilled  water  with 
which  the  solution  is  diluted  previ- 
ously boiled  in  order  to  drive  off  any 
dissolved  free  oxygen. 


FIG.  56. 

ioFeSO4  +  2KMnO4 

ioo)555  100)313.96 

5-55  gms- 


8H2SO 


N 


3.1396  gms.  or  1000  cc.  —  V.  S. 
10 

=  5Fe2(SO4)3+ K2SO4+  2MnSO4+ 8H2O. 


N 

This  equation,  etc.,  shows  that  each  cc.  of  —  permanganate  rep- 
resents 0.00555  gm.  of  metallic  iron. 

Standardization  by  Means  of  Oxalic  Acid.  0.06255  gm-  °f tne 
pure  crystallized  acid  is  weighed  (or  10  cc.  of  decinormal  oxalic  acid 
carefully  measured)  and  placed  in  a  flask,  with  some  dilute  sulphuric 
acid  and  considerable  water,  the  mixture  warmed  to  about  60°  C. 
(140°  F.),  and  the  permanganate  added  from  a  burette. 

The  action  is  in  this  case  less  decisive  and  rapid  than  in  the  titra- 
tion \vith  iron,  and  more  care  should  be  used.  The  color  disappears 
slowly  at  first,  but  afterwards  more  rapidly. 

Note  the  number  of  cc.  of  the  permanganate  solution  used,  and 
then  dilute  the  remainder  so  that  equal  volumes  of  decinormal  oxalic 
acid  and  decinormal  permanganate  solution  will  exactly  correspond. 


STANDARDIZATION  BY  MEANS  OF  OXALIC  ACID         145 

Example.  Assuming  that  9  cc.  of  the  permanganate  solution  first 
prepared  had  been  required  to  produce  a  permanent  pink  tint  when 

N 
titrated  into  10  cc.  of  -  -  oxalic-acid  solution,  then  the  permanganate 

must  be  diluted  in  the  proportion  of  9  of  permanganate  and  i  of  dis- 
tilled water,  or  900  and  100. 

The  U.  S.  P.,  1890,  gives  the  following  method  for  the  preparation 
of  this  solution: 

A  stronger  and  a  weaker  solution  is  made  and  mixed  in  certain 
proportions  to  form  a  solution  of  the  proper  strength.  It  is  said  that 
when  thus  prepared  the  solution  will  keep  its  titer  for  months  if  prop- 
erly preserved. 

The  Stronger  Solution.  3.5  gms.  of  pure  crystallized  permanga- 
nate are  dissolved  in  1000  cc.  of  water  by  the  aid  of  heat,  and  the  solu- 
tion then  set  aside  in  a  closed  flask  for  two  days,  so  that  any 
suspended  matters  may  deposit. 

The  Weaker  Solution.  Dissolve  6.6  gms.  of  the  salt  in  2200  cc. 
of  water  in  the  same  manner  as  above,  and  set  this  solution  aside  for 
'two  days. 

These  two  solutions  are  then  separately  titrated  in  the  following 
manner: 

Introduce  10  cc.  of  decinormal  oxalic -acid  solution  into  a  flask, 
add  i  cc.  of  pure  concentrated  sulphuric" acid,  and  before  the  mixture 
cools  add  the  permanganate  solution  slowly  from  a  burette,  shaking 
the  flask  after  each  addition,  and  towards  the  end  of  the  operation 
reducing  the  flow  to  drops.  When  the  last  drop  is  no  longer  decolorized, 
but  imparts  a  pinkish  tint  to  the  liquid,  the  reaction  is  completed. 
Note  the  number  of  cc.  consumed.  Finally,  mix  the  two  solutions  in 

N 
such  proportions  that  equal  volumes  of  the  mixture  and  of  —  oxalic 

acid  V.  S.  will  exactly  correspond. 

To  obtain  the  accurate  proportions  for  mixing  the  two  solutions, 
deduct  10  from  the  number  of  cc.  of  the  weaker  solution  consumed  in 
the  above  titration;  with  this  difference  multiply  the  number  of  cc. 
of  the  stronger  solution  consumed:  the  product  shows  the  number 
of  cc.  of  the  stronger  solution  needed  for  the  mixture. 

Then  deduct  the  number  of  cc.  of  the  stronger  solution  consumed 
in  the  titration  from  10,  and  with  the  difference  multiply  the  number 
of  cc.  of  the  weaker  solution  consumed:  the  product  shows  the  number 
of  cc.  of  the  weaker  solution  needed  for  the  mixture. 


146  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Or,  designating  the  number  of  cc.  of  the  stronger  solution  by  S, 
and  the  number  of  cc.  of  the  weaker  solution  by  W,  and  using  the  fol- 
lowing formula,  the  proportions  in  which  the  solutions  must  be  mixed 
are  obtained: 

Stronger  Solution.       Weaker  Solution. 

(W  -  io)S    +    (10  -  S)W. 

Example.     Assuming  that  9  cc.  of  the  stronger  and  10.5  cc.  of 

N 
weaker  had  been  consumed  in  decomposing  10  cc.  of  —  oxalic  acid 

V.  S.;   then,  substituting  these  values  in  the  above  formula,  we  obtain 
(10.5  -10)9+  (10-9)10.5,       or      4.5  +10.5, 

making  15  cc.  of  final  solution. 

The  bulk  of  the  two  solutions  is  now  mixed  in  the  same  proportion  : 
450  cc.  of  the  stronger  and  1050  cc.  of  the  weaker,  or  900  cc.  of  the 
stronger  and  2100  cc.  of  the  weaker. 

After  the  solutions  are  thus  mixed  a  new  trial  should  be  made, 

N 
when  10  cc.  of  the  solution  should  exactly  decompose  10  cc.  of  —  oxalic 

acid  V.  S. 

The  reaction  between  potassium  permanganate  and  oxalic  acid  is 
illustrated  by  the  following  equation  : 

Ct, 
2KMnO4+5(H2C2O4.2H2O)+3H2SO4= 

K2SO4+  2MnSO4+  ioCO2+  i8H2O. 

Standardization  by  the  lodometric  Method.  This  method, 
which  was  proposed  by  Volhard,  is  the  most  accurate  and  rapid  for  the 
standardization  of  permanganate.  It  is  based  upon  the  fact  that 
potassium  permanganate  reacts  with  potassium  iodid  in  solutions 
acidulated  with  either  hydrochloric  or  sulphuric  acid,  and  liberates 
an  equivalent  quantity  of  iodin,  which  may  be  estimated  by  standard 
solution  of  sodium  thiosulphate.  The  reactions  are  illustrated  by  the 
equations 


(a) 

313.96  1259 

(6) 


251.9      492.92 


FERROUS  AMMONIUM  SULPHATE  147 

Thus  it  is  seen  that  2KMnO4  (313.96  gms.)  containing  five  atoms 
of  available  oxygen,  has  the  power  of  liberating  its  equivalent  of  iodin, 
i.e.,  10  atoms  or  1259  gms-  (see  equation  a)  and  that  492.92  gms.  of 
sodium  thiosulphate  will  reduce  251.9  gms.  of  iodin  (see  equation  b). 

N 
Hence  1000  cc.  of  —  sodium  thiosulphate  (containing  24.646  gms.) 

will  reduce,  and  therefore  be  equivalent  to  12.59  gms-  °f  i°din  which 
in  turn  represents  3.1396  gms.  of  potassium  permanganate.  Therefore 

N 
i  cc.  of  the  —  thiosulphate  represents  0.01259    gm.  of   iodin   and 

0.0031396  gm.  of  potassium  permanganate,  which  latter  is  the  quantity 

N 
of  potassium  permanganate  present  in  i  cc.  of  its  —  V.  S. 

The  process  is  conducted  as  follows:  Into  a  200  cc.  flask  place 
about  0.5  gm.  of  potassium  iodid  and  10  cc.  of  diluted  sulphuric  acid, 
add  to  this  (slowly  from  a  burette)  exactly  10  cc.  of  the  permanganate 
solution  to  be  standardized  and  dilute  the  mixture  (which  is  brown  in 
color,  because  of  the  liberated  iodin)  with  distilled  water  to  about 
150  cc.  Then  slowly  titrate  (with  constant  stirring)  with  an  accu- 

N 
rately  standardized  -  -  sodium  thiosulphate  until  the  color  of  the 

solution  is  a  faint  yellow,  then  add  a  few  drops  of  starch  solution 
and  continue  the  titration  until  the  color  is  discharged.  Note  the 
number  of  cc.  consumed  and  diltue  the  permanganate  with  distilled 
water  so  that  equal  volumes  of  the  two  solutions  correspond  to  each 
other. 

Example.  If  13  cc.  of  the  thiosulphate  solution  were  required, 
then  each  10  cc.  of  the  permanganate  solution  must  be  diluted  to 
13  cc. 

Standardization  with  Ferrous  Ammonium  Sulphate  (Mohr's 
salt)  (FeSO4.(NH4)2SO4.6H2O).  389.34  gms.  of  this  salt  contains 
55.5  gms.  iron  (3.507  gms.  contain  0.5  gm.  of  iron).  3.507  gms.  of 
the  salt  are  accurately  weighed  out  and  dissolved  in  sufficient  recently 
boiled  distilled  water  to  make  250  cc.  50  cc.  of  this  solution  containing 
o.i  gm.  of  iron  are  transferred  to  a  small  flask,  10  cc.  of  diluted  sul- 
phuric acid  added,  and  then  the  permanganate  solution  to  be  standard- 
ized is  run  in  slowly  until  a  faint  pinkish  tint  appears.  Whatever 
number  of  cc.  is  consumed  that  number  represents  o.i  gm.  of  iron, 
and  must  be  diluted  to  18.02  cc.  to  make  the  solution  exactly  deci- 
normal. 


148  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

Standardization  by  Means  of  Sodium  Oxalate.  Sodium  oxalate 
may  be  used  in  the  same  manner  as  oxalic  acid,  the  salt  is  anhydrous  and 
may  be  accurately  weighed,  but  it  offers  no  advantage  over  oxalic  acid. 

Standardization  with  Hydrogen  Peroxid  in  the  Nitrometer 
is  proposed  by  Lunge,  J.  S.  C.  I.,  IX,  21.  The  volume  of  oxygen 
given  off  corrected  for  temperature  and  pressure,  and  calculated  into 
weight,  gives  the  weight  of  the  permanganate  present. 

On  the  Use  of  Empirical  Permanganate  Solutions. 
A.  If  the  standardization  of  the  solution  is  done  by  means  of  iron  as 
described  on  page  142,  o.i  gm.  of  iron  wire  (representing  0.0996  gm.  of 
pure  iron)  will  require  17.94  cc.  of  the  permanganate  solution  if  the 
latter  is  exactly  decinormal.  If  less  than  this  quantity  of  solution  is 
used  (say  17.5  cc.)  it  indicates  that  the  solution  is  stronger  than  deci- 
normal, and  may  either  be  diluted  so  that  each  17.5  cc.  will  measure 
17.94,  or  it  may  be  used  as  it  is.  This  in  most  cases  is  preferable. 
The  value  of  one  cc.  of  the  solution  in  iron  is  calculated  thus: 

17.5  cc.  :  i  cc.  :  :  0.0996  gm.  :  x.        #=0.00569  gm. 

If  a  solution  of  this  strength  is  to  be  used  for  the  estimation  of 
iron,  simple  multiplication  of  the  number  of  cc.  used  by  0.00569  gm. 
gives  the  weight  of  Fe  present.  If,  however,  this  solution  is  employed 
for  the  titration  of  other  oxidizable  substances,  the  number  of  cc. 
consumed  is  multiplied  by  0.00569  gm.  and  then  by  a  fraction  in  which 
the  numerator  represents  a  quantity  of  the  substance  examined,  equiva- 
lent in  grams  to  an  atom  of  iron  in  its  reaction  with  permanganate, 
and  the  denominator  is  the  atomic  weight  of  iron. 

Example.  If  in  a  titration,  we  use  40  cc.  of  a  permanganate 
solution,  the  titer  of  which  has  been  found  to  be  i  cc.  =  0.00569  gm., 
the  calculation  would  be: 

in  the  case  of  ferrous  sulphate  (FeSO4,  150.85), 

40X0.00569  gm.X—  —  =0.6184  gm.; 


in  the  case  of  oxalic  acid  (H.2^2^4-  2H2O  =  125.10), 

40X0.00569  gm.X  —  :  —  =  0.2564  gm.; 

in  the  case  of  hydrogen  dioxid  (11202  =  33.76.), 

16.88 
40X0.00569  gm.X—  —  -  =  0.0692  gm. 


USE  OF  EMPIRICAL  PERMANGANATE  SOLUTIONS        149 

B.  Another  way.  The  solution  just  mentioned,  of  which  17.5000. 
are  consumed  in  titrating  o.i  gm.  of  iron  wire,  is  compared  with  a 
true  decinormal  permanganate  solution,  of  which  17.94  cc.  are  con- 
sumed in  the  same  reaction.  The  strength  of  the  former  solution  is 

therefore  —  -  as  compared  with  a  decinormal  solution. 
1750 

In  titrating  with  this  solution  the  number  of  cc.  consumed  are  to 


be  multiplied  by  --  and  then  by  the  true  decinormal  factor  for  the 

substance  being  analyzed. 

Example.     40  cc.  of  the  solution  are  consumed. 

In  the  case  of  ferrous  sulphate  (FeSO4  =  150.85),  the  decinormal 
factor  (i.e.,  the  weight  of  ferrous  sulphate  represented  by  i  cc.  of  a 
true  decinormal  solution)  is  0.015085  gm. 

—  —  Xo.oi5o85  gm.  =  o.6i84  gm. 

In  the  case  of  oxalic  acid  (H2C2O4,2H2O  =  125.10)-,  the  decinormal 
factor  is  0.006255  gm. 

1704 
40  X  —  —  X  0.006255  gm.  =  0.2564  gm. 

In  the  case  of  hydrogen  dioxid  (H2O2=33.76),  the  decinormal  factor 
is  0.001688  gm. 

4oX—  -X  0.001688  gm.  =  0.0692  gm. 

C.  If  the  standardization  is  done  by  means  of  oxalic  acid,  as 
described  on  page  144  in  which  10  cc.  of  a  strictly  decinormal 
oxalic  acid  solution  are  titrated  with  the  permanganate  solution  which 
is  being  standardized,  exactly  10  cc.  of  the  latter  will  be  consumed 
if  it  is  of  decinormal  strength.  If  in  the  trial,  however,  it  is  found 
that  only  9.6  cc.  are  consumed  it  indicates  that  the  solution  is  stronger 

than   decinormal;    its  strength  being  expressed  by  —  —  .     If,  on  the 
other  kand,  more  than  10  cc.  of  the  solution  are  consumed  (say  10.4  cc.) 

the  solution  is  below  decinormal  strength,  namely,  •  -  . 

104 

In  using  a  solution  of  the  first  strength  the  number  of  cc.  of  it 
consumed  in  any  titration  is  to  be  multiplied  by  —  —  and  then  by  the 


150  A    MANUAL   OF   VOLUMETRIC    ANALYSIS 

decinormal  factor  for  the  substance  examined.     In  the  case  of  the 

100 

weaker  solution  the  number  of  cc.  consumed  is  multiplied  by  — -  and 

104 

then  by  the  decinormal  factor  for  the  substance  being  analyzed. 

Examples.  Ferrous  sulphate  (FeSO4  =  150.85),  is  titrated  with 
the  stronger  solution,  40  cc.  of  the  latter  being  consumed. 

Then,  40 X— -r X 0.015086  gm.  =  o.628  gm. 

Oxalic  acid  (H2C2O4 .  2H2O  =  125.10),  40  cc.  are  consumed. 
Then,  4oX  -7- X 0.006255  gm.  =  0.260  gm. 

Hydrogen  dioxid  (11202=33.76),  40  cc.  are  consumed. 

Then,  40 X— 7-  X  0.001688  gm.  =  0.0703  gm. 

90 

D.  If  the  checking  of  the  permanganate  solution  is  done  by  the 
iodometric  method  (page  146)  and  it  is  found  that  10  cc.  of  the  per- 
manganate requires  the  use  of  13  cc.  of  decinormal  thiosulphate  solu- 
tion; the  titer  of  the  solution  is  expressed  with  reference  to  deci- 
normal as  — .  In  using  a  solution  of  this  strength,  the  number  of  cc. 

of  it  consumed  in  an  analysis  is  multiplied  by  —  and  then  by  the 

10 

decinormal  factor  for  the  substance  analyzed. 


TYPICAL  ANALYSES   WITH   PERMANGANATE 

A.  Direct  Ti< 'rations. 

a.  Estimation  of  Ferrous  Sulphate  (FeSO4+7H2O  =276.01). 
i  gm.  of  ferrous  sulphate  is  dissolved  in  25  cc.  of  water  and  the 
solution  strongly  acidulated  with  sulphuric  acid.  Decinormal  potas- 
sium permanganate  is  then  delivered  from  a  burette  until  a  per- 
manent pink  tint  is  obtained,  indicating  the  complete  oxidation  of  the 
ferrous  salt. 


FERROUS  SULPHATE  151 

The  reaction  is  as  follows: 


(ioFeSO4  +  7H2O)  +  2KMnO4  +  8H2SO4 

100)2760.1  100)313.96  ^ 

27.601  gms.       =  3.1396  gm.  =  1000  cc. — V.  S. 

10 

N 

2."6oi  gms.     =  100  cc.  —  V.  S. 

10 

N 

0.027601  gm.  *=  i  cc.  —  V.  S. 

10 


Thus  313.96  gms.  of  permanganate  =2 760. i  gms.  of  crystallized 

N 

ferrous  sulphate,  which  equals  5^5  gms.  of  metallic  iron,     i  cc.  of  — 

10 

permanganate     solution      therefore      represents     0.027601     gm.     of 
FeSO4  +  7H2O    or    0.00555  &m-  °f  Fe. 

N 
In  the  analysis   36  cc.  of  the  —  permanganate  were  consumed. 

The  i  gm.  taken  then  contains  36X0.027601=0.993636  gm.  or  99.36 
per  cent. 


i  :  0.993636  : :  100  :  x.        #=99.36+  per  cent. 

If  it  is  desired  that  each  cc.  of  the  permanganate  solution  should 
represent  a  certain^  percentage  of  pure  salt,  a  molecular  quantity  of 
the  salt  should  be  taken  for  analysis  instead  of  i  gm.  For  example, 
if  2.7601  gms.  be  taken,  each  cc.  of  the  decinormal  solution  con- 
sumed will  correspond  to  i  per  cent,  because  2.7601  gms.  is  the  weight 
of  crystallized  ferrous  sulphate  which  can  be  oxidized  by  100  cc.  of 
the  decinormal  solution.  If  half  of  this  weight  be  taken,  i.e.,  1.38  gm. 
each  cc.  of  the  permanganate  solution  compound  will  represent  2  per 
cent  of  pure  salt. 

Granulated  Ferrous  Sulphate  (FeSO4+7H2O)  is  estimated  in 
the  same  way  as  the  foregoing,  and  should  correspond  with  it  in 
strength. 

Exsiccated  (Dried)  Ferrous  Sulphate.  This  salt  is  tested  in  the 
same  manner  as  the  other  two  sulphates.  It  contains  a  larger 
percentage  of  ferrous  sulphate  than  the  other  two,  having  less 


152  A    MANUAL  OF    VOLUMETRIC   ANALYSIS 

water      of      crystallization.       Its      composition      is      approximately 
FeS04+3H20. 

ioFeSO4     +     2KMnO4     +     8H2SO4 

100)1508.5  10    313.96 

15-085  gms-  3-1396  gms-  or  1000  cc.  —  standard  solution. 

10 


Each  cc.  of  the  standard  solution  represents  0.015085  gm.  of  anhy- 
drous (real)  ferrous  sulphate.  If  one  gm.  of  the  dried  salt,  treated 

N 
as  above  described,  requires  48  cc.  of  -  -  permanganate  solution,  it 

contains 

0.015085X48  =  0.72408  gm., 

or  72.40  per  cent  of  real  ferrous  sulphate,  and  100.00  —  72.40=27.60 
per  cent  of  water  of  crystallization. 

Saccharated  Ferrous  Carbonate.  The  U.  S.  P.  directs  that  1.15 
gms.  of  saccharated  ferrous  carbonate  be  dissolved  in  10  cc.  of 
diluted  sulphuric  acid,  then  diluted  with  water  to  about  100  cc.,  and 

N 
then  titrated  with  —  potassium  permanganate  V.  S.  until  a  pink  tint 

is  produced  in  the  liquid.  15  cc.  should  be  required.  This  method 
is  not  an  exact  one,  especially  if  heat  is  applied  for  solution  of  the 
powder,  in  that  permanganate  is  reduced  by  the  sugar  present. 

b.  Estimation  of  Metallic  Iron  in  Ferrum  Reductum.  Ferrum 
reductum  (reduced  iron)  always  contains  besides  metallic  iron  a  vary- 
ing quantity  of  oxid.  Therefore,  in  assaying  this  preparation  a 
method  must  be  employed  which  will  estimate  the  iron  only,  which  is 
present  as  metallic  iron.  This  may  be  done  by  means  of  a  solution 
of  mercuric  chlorid  which  reacts  with  metallic  iron  only  and  not  with 
the  oxid. 

The  method  is  as  follows: 

0.555  gm.  of  reduced  iron  is  introduced  into  a  glass-stoppered 
bottle,  50  cc.  of  mercuric  chlorid  solution  (5  gms.  in  100  cc.)  ate  added 
and  the  bottle  heated  on  a  water  bath  for  one  hour,  agitating  fre- 
quently, but  keeping  the  bottle  well  stoppered. 

2HgCl2  +  Fe2  =  2FeCl2  +  2Hg, 

then  allow  it  to  cool,  dilute  the  contents  with  water  to  100  cc., 
and  filter.  Take  10  cc.  of  the  filtrate  (representing  0.0555  gm.  of 


OXALIC  ACID  AND  OXALATE  153 

reduced  iron)  add  to  it  10  cc.  of  diluted  sulphuric  and  10  cc.  of  a  solu- 
tion of  manganous  sulphate  (1:5),  introduce  the  mixture  into  a  glass  - 
stoppered  bottle  (having  a  capacity  of  100  cc.),  and  titrate  with  deci- 
normal  permanganate  until  a  permanent  pink  color  is  obtained.  Each 
cc.  of  the  permanganate  solution  represents  0.00555  gm.  of  metallic 
iron  or  10  per  cent. 


ioo)555  100)313.96  ^ 

5.55  gms.       =      3-I396  gms.=  1000  cc.  —  V.  S. 

10 

°-00555  gms.=     0.0031396  gms.  =  i  cc.  —  V.  S. 

N 
If  9  cc.  of  the  —  permanganate  are  consumed,  then  9X0.00555 

=0.04995  gm.,  and  since  the  10  cc.  of  the  iron  solution  taken  for 
analysis  represented  0.0555  gm.,  the  per  cent  of  metallic  iron  present  is 
90. 

0.0555  :  IO°  :  :  °-°4995  :  x'        x=go. 

The  use  of  manganous  sulphate  in  this  process  is  to  prevent  de- 
composition of  permanganate  by  the  hydrochloric  acid.  The  quantity 
is,  however,  so  small,  that  if  the  titration  be  conducted  cold,  its  use 
is  unnecessary. 

Titration  with  an  Empirical  Permanganate  Solution.  A  solution 
of  permanganate  which  is  found  upon  standardization  to  be  of  a 
strength  in  which  i  cc.  is  equivalent  to  0.00512  gm.  of  Fe  is  to  be 
used. 

Each  cc.  of  this  solution  is  equivalent  to  the  following  quantities: 

FeSO4  ...................  0.013916  gm. 

FeSO4+7H20  ............  0.025462  " 

FeC03  ...................  0.013934  " 

FeCl2  ..................  --  0.01161  " 

Fe  .......................  0.00572  " 

c.  Estimation  of  Oxalic  Acid  and  Oxalates  with  Potassium 
Permanganate  Solution  (H2C2O4+2H2O=i25.io;H2C2O4=89.34). 
The  estimation  of  oxalic  acid  may  be  accurately  made  either  by 
neutralization  with  a  standard  alkali  or  by  oxidation  with  standard 
permanganate.  The  latter  method  is,  however,  the  one  to  be  employed 
in  the  case  of  oxalates. 


154  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

The,  oxidimetric  estimation  of  oxalic  acid  is  carried  out  as  follows: 

i  gm.  of  the  acid  (accurately  weighed)  is  dissolved  in  sufficient 
water  to  make  100  cc.  Of  this  solution,  10  cc.  (representing  o.i  gm. 
of  the  acid)  is  taken  for  analysis.  2  cc.  of  diluted  sulphuric  acid  are 
added,  the  solution  is  heated  to  between  40°  C.  and  60°  C.,  and  keeping 
it  at  about  this  temperature  is  titrated  with  decinormal  potassium 
permanganate,  agitating  constantly,  until  a  faint  rose  tint  marks  the 
completion  of  the  reaction. 

Each  cc.  of  the  permanganate  solution  consumed  represents 
0.006255  gm.  of  crystallized  oxalic  acid. 

The  reaction  is  as  follows: 


100)625.5  100)313.96  N 

6.255  gms.       =  3-1396  gms.       =iooocc.  —  V.  S. 

10 

N 
0.6255  gm.      =  0.31396  gm.       =    100  cc.  —  V.  S. 

N 
0.006255  gm.  =  0.0031396  gm.  =        i  cc.  —  V.  S. 


Direct  Percentage  Titration.     0.6255  gm.  of  crystallized  oxalic  acid 

N 
is  oxidized  by  100  cc.  of  —  permanganate.     Therefore  if  0.6255  gm. 

N 
of  the  acid  is  taken   for  analysis,  each  cc.  of  —  permanganate  will 

represent  i  per  cent. 

Titrating  with  an  Empirical  Solution.  If  the  permanganate  is 
checked  with  iron,  we  take  into  consideration  that  2KMnO4  will 
oxidize  10  atoms  of  iron  (555  parts),  and  on  the  other  hand  5  molecules 
of  oxalic  acid  (625.5  parts).  If  the  titer  of  the  permanganate  be 
found  on  experiment  to  be  i  cc.  =0.00569,  whatever  number  of  cc. 
of  this  solution  is  consumed,  is  to  be  multiplied  by  0.00569  and 

then  by  ^« 
55-5 

Example.  0.3  gm.  of  oxalic  acid  require  for  oxidation  40  cc.  of  a 
permanganate  solution  whose  titer  is  i  cc.  =0.00569  gm.  Fe,  the  cal- 
culation is  made  as  follows  : 

40X0.00569  gm.X  —  —  =0.2564  gm. 

D  D  *  o 


OXALATES  155 

0.2564  gm.  is  the  quantity  of  pure  crystallized  oxalic  acid  present 
in  the  0.3  gm.  taken  for  analysis.     This  is  85.46  per  cent. 


0.2564X100 
0.3 


If  the  standardization  of  the  permanganate  is  done  by  means  of 
a  decinormal  oxalic  acid,  or  by  the  iodometric  method,  the  calcula- 
tion is  as  described  on  pages  149  and  150. 

Oxalates  are  estimated  in  the  same  manner;  a  much  larger  quantity 
of  sulphuric  acid  is,  however,  required.  This  serves  to  liberate  the 
oxalic  acid  from  its  combination. 

The  presence  of  precipitates  of  sulphates  of  calcium,  barium,  or 
lead  does  not  interfere  with  the  recognition  of  the  end-point. 

N 
Each  cc.  of  —  potassium  permanganate  represents 

Oxalic  acid  anhydrous  (H2C2C>4) 0.004467  gm. 

Oxalic  acid  crystallized  (H2C2O4+2H2O) 0.006255   " 

This  method  may  be  applied  to  calcium  salts  which  are  soluble 
in  water  or  in  acetic  acid,  as  well  as  to  other  metals  which  are  pre- 
cipitable  as  oxalates.  It  is,  however,  especially  applicable  to  cal- 
cium, because  of  the  readiness  with  which  this  metal  may  be  separated 
from  others  as  oxalate.  The  precipitation  may  be  accomplished 
in  either  an  ammoniacal  or  a  weak  acetic  acid  solution.  If  it  is  neces- 
sary to  dissolve  the  calcium  salt  with  the  aid  of  hydrochloric  acid, 
the  solution  must  be  rendered  strongly  alkaline  with  ammonia,  and 
the  precipitation  effected  with  ammonium  oxalate. 

Example.  0.2  gm.  of  calcium  carbonate  is  dissolved  in  the  smallest 
necessary  quantity  of  dilute  acetic  acid,  then  sufficient  oxalic  acid  is 
added  to  completely  precipitate  the  calcium  as  oxalate  (CaC2O4). 
This  precipitate  is  then  thoroughly  washed  on  the  filter.  A  hole  is 
then  made  in  the  filter  and  the  precipitate  washed  through  the  funnel 
into  a  flask  (about  100  cc.  of  water  being  used)  a  small  quantity,  say 
about  2  cc.  of  dilute  sulphuric  acid  are  added,  the  mixture  warmed  to 
between  40°  C.  and  60°  C.  and  titrated  with  decinormal  potassium 
permanganate  until  a  permanent  rose  tint  appears. 

Each  cc.    of  the    decinormal   permanganate    represents   0.004467 


156  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

gm.    of   oxalic   acid  (anhydrous)  or  0.0049675  gm.   of  calcium  car- 
bonate. 

N 
If  in  the  above  assay,  39.5  cc.  of  the  —  permanganate  were  em- 

ployed, then  0.0049675  gm.  X  39.6  =0.195  7  -f-gm.;    this  is  the  quanity 
of  pure  CaCOs  in  the  0.2  gm.  taken  for  analysis. 


o.io<;7ioo 

2&$  --  =  97  .85  Per  cent. 

If  the  substance  to  be  examined  contains  iron,  aluminum,  man- 
ganese, etc.,  these  metals  must  be  removed  before  titrating. 

Example.  Dolomite  (which  contains  besides  calcium  carbonate 
some  iron,  aluminum,  and  magnesium)  is  to  be  analyzed.  A  weighed 
quantity  is  dissolved  in  hydrochloric  acid,  and  an  excess  of  ammonia 
water  is  added.  (If  the  ammonia  water  contains  any  carbonate,  the 
precipitate  must  be  again  dissolved  and  reprecipitated).  The  strongly 
ammoniacal  nitrate  is  treated  with  excess  of  oxalic  acid,  to  induce 
complete  precipitation.  The  precipitate  is  then  collected  on  a  filter, 
thoroughly  washed,  and  oxalic  acid  estimated  by  titration  with 
permanganate  solution. 

The  calculation  is  based  upon  the  fact  that  a  molecular  weight  of 
oxalic  acid  (125.1)  will  precipitate  a  molecule  of  CaO  (55.68).  The 

quantity  of  oxalic  acid  found  is  then  multiplied  by  —  -  —  . 

If  an  empirical  permanganate  solution  is  employed  the  titer  of 
which  is  i  cc.  =0.00569  gm.  Fe,  and  42  cc.  of  this  be  consumed,  we 
calculate  as  follows: 

0.00569X42X^^X^^=0.139+  gm.  CaO. 
125.10      55.5 

If,  as  in  the  case  of  dolomite  a  large  quantity  of  magnesium  is 
present,  some  of  the  difficultly  soluble  magnesium  oxalate  will  be  in 
the  precipitate,  mixed  with  the  calcium  oxalate;  this  may  be  removed 
by  dissolving  the  precipitate  in  diluted  acetic  acid  and  reprecipitating 
with  pure  ammonia. 

d.  Estimation  of  Hydrogen  Dioxid  and  Barium  Dioxid  with 
Standard  Potassium  Permanganate.  Hydrogen  Dioxid  (Hydro- 
gen Peroxid;  Aqua  Hydrogenii  Dioxidi,  U.  S.  P.)  (^02=33.76). 


HYDROGEN    DIOXIDE  157 

This  substance  was  official  for  the  first  time  in  the  U.  S,  P.,  1890, 
in  which  methods  for  its  preparation,  preservation,  and  assay  were 
given.  The  official  solution  contained  3  per  cent  by  weight  of  pure 
hydrogen  dioxid,  which  corresponds  to  10  volumes  of  available  oxygen. 

Solution  of  hydrogen  dioxid  is  an  important  commercial  product, 
being  used  in  the  arts  as  well  as  in  medicine. 

It  is  sold  as  containing  5,  10,  15,  or  20  volumes  of  oxygen  in  solu- 
tion. This  should  mean  that  a  given  volume  of  the  solution  yields 
,from  itself  5,  10,  15,  or  20  times  its  own  volume  of  oxygen. 

Thus,  i  cc.  of  a  5-volume  solution  yields  5  cc.  of  oxygen;  a  lo-volume 
solution  is  one  of  which  i  cc.  will  yield  10  cc.  of  oxygen,  etc. 

Many  solutions  of  hydrogen  dioxid  are  sent  into  the  market  under 
false  pretences,  being  labeled  as  containing  10,  15,  or  20  volumes 
of  oxygen. 

It  is  true  a  given  volume  of  these  solutions  will  yield  the  specified 
volume  of  oxygen  when  decomposed  with  potassium  permanganate, 
but  half  of  this  oxygen  comes  from  the  permanganate  itself.  There- 
fore the  hydrogen  dioxid  solution  contains  only  half  as  much  avail- 
able oxygen  as  is  given  off  in  this  decomposition. 

Freshly  bought  samples  of  the  five  largest  manufacturers,  accord- 
ing to  the  analyses  of  Dr.  Edward  R.  Squibb  (Ephemeris,  vol.  iv. 
No.  2),  gave  9.2,  8.7,  8.4,  10.9,  9.7,  8.6,  8.5,  7.3,  and  7.4  volumes. 
All  of  these  were  labeled  as  being  of  15  volumes  strength.  The 
author  has  had  a  similar  experience. 

In  its  purest  and  most  concentrated  form  hydrogen  dioxid  is 
a  syrupy  colorless  liquid,  having  an  odor  resembling  that  of  chlorin  or 
ozone. 

One  cc.  of  this  concentrated  hydrogen  dioxid  when  decom- 
posed at  o°  C.  evolves  330.3  times  its  own  volume  of  oxygen,  at  a  pres- 
sure of  760  mm.  at  45°  N.  latitude. 

At  a  temperature  of  100°  C.  (212°  F.)  H2O2  decomposes  rapidly 
into  water  and  oxygen.  This  change  also  takes  place  at  ordinary 
temperatures,  but  more  slowly.  In  diluted  solutions  it  is  more  stable, 
and  may  be  concentrated  by  boiling  without  suffering  much  decom- 
position. 

Dr.  Squibb  made  a  series  of  experiments  in  order  to  prove  this, 
as  well  as  the  fact  that  solutions  of  hydrogen  dioxid  when  kept  in 
o^en  vessels  at  the  ordinary  temperature  become  stronger  instead  of 
weaker,  as  was  generally  supposed.  The  water  evaporates  more 
rapidly  than  the  dioxid  decomposes.  Part  of  the  results  of  these 


158  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

experiments  as  published  in  the  Ephemeris,  vol.  iv,  No.  2,  is  as 
follows : 

A  freshly  made  solution  that  yielded  10.3  volumes  of  available 
oxygen  was  taken  as  the  basis  of  the  experiment.  The  evaporation 
was  done  on  a  water-bath,  at  temperatures  varying  from  55°  to  62°  C. 
(131°  to  143.6°  F.),  one  cc.  of  the  concentrated  solution  being  taken 
out  for  testing  after  each  evaporation. 

200  cc.  evaporated  in  2  hours  to  100  cc.  tested  20.6  volumes:  no 
apparent  loss. 

100  cc.  of  the  io.3-volume  solution  were  added,  and  evaporated 
in  2  hours  to  100  cc.,  tested  29.6  volumes:  1.3  volumes  loss. 

100  cc.  of  the  io.3-volume  solution  were  added,  and  evaporated 
in  2  hours  to  100  cc.,  tested  36.5  volumes:  4.7  volumes  loss. 

100  cc.  of  the  1 0.3 -volume  solution  were  added,  and  evaporated  in 
2.5  hours  to  23  cc.,  tested  146.8  volumes. 

Another  series  of  evaporations  were  made  at  higher  temperatures, 
which  also  showed  an  increase  in  strength,  but  the  loss  was  a  little 
larger. 

Hydrogen  dioxid  and  potassium  permanganate,  though  both 
oxidizing  agents,  will,  when  mixed  in  an  acid  solution,  reduce  each 
other.  The  reaction  which  occurs  is  probably  primarily  an  oxidation 
of  the  H2C>2  to  a  higher  oxid  (H^C^  (?))  which,  however,  immedi- 
ately breaks  up  with  the  liberation  of  oxygen.  The  method  of  assay- 
ing hydrogen  dioxid  by  means  of  permanganate  is  applicable  not 
only  to  this  substance  but  also  to  the  estimation  of  barium  dioxid 
and  the  soluble  alkali  peroxids.  The  method  is  usually  carried  out 
by  adding  the  permanganate  solution  to  the  dioxid  in  a  solution  acidu- 
lated with  sulphuric  acid.  Immediate  decolorization  of  the  perman- 
ganate occurs,  as  long  as  any  hydrogen  dioxid  is  present.  When  the 
latter  has  been  entirely  taken  up  the  permanganate  is  no  longer  de- 
colorized and  a  faint  pink  tint  marks  the  end-point.  In  the  estima- 
tion of  the  pharmacopceial  or  commercial  dioxid  solutions,  containing 
2  or  3  per  cent  of  H2O2,  a  measured  quantity  is  taken  for  analysis. 
The  specific  gravity  of  the  solution  being  nearly  that  of  water  i  cc. 
is  taken  to  represent  i  gm.  In  the  case  of  solutions  of  hydrogen 
dioxid  of  high  percentage  strength  it  is  advisable  to  take  a  weighed 
quantity  for  analysis.  If  hydrochloric  acid  is  present  a  small 
quantity  of  manganese  sulphate  should  be  introduced  before 
titrating. 

The  assay  is  conducted  as  follows: 


HYDROGEN    DIOXID  159 

10  cc.  of  the  solution  are  accurately  measured  and  diluted  (in  a 
graduated  cylinder)  with  water  to  make  100  cc.  10  cc.  of  this  diluted 
liquid  (containing  i  cc.  of  hydrogen  dioxid  solution)  are  transferred 
to  a  beaker,  5  cc.  of  diluted  sulphuric  acid  (1-8)  are  added  and  then 

N 
the  -  -  permanganate  solution  run  in  from  a  burette,  stirring  after 

each  addition  until  a  permanent  faint  pink  tint  appears.  The  reaction 
is  as  follows: 

5H2O2    +    2KMnO4  +  3H2SO4=  K2SO4  +  2MnSO4  +  8H2O  +  sO2. 

100)168.8         100)313.96 

1.688  gms.         3-1396  gms.=  1000  cc.  —  permanganate  V.  S. 

10 

N 
0.001688  gm.  =  0.003 1396  gm.=   i  cc.  —  permanganate  V.  S. 

N 
Thus  each  cc.  of  —  permanganate   represents   0.001688    gm.   of 

absolute  hydrogen  dioxid.  Assuming  that  in  the  above  estimation 
19  cc.  of  the  permanganate  solution  were  required,  then  the  i  cc. 
taken  for  analysis  contained  0.001688  gm.  Xip,  which  is  0.032072  gm. 
of  absolute  H2O2.  This  corresponds  to  3.2072  per  cent. 

The  Direct  Percentage  Method.  10  cc.  of  the  solution  are  diluted 
with  water  to  measure  100  cc.  16.88  cc.  of  this  diluted  solution  (con- 
taining 1.688  cc.  of  hydrogen  dioxid)  are  acidulated  with  sulphuric 
acid  and  titrated  with  decinormal  permanganate  as  above  described. 
Each  cc.  of  the  permanganate  solution  consumed  will  represent  o.i  per 
cent  by  weight  of  H2O2. 

Titration  with  an  Empirical  Solution.  A  permanganate  solution  is 
on  hand  which  is  found  upon  standardization  with  iron  to  be  i  cc. 
=0.00569  gm.  Fe.  To  use  this  solution  as  it  is,  we  take  into  con- 
sideration that  2KMnO4  =  (313.96)  =10  atoms  of  Fe  (555)  and  also 
5  molecules  of  H2O2  (168.8).  31.396  gm.  KMnO4=55.5gm.Fe  =  i6.88 
gm.  H2O2.  Whatever  number  of  cc.  of  this  permanganate  solution  is 

^  f\  88 

used  multiplied  by  0.00569  gm.  and  then  by  —     —  will  give  the  weight 

of  H2O2  present  in  the  sample  analyzed.     See  page  148. 

Estimation  oj  Volume  Strength.  Let  us  look  at  the  above 
equation  in  a  different  light. 

We  there  see  that  when  potassium  permanganate  and  hydrogen 
dioxid  react,  10  atoms  of  oxygen  are  liberated. 

The  permanganate  itself  when  decomposed  liberates  five  atoms  of 


160  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

oxygen.     Therefore  of  the  above  ten  atoms  only  five  come  from*  the 
hydrogen  dioxid. 


In  order  to  find  the  factor  for  volume  of  available  oxygen,  see  the 
following  equation,  etc.: 


100)313.96  N  100)79.40 

3.1396  gms.  or  1000  cc.  —  V.  S.=  °-794  gm. 

10 

N 
i  cc.  —  V.  S.=  0.000794  gm. 

N 
Thus  it  is  seen  that  each  cc.  of  -  -  potassium  permanganate  V.  S. 

represent  0.000794  gm.  of  oxygen.     But  we  require  to  find  the  volume 

N 
of  oxygen,  not  the  weight  represented  by  i  cc.  of  -  -  permanganate. 

1000  cc.  of  oxygen  at  o°  C.  and  760  mm.  pressure,  weigh  1.427612 
gms.  Therefore,  if  1.427612  gms.  measure  1000  cc.,  0.000794  gm. 
will  measure  x.  #=0.556  +  0:. 

1000X0.000794 

—  —  ^^=0.556  cc. 
1.427612 

The  factor,  then,  for  volume  of  oxygen,  liberated  when  hydrogen 

.N 
dioxid    is    titrated   with    —   potassium   permanganate  is  0.556,  and 

N 
the  number  of  cc.  of  the  —  potassium  permanganate  consumed  in  the 

titration  gives  the  volume  of  oxygen  liberated  by  the  quantity  of  hy- 
drogen dioxid  taken. 

N 

Thus  if  19  cc.  of  the  —  V..  S.  were  required, 
10 

0.556  X  19=  10.564  cc.  of  oxygen, 

It  is  convenient  to  operate  upon  i  cc.  hydrogen  dioxid  solution. 
Then  each  cc.  of  potassium  permanganate  V.  S.  used  will  represent 


BARIUM   DIOXID  161 

0.556  cc.  of  available  oxygen,  or  0.000794  gm.  of  oxygen,  and  it  is  only 
necessary  to  multiply  the  cc.  by  these  numbers  to  obtain  the  volume 
or  weight  of  available  oxygen. 

If  any  other  quantity  than  i  cc.  of  dioxid  be  taken  for  analysis, 
it  will  be  necessary  after  multiplying  by  0.556  to  divide  the  result  by 
the  quantity  of  dioxid  solution  taken,  in  order  to  find  volume 
strength. 

Hydrogen  dioxid  solution  may  also  be  volumetrically  assayed  by 
Kingzetf  's  method,  which  is  described  under  lodometry. 

The  gasometric  estimation  is  also  described  further  on. 

Barium  Dioxid  (Barium  Peroxid)  (BaO2  =  i68.i6).  This  sub- 
stance is  assayed  by  treating  it  with  an  acid,  and  then  estimating  the 
liberated  hydrogen  dioxid,  as  follows: 

Weigh  off  2  gms.  of  the  coarse  powder,  put  it  in  a  porcelain  capsule, 
add  about  10  cc.  of  ice-cold  water,  then  7.5  cc.  of  phosphoric  acid, 
(85  per  cent),  and  sufficient  ice-cold  water  to  make  25  cc.  Stir  and 
break  up  the  particles  with  the  end  of  the  stirrer  until  a  clear 
or  nearly  clear  solution  is  obtained  and  all  that  is  soluble  is  dis- 
solved. 

5  cc.  of  this  solution  (which  corresponds  to  0.4  gm.  of  barium 
dioxid)  is  measured  off  for  assay. 

Drop  into  this  from  a  burette,  with  constant  stirring,  decinormal 
potassium  permanganate  until  a  final  drop  gives  the  solution  a 
permanent  pink  tint. 

About  40  cc.of  the  decinormal  permanganate  should  be  required 
to  produce  this  result. 

In  this  process,  the  first  step  is  the  formation  of  hydrogen  diroxid 
by  treating  the  barium  dioxid  with  phosphoric  acid,  as  illustrated 
by  the  following  equation  : 


168.16  33.76 

The   hydrogen   dioxid    is    then    estimated  with    decinormal    per- 
manganate as  described  above. 

5(Ba02)     =     5H202     +     2KMnO4     +     3H2SO4 

100)840.8  100)168.8  100)313.96 

8.408  gms.=         1,688  gms.=         3.1396=1000  cc.  —  permanganate. 

10 

=  K2SO4+2MnSO4+8H2O-r-5O2. 


162  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

N 
Thus  each  cc.  of  —  permanganate   represents   0.001688   gm.   of 

H2O2,  which    is    equivalent  to  0.008408    gm.  of  BaC>2,  as  shown  in 
above  equation. 

Assuming  that  40  cc.  of  the  decinormal  solution  of  permanganate 
were  required,  then  0.008408X40=0.33632  gm. 

0.33632  X  ioo 
0.4 

//  an  Empirical  Solution  is  used,  which  was  set  with  iron  and 
whose  titer  is  i  cc.  =0.00569  gm.  Fe,  the  quantity  of  the  latter  used, 

multiplied  by  0.00569  gm.  and  then  by  -      -  will  give  the  quantity 

of  BaC>2  present  in  the  sample  taken. 

Example.  0.4  gm.  were  taken  for  analysis,  and  38  cc.  of  the 
above  ompirical  solution  were  consumed,  then 

38X0.00569  gm.X~-= 0.3276. 

D  J  *0 

If  the  empirical  solution  is  checked  with  a  decinormal  oxalic  acid, 
and  9.6  cc.  are  required  to  oxidize  the  10  cc.  of  standard  oxalic  acid, 
the  permanganate  solution  is  stronger  than  it  should  be.  If  this 
solution  is  used  in  the  above-described  assay  and  38  cc.  are  employed, 
the  calculation  is  made  as  follows : 

Multiply  38  by  —  and  then  by  the  true  decinormal  factor  for 
BaC>2  (0.008408  gm.). 

38X^X0.008408=0.3328  gm. 

e.  Estimation  of  Ferric  Salts  by  Means  of  Potassium  Per- 
manganate (After  reduction).  It  is  frequently  necessary  to  esti- 
mate ferric  salts  by  means  of  permanganate  solution;  this  is  particu- 
larly the  case  in  compounds  where  ferric  and  ferrous  salts  are  both 
present. 

The  ferric  salts  must  of  course  be  reduced  to  the  ferrous  state 
in  order  to  estimate  them  with  permanganate,  or  in  fact  with  any 
oxidizing  agent.  There  are  many  ways  of  affecting  this  reduction, 
but  the  best  way  (where  permanganate  is  to  be  used)  is  no  doubt  by 


FERRIC  SALTS  163 

the  use  of  metallic  zinc  or  magnesium  in  sulphuric  acid  solutions. 
Hydrochloric  acid  may  be  used  instead  of  sulphuric,  but  in  that  case 
the  solution  must  be  very  dilute  and  of  low  temperature,  in  order  to 
avoid  the  liberation  of  chlorin,  which  would  spoil  the  analysis. 

In  concentrated  or  hot  solutions  hydrochloric  acid  acts  upon  per- 
manganate as  a  reducing  agent,  as  shown  in  the  equation 

KMnO4+8HCl=KCl+MnCl2+4H2O  +  5a. 

The  irregularities  due  to  the  liberation  of  chlorin  may  be  obviated 
by  the  addition  of  an  excess  of  mercuric  sulphate  before  titration,  as 
suggested  by  Cady  and  Ruediger,  or  by  the  use  of  magnesium  or 
manganous  sulphate,  as  suggested  by  Kessler  and  Zimmermann  (see 
page  140). 

The  reduction  is  effected  by  adding  to  the  warm  diluted  solution 
of  the  ferric  salt  acidulated  with  sulphuric  acid  small  pieces  of  pure 
metallic  zinc  or  coarsely  powdered  magnesium,  and  setting  aside 
in  a  covered  vessel  until  the  solution  is  colorless,  or  until  it  fails  to 
produce  a  red  color  when  a  drop  of  it  is  brought  in  contact  with  a 
drop  of  sulphocyanate.  The  zinc  used  must  be  free  from  iron,  or  if 
the  latter  metal  is  present  its  quantity  must  be  known.  All  of  the 
zinc  or  magnesium  must  be  dissolved  before  the  titration  is  begun, 
otherwise  the  reduction  would  continue  whilst  the  titration  is  being 
done.  When  the  ferric  salt  is  completely  reduced  the  titration  should 
be  carried  out  without  delay  in  order  to  avoid  reoxidation  through 
exposure  to  the  air.  Reoxidation  takes  place  more  readily  in  the 
presence  of  hydrochloric  acid  than  when  sulphuric  acid  is  used.  Ac- 
cording to  D.  J.  Carnegie,  reduction  takes  place  much  more  rapidly 
in  neutral  than  in  acid  solutions.  He  suggests  neutralization  of  the 
solution  with  ammonia,  and  after  reduction  the  addition  of  sulphuric 
acid  to  keep  the  iron  in  solution.  Other  methods  for  the  reduction 
of  ferric  salts  are  described  further  on. 

The  solution  of  the  ferric  salt  to  be  estimated  should  not  contain 
more  than  0.15  gm.  of  metallic  iron  in  250  cc.  To  this  quantity  of 
solution  about  10  gms.  of  metallic  zinc  and  25  cc.  of  sulphuric  are 
taken.  The  solution  is  kept  at  a  temperature  between  60°  and  80°  C. 
until  the  zinc  is  entirely  dissolved,  then  the  mixture  is  boiled  in  a  flask 
provided  with  a  valve  stopper,  as  shown  in  Fig.  54  (in  order  to  exclude 
air  and  prevent  reoxidation).  It  is  then  rapidly  cooled,  and  titrated 
with  permanganate  without  delay. 


164  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Example.  Estimation  of  Ferric  Chlorid.  0.35  gin.  of  the  dried 
salt  are  dissolved  in  250  cc.  of  water  in  a  flask,  25  cc.  of  sulphuric  acid 
are  added,  and  then  10  gms.  of  metallic  zinc  are  introduced.  The 
flask  is  then  gently  warmed  until  the  zinc  is  entirely  dissolved  and 
the  solution  is  colorless  and  fails  to  give  a  red  color  when  a  small  por- 
tion of  it  removed  with  a  glass  rod,  is  brought  in  contact  with  a 
drop  of  potassium  sulphocyanate.  The  solution  is  then  brought  to 
a  boil,  and  after  this  rapidly  cooled,  avoiding  entrance  of  air,  and 

N 
when    cool  titrated  with  —  potassium   permanganate,  until  a  faint 

permanent  pink  color  appears. 

N 
19.5  cc.  of  the  permanganate  were  required.     Each  cc.  of  —  per- 

manganate represents  0.00555  Sm-  °f  metallic  iron,  which  is  equiva- 
lent to  0.016104  gm.  of  anhydrous  ferric  chlorid.  Then  if  19.5  cc. 
of  the  permanganate  were  employed,  the  quantity  of  real  ferric  chlorid 
present  in  the  sample  is  0.016104  gm-Xi9-5,  which  is  0.314028  gm., 
or  89.7  per  cent,  while  the  quantity  of  metallic  iron  is  0.00555  gm.  Xi9-S 
=0.108225  gm.  or  30.92  per  cent. 

The  reactions  are  represented  by  the  following  equations: 

5Fe2Cl6  +  5Zn  =  ioFeCl2  +  5ZnCl2. 
100)1610.40  1258.6 

16.104 

ioFeCl2  +  2KMnO4  +  i6HCl  =  5Fe2Cl6  +  2KC1  +  2MnCl2+  8H2O. 
100)1258.6  100)313.96 

12.586  gms.=     3.1396  gms.=  1000  cc.  —  permanganate. 


N 
Here  it  is  shown  that   1000  cc.  of  —  permanganate  containing 

3.1396  gm.  of  the  salt  will  oxidize  12.586  gms.  of  ferrous  chlorid  which 
represents  16.104  gms.  of  ferric  chlorid  or  5.55  gms.  of  metallic  iron. 

Therefore  each  i  cc.  of  the  —  permanganate  will  represent  TffVir  part  of 

each  of  these  quantities,  namely,   0.012586  gm.  of  FeCl2,  0.016104 
gm.  of  Fe2Cl6,  and  0.00555  gm.  of  metallic  iron. 

Because  of  the  frequent  and  almost  invariable  presence  in  zinc 
of  carbon  and  iron,  which  have  a  reducing  action  upon  permanganate, 
it  is  necessary  to  carry  out  a  blank  experiment,  to  determine  the  quan- 
tity of  permanganate  solution  used  up  by  these  impurities  in  the  zinc. 


NITROUS  ACID   AND   NITRITES  165 

This  blank  experiment  must  be  conducted  under  the  same  con- 
ditions as  the  assay,  and  differs  only  in  that  the  iron  is  left 
out. 

Example.  10  gms.  of  zinc  from  the  same  lot  as  used  for  the  assay 
is  treated  with  250  cc.  of  water  and  25  cc.  of  sulphuric  acid,  and 
when  it  is  completely  dissolved,  the  potassium  permanganate  solution 
is  added,  until  a  permanent  pale  pink  tint  results.  The  number  of 
cc.  consumed  is  deducted  from  the  quantity  employed  in  the  assay;  the 
difference  is  the  quantity  of  the  permanganate  solution  which  was  con- 
sumed by  the  ferrous  chlorid.  Another  method  for  the  reduction  of 
ferric  salts,  previous  to  titration  with  permanganate,  is  that  of  N. 
Matolcsy.  In  this  the  ferric  salt  is  precipitated  with  ammonium 
sulphid  and  the  precipitated  ferrous  sulphid  dissolved  in  sulphuric 
acid,  which  converts  it  into  ferrous  sulphate.  This  is  then  titrated 
with  permanganate  after  the  H2S  has  been  driven  off.  The  method 
is  described  in  detail  on  page  382. 

If  permanganate  solution  of  the  titer  i  cc.  =0.00569  gm.  Fe  be  used, 
the  calculation  is 

0.00569X19.5X^-^=0.3219  gm.  Fe2Cl6, 

or  91.9  per  cent. 

/.  Estimation  of  Nitrous  Acid  and  Nitrites.  Nitrous  acid, 
when  brought  in  contact  with  a  potassium  permanganate  solution 
acidulated  with  sulphuric  acid,  is  oxidized  to  nitric  acid.  Two 
molecules  of  KMnC>4  reacting  with  5  molecules  of  HNC>2,  as  the 
equation  shows, 


In  the  case  of  nitrites,  as  for  example  sodium  nitrite,  the  oxidation 
takes  place  in  the  same  manner,  and  the  process  may  be  applied  with 
equally  good  results  to  the  salts,  as  well  as  to  free  HNO2.  At  or- 
dinary temperatures  the  oxidation  proceeds  very  slowly,  but  at  a  tem- 
perature of  40°  C.  (104°  F.)  rapid  reaction  occurs.  But  because  of 
the*  volatility  of  nitrous  acid  in  acidulated  solutions  of  its  salts  it  is 
impossible  to  accurately  estimate  them  by  direct  titration  with  per- 
manganate at  a  raised  temperature. 

It  is  customary  to  add  the  nitrite  solution  to  a  measured  volume 
of  warm  acidulated  standard  permanganate  solution.  The  nitrite  is 


166  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

then  oxidized  immediately  as  it  comes  in  contact  with  the  permanganate, 
and  each  drop  added  makes  the  permanganate  lighter  in  color,  and 
when  complete  decolorization  of  the  permanganate  is  attained,  the 
reaction  is  at  an  end. 

The  process  in  detail  is  as  follows: 

N 
100  cc.  of  —  potassium  permanganate  are  measured  into  a  flask, 

5  cc.  of  diluted  sulphuric  acid  (i  :  5)  are  added,  and  the  solution  warmed 
to  40°  C.  (104°  F.).  A  solution  of  the  nitrite  (say  sodium  nitrite)  is 
now  prepared  by  dissolving  i  gm.  of  the  salt  in  sufficient  water  to 
make  100  cc.  This  solution  is  placed  in  a  burette  and  delivered  slowly 
into  the  acidulated  permanganate  solution,  with  constant  shaking, 
and  reducing  the  flow  to  drops  towards  the  end  of  the  titration,  until 
the  permanganate  is  completely  decolorized.  We  will  assume  that 
38  cc.  of  the  NaNO2  solution  were  used. 


5NaNO2 

100)342.85        100)313.96 

3.4285  gms.       3-1396  gms.=  iooo  cc.  —  permanganate. 

10 

N 
Thus  100  cc.  of  —  permanganate  represent  0.34285  gm.  of  pure 

NaNO2.     Therefore  if  38  cc.  of  the  sodium  nitrite  solution  decolorized 

N 
ico  cc.  of  —  permanganate,  the  38  cc.  must  contain  0.34285  gm. 

of  NaNO2.     If  38  cc.  contains  0.34285,  100  cc.  contains  0.902  gm. 
38  :  0.34285  .  :  TOO  :  x'}        x=  0.902  gm., 

and  since  i  gm.  of  salt  was  dissolved  in  100  cc.  of  solution  the  per- 
centage of  pure  NaNO2  is  90.2  per  cent. 

Nitrites  may  also  be  estimated  by  adding  an  excess  of  acidulated 
permanganate  solution,  warming  the  mixture,  and  retitrating  with 
standard  oxalic  acid.  This  is  the  method  which  is  directed  in  the 
U.  S.  P.  for  the  assay  of  sodium  nitrite. 

The  U.  S.  P.  method  is  as  follows: 

N  • 

To  30  cc.  of  —  potassium  permanganate   add   water   to   make 

about  150  cc.  of  solution,  then  add  5  cc.  of  sulphuric  acid  and  10  cc. 
of  a  solution  (i  gm.  in  100  cc.)  of  the  sodium  nitrite  to  be  assayed, 
warm  the  mixture  to  40°  C.  (104°  F.)  and  allow  to  stand  for  five 


HYPOPHOSPHOROUS  ACID  167 

9 

N 

minutes,  then  titrate  with  —  oxalic  acid  solution  until  complete  de- 

10 

colorization  is  effected.     Not  more  than  3.75  cc.  of  the  latter  should 
be  required. 

N 
The  volume  o_  —  oxalic  acid  solution,  deducted  from  the  30  cc. 

N 
of  —  potassium  permanganate  solution  used  gives  the  quantity  of 

the  latter  which  reacted  with  the  one  gram  of  sodium  nitrite,  each 

N 
cc.  of  —  permanganate  represents  0.0034285  gm.  of  NaNC>2. 

30  cc.— 3.75  cc.  =26.25  cc* 
then 

0.0034285X  26.25  =  0.08999+ . 

0.08999X100 

—=89.99+  Per  cent- 


B.  Residual  Titrations 

a  METHODS  IN  WHICH  AN  EXCESS  OF  STANDARD  PERMANGANATE 
is  ADDED,  AND  THE  EXCESS  DETERMINED  BY  RESIDUAL  TITRA- 
TION  WITH  STANDARD  OXALIC  ACID. 

Estimation  of  Hypophosphorous  Acid  and  Hypophos- 
phites. — An  accurately  weighed  quantity  of  the  acid  or  its  salt  is 
dissolved  in  water,  the  solution  is  strongly  acidulated  with  sulphuric 

N 
acid,  and  then  a  measured  excess  of  —   potassium  permanganate 

solution  added.  The  mixture  is  boiled  for  fifteen  minutes  to  hasten 
and  facilitate  the  oxidation,  and  then  the  excess  of  the  permanga- 
nate solution  estimated  by  residual  titration  with  —  oxalic  acid  solu- 

10 

tion. 

Hypophosphorous  Acid  (HPH2O2  =65.53).  3  gms.  of  the  acid 
accurately  weighed  are  diluted  with  water  to  make  60  cc.  Of  this 
solution,  6  cc.  (containing  0.3  gm.  of  the  acid)  are  carefully  removed 
with  a  pipette,  and  introduced  into  a  flask.  7  cc.  of  sulphuric  acid  are 


168  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

N 
added,  and  then  50  cc.  of  —  potassium  permanganate  solution,  and 

the  mixture  boiled  for  fifteen  minutes. 

The  potassium  permanganate,  in  the  presence  of  sulphuric  acid, 
oxidizes  the  hypophosphorous  acid  to  phosphoric,  as  the  equation 
shows  : 

5HPH202+6H2S04+2(2KMnO4) 

2)327.65  2)627.92 

100)163.82  100)313.96 

1.6382  gms.  3-J396  gms.  or  1000  cc.  —  V.  S. 

10 


Each  cc.  of  the  decinormal  V.  S.  represents  0.0016382  gm.  of 
absolute  hypophosphorous  acid.  The  quantity  of  permanganate 
solution  directed  to  be  added  is  slightly  in  excess.  The  excess  is 
then  ascertained  by  retitration  with  decinormal  oxalic  acid.  Each 
cc.  of  oxalic  acid  required  corresponds  to  one  cc.  of  decinormal 
permanganate  which  has  been  added  in  excess  of  the  quantity 
actually  required  for  the  oxidation. 

The  excess  of  permanganate  colors  the  solution  red,  and  the  oxalic 
acid  V.  S.  is  then  added  until  the  red  color  just  disappears,  which 
indicates  that  the  excess  of  permanganate  is  decomposed. 


=  K2SO4+  2MnSO4+  i8H2O+  ioCO2. 

If  4.7  cc.  of  decinormal  oxalic  acid  are  required,  it  indicates  that 
50  cc.—  4.7-  cc.  =45.3  cc.  of  decinormal  permanganate  were  actually 
used  up  in  oxidizing  the  hypophosphorous  acid;  therefore 

0.0016382  gm.X  45-3  =0.0742+  gm. 
of  absolute  hypophosphorous  acid,  HPH2O2,  or 

0.0742X100 
—  -  -  =24.7  per  cent. 

In  the  above  process,  boiling  facilitates  the  oxidation,  but  if  the 
acid  is  boiled  before  sufficient  permanganate  has  been  added  to  com- 
pletely oxidize  it,  decomposition  will  take  place.  Hence  direct  titra- 


CALCIUM  HYPOPHOSPHITE  169 

tion  with  the  permanganate  is  impossible,  and  the  residual  method 
must  be  resorted  to. 

Calcium  Hypophosphite  (Ca(PH2O2)2  =  168.86).  o.i  gm.  of  the 
salt  is  dissolved  in  10  cc.  of  water,  then  10  cc.  of  sulphuric  acid  and 
50  cc.  of  decinormal  potassium  permanganate  are  added,  and  the 
mixture  boiled  for  fifteen  minutes. 

The  excess  of  permanganate  is  then  found  by  retitrating  with 
decinormal  oxalic-acid  solution. 

The  reactions  which  take  place  are  expressed  by  the  following 
equations  : 


.    .    .     (i) 
ioHPH2O2+i2H2SO4+4(2KMnO4) 


.     ...     (2) 

These  two  reactions  may  be  written  together  thus: 

5Ca(PH2O2)2+i7H2SO4+4(2KMnO4) 

4)844.30  4)1255.84 

100)211.07  100)313.96 

2.1107  gms.  3-1396  gms.  or  1000  cc.  standard  V.  S. 

=  5CaSO4+4K2SO4+8MnSO4+ioH3PO4+i2H2O. 

Thus  each  cc.  of  the  standard  permanganate  represents  0.0021107 
gm.  of  pure  Ca(PH2O2)2.  50  cc.  of  decinormal  potassium  perman- 
ganate are  about  3  cc.  more  than  is  necessary  to  oxidize  o.i  gm.  of 
pure  calcium  hypophosphite.  Therefore  not  more  than  3  cc.  of  the 
standard  oxalic  acid  solution  should  be  required  to  decolorize  the 
solution  to  which  50  cc.  of  permanganate  has  been  added. 

Then 

5occ.-3  cc.  =  47  cc. 

N 
the  quantity  of  —  permanganate  which  was  required  for  the  oxidation 

of  o.i  gm.  of  the  salt. 

0.002  1  107  X  47  =  0.0992  +  gm., 

the  quantity  of  pure  Ca(PH2O2)2  in  the  o.i  gm.  taken. 
0.0992  X  100 


o.i 


99.2  per  cent  pure. 


170  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  U.  S.  P.  assay  for  sodium  nitrite  is  a  residual  method  coming 
under  this  head;   it  is  described  under  A,  f. 


b.  METHODS    INVOLVING   A    PRECIPITATION   BY  OXALIC  ACID  AND 

THE  TlTRATION  OF  THE  EXCESS  OF  THE  LATTER  WITH  STANDARD 

PERMANGANATE. 

Estimation  of  Soluble  Calcium  Salts.  To  a  weighed  quantity  of 
the  calcium  salt  dissolved  in  water,  a  measured  excess  of  normal  oxalic 
acid  is  added.  The  mixture  is  then  made  alkaline  with  ammonia  and 
boiled,  to  facilitate  the  separation  of  the  precipitate.  The  mixture  is 
then  cooled  and  diluted  with  water  to  an  accurately  measured  volume, 
and  after  nitration  an  aliquot  portion  removed,  acidulated  with  sul- 

N 
phuric  acid,  and  carefully  titrated  with  —  potassium  permanganate. 

Example.  0.4  gm.  of  calcium  chlorid  is  dissolved  in  water,  10  cc. 
of  normal  oxalic  acid  added,  the  mixture  made  alkaline  with  ammonia 
water,  and  boiled  for  a  few  minutes.  It  is  then  filtered,  the  residue 
and  filter  washed  with  water,  and  after  cooling,  the  combined  filtrate 
and  washings  are  diluted  to  make  100  cc. 

Of  this  solution  50  cc.  are  taken  for  analysis  (representing  0.2  gm. 
of  the  salt),  acidulated  with  sulphuric  acid,  and  then  titrated  with 

N 

—  potassium  permanganate  to  a  faint  rose  tint. 

The  50  cc.  of  solution  represent  5  cc.  of  normal  oxalic  acid 
which  is  equivalent  to  50  cc.  of  decinormal  oxalic  acid,  so  that  whatever 
number  of  cc.  of  decinormal  permanganate  solution  is  required  in  the 
titration,  that  quantity  is  to  be  deducted  from  50  cc.  and  the  difference 

N 
multiplied  by  the  -  -  factor  for  calcium  chlorid  to  find  the  quantity 

of  pure  CaCl2  present  in  0.2  gm. 

N 
If  14  cc.  of  :  -  permanganate  are  employed,  then  14  from  50  cc. 

leaves    36  cc.,  the  quantity  of  decinormal  oxalic  acid  solution  which 
combined  with  the  0.2  gm.  of  calcium  chlorid.     Then 

0.005508  gm.  X  36  =  0.198288  gm., 

the  quantity  of  pure   CaCl2  present   in   the  0.2   gm.   or  99.144  Per 
cent. 


ESTIMATION   OF  MANGANESE  DIOXID  171 

Calcium  salts  to  be  estimated  by  this  method  must  be  tolerably 
pure,  and  free  at  least  from  impurities  which  would  react  with  oxalic 
acid  or  which  would  reduce  permanganate. 

Many  of  the  less  soluble  calcium  salts  may  be  estimated  by  this 
method,  but  they  must  be  subjected  to  longer  treatment  with  the 
oxalic  acid. 

Gold  and  lead  salts  may  also  be  estimated  by  the  same  method. 

c.  METHODS  INVOLVING  A  REDUCTION  BY  MEANS  OF  OXALIC  ACID, 
AND  RETITRATION  OF  THE  EXCESS  OF  THE  LATTER  WITH  POTAS- 
SIUM PERMANGANATE. 

Estimation  of  Manganese  Dioxid  (MnO2).  The  estimation  of 
manganese  dioxid  depends  upon  the  fact  that  when  it  is  boiled  with 
oxalic  acid  in  the  presence  of  sulphuric  acid  a  definite  reaction  takes 
place,  as  the  equation  shows  : 


In  the  estimation  a  measured  excess  of  oxalic  acid  solution  is  added, 
together  with  some  sulphuric  acid,  and  the  mixture  heated  until  solu- 
tion is  complete. 

The  excess  of  oxalic  acid  is  then  found  by  retitration  with  standard 
permanganate  solution.  It  is  well  to  use  a  normal  oxalic  acid  solution 
and  a  decinormal  permanganate  solution. 

0.5  gm.  of  the  dioxid  is  a  convenient  quantity  to  operate  upon. 
Each  cc.  of  decinormal  solution  represents  0.004318  gm.  of  MnO2. 

Example.  0.5  gm.  of  MnC>2  is  treated  with  sulphuric  acid  and 
10  cc.  of  normal  oxalic  acid  solution,  which  is  equivalent  to  100  cc. 
of  decinormal  oxalic  acid  solution,  the  mixture  treated,  as  described 
above,  and  upon  retitrating  25  cc.  of  decinormal  permanganate  are 
required.  Thus 

100  cc.—  25  cc.=75  cc. 

N 

of  —  oxalic  acid  went  into  reaction  with  the  MnO2. 
10 

Then 

75X0.004318=0.32385  gm.; 

0.32385X100 

~      =64.77  Per  cent. 


172  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

d.  METHODS  INVOLVING  A  REDUCTION  BY  MEANS  OF  A  STANDARDIZED 
SOLUTION  OF  A  FERROUS  SALT,  AND  TITRATION  OF  THE  REMAINING 
UNOXIDIZED  FERROUS  SALT,  BY  PERMANGANATE. 

Estimation  of  Nitrates  (Pelouze).  This  method  consists  in 
adding  a  weighed  quantity  of  the  nitrate  to  an  acidulated  solution  of  a 
ferrous  salt  of  known  strength,  and,  when  reaction  is  complete,  esti- 
mating the  ferrous  salt  remaining,  by  titration  with  permanganate  or 
in  certain  cases  by  means  of  dichromate  V.  S.  The  principle  upon 
which  the  method  is  based  is,  that  when  nitric  acid  or  a  nitrate  is  brought 
in  contact  with  a  highly  acidulated  solution  of  a  ferrous  salt,  the  former 
gives  off  oxygen,  which,  passing  over  to  the  ferrous  salt,  oxidizes  it  to 
the  ferric  state,  while  at  the  same  time  NO  is  evolved.  The  reaction 
is 


2HNO3  +  6HC1  +  6FeCl2  =  3F2C16  +  4H2O  +  2NO. 

Nitric  Acid.  Ferrous  Chlorid 

2X62.57  =  125.14  6X125.86  =  755.16 

Iron. 

6X55-5  =  333-0 


Thus  one  molecular  weight  of  nitric  acid  (62.57)  wiH  oxidize  three 
molecular  weights  of  ferrous  salt,  or  three  atoms  of  iron  (166.5). 

Either  hydrochloric  or  sulphuric  acid  may  be  employed.  The 
former  is  preferred  by  most  operators,  and  it  is  generally  agreed  that 
in  order  to  attain  results  of  sufficient  precision  the  estimation  should 
be  done  in  the  presence  of  hydrochloric  acid  only.  In  using  hydro- 
chloric acid,  however,  where  the  titrations  are  to  be  made  with  per- 
manganate, certain  precautions  (previously  mentioned)  must  be 
observed,  because  of  the  evolution  of  chlorin  which  will  otherwise 
take  place  and  spoil  the  analysis.  This  may  be  obviated  by  adding 
to  the  solution  to  be  titrated  an  excess  of  manganese  sulphate. 

The  NO  which  is  produced  during  the  reaction  must  be  removed 
by  boiling  before  titration  with  permanganate  is  begun.  Air  must  be 
absolutely  excluded  during  the  entire  process  to  prevent  oxidation  of 
ferrous  salt  by  the  atmospheric  oxygen,  as  well  as  to  prevent  oxidation 
of  NO  to  HNO3,  which  will  oxidize  more  ferrous  salt.  The  exclusion 
of  air  may  be  partially  affected  by  the  use  on  the  flask  of  a  Bunsen 
valve  stopper  (see  Fig.  54),  but  the  best  method  is  to  employ  an  appa- 
ratus so  arranged  that  a  constant  stream  of  CO2  or  H  gas  may  be 
passed  through  it  (see  Fig.  57). 

This  method,  although  theoretically  perfect,  is  in  practice  liable  to 
great  irregularities,  and  will  give  fairly  good  results  only  if  the  directiona, 


ESTIMATION   OF  NITRATES 


173 


FIG.  57. 


especially  those  as  to  exclusion  of  air,  are  faithfully  carried  out.     The 
method  of  Kjeldahl  is  to  be  preferred. 

To  conduct  the  process,  weigh  accurately  1.5  gm.  of  flower  wire  * 
free  from  rust  (the  iron  content  of  which  is  known),  place  it  in  an 
Erlenmeyer  flask  which  is  provided  with  a  double  perforated  stopper 
fitted  with  two  glass  tubes,  one  of 
which  should  reach  just  to  the  sur- 
face of  the  liquid  in  the  flask  when 
in  place,  and  the  other,  which  is  the 
outlet  tube,  should  reach  no  lower 
than  the  bottom  of  the  stopper. 
The  first  of  these  tubes  is  con- 
nected with  an  apparatus  generating 
carbon  dioxid  or  hydrogen,  while 
the  outlet  tube  serves  to  convey 
the  gas  into  the  air  or  into  another  flask  containing  water  or  an  alka- 
line solution.  30  to  40  cc.  of  pure  fuming  hydrochloric  acid  are  added 
to  the  iron  wire  in  the  flask,  gentle  heat  is  applied,  and  a  stream  of 
either  CC>2  or  H  passed  through  the  flask  and  maintained  throughout 
the  entire  process.  When  the  iron  is  completely  dissolved,  the  stopper 
is  raised  just  long  enough  to  introduce  a  small  glass  tube  open  at 
one  end  and  containing  the  nitrate  to  be  estimated.  The  quantity 
of  nitrate  taken  must  be  equivalent  to  not  more  than  0.2  gm.  of  HNOs. 
The  stopper  is  then  reinserted,  heat  applied,  and  gradually  increased 
until  the  reaction  is  complete.  The  free  hydrochloric  acid  liberates 
nitric  acid  from  the  nitrate  and  oxidation  of  a  portion  of  the  iron  is 
effected.  The  ferrous  chlorid  is  oxidized  to  ferric  chlorid,  as  the  equa- 
tion shows,  and  the  solution  becomes  at  first  dark  brown  through  the 
presence  of  NO.  As  the  heat  is  increased,  the  dark-brown  color  of  the 
solution  is  gradually  changed  to  yellow,  as  ferric  chlorid  is  formed, 
and  increases  in  intensity  until  the  reaction  is  complete,  then  the  color 
remains  stationary  and  indicates  completion  of  oxidation.  The 
solution  is  now  allowed  to  cool,  but  the  stream  of  CC>2  or  H  gas  is 
maintained.  40  cc.  of  a  solution  of  manganese  sulphate  are  now 
added  (this  is  not  necessary  if  sulphuric  acid  is  used  instead  of  hydro- 

N 

chloric),    and   titration    with     —  potassium    permanganate  solution 

10 

begun,  in  order  to  determine  the  quantity  of  unaltered  ferrous  salt 


*  Or  fine  piano-forte  wire. 


174  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

remaining  in  the  solution.     Assuming  that  89  cc.  were  required,  the 
calculation  is  made  as  follows: 

Since  one  molecule  of  HNOs  (62.57)  reacts  with  three  atoms  of 
iron  (166.5)  tne  quantity  of  iron  found  to  have  been  oxidized  if  multi- 
plied by  — '- —  or  by  the  factor  0.03758  (obtained  by  dividing  62.57 

by  166.5),  wiM  giye  tne  quantity  of  nitric  acid  present. 

Example.  1.5  gm.  of  iron  wire,  99.6  per  cent  Fe  =  (1.494  gm.  of 
iron),  is  dissolved  in  hydrochloric  acid  as  above  described,  and 
0.6  gm.  of  potassium  nitrate,  KNOs  (100.43),  added.  After  oxida- 
tion, 98  cc.  of  decinormal  permanganate  were  required.  Each  cc.  of 

-  KMnO4 =0.00555  gm.  of  Fe. 
10 

0.00555  gm.  X98  =0.5439  gm.  of  oxidized  iron.     1.494  gm.  of  iron 
were  originally  taken. 
Therefore, 

1.4940 
Q.5439 
0.9501  gm.  =  the  quantity  of  iron  oxidized. 

Then 

0.9501X62.57 


=  0.357  gm-  of  HNO3, 
0.573  gm.  of 


166.5 
which  equals 

0.9501X62.57X100.43 
166.5X62.57 

or  95.5  per  cent,  pure. 

N  N 

It  is  usually  advisable  to  use  an  —  instead  of  an  —  KMnC>4  solu- 
tion. 

Chromic  Acid  and  Chromates.  Chromic  acid  oxidizes  ferrous 
salts  in  the  same  manner  as  nitric  acid  does.  The  reaction  is  thus 
expressed : 


To  an  accurately  weighed  quantity  of  ferrous  ammonium  sulphate 
(Mohr's  salt)   FeSO4+(NH4)2SO4+6H2O,    (the  permanganate  titer 


CHROMIC  ACID   AND   CHROMATES  175 

of  which  is  known)  which  is  dissolved  in  a  sufficient  quantity  of  diluted 
sulphuric  acid  in  an  Erlenmeyer  flask,*  add  a  weighed  quantity  of 
the  chromate  or  chromic  acid  in  a  concentrated  aqueous  solution. 
Warm  the  mixture  on  a  water-bath,  under  a  constant  stream  of  carbon 
dioxid  until  the  liquid  assumes  a  clear  green  color.  This  occurs  in  a 
few  minutes,  and  indicates  complete  reduction  of  the  chromate. 

Now  allow  the  solution  to  cool,  continuing  the  passage  of  carbon 
dioxid  through  the  flask,  and  transfer  the  cold  solution  to  a  large 
beaker,  and  after  diluting  it  to  about  300  cc.  and  strongly  acidifying 
it  with  sulphuric  acid,  titrate  it  for  unoxidized  ferrous  salt  by  means 

,  N 
of  —  potassium  permanganate. 

It  is  usually  sufficient  to  mix  the  solutions  cold,  but  it  is  better 
to  employ  heat  after  mixing.  A  large  excess  of  .ferrous  salt  is  unnec- 
cessary.  It  is  imperative  to  dilute  the  solution  highly  before  titration, 
as  then  only  can  the  end  color  point  be  accurately  determined  in  the 
green  solution.  The  use  of  an  excess  of  sulphuric  acid  before  titration 
is  likewise  demanded.  A  violet -red  color  marks  the  end -point,  and 
unless  too  great  a  quantity  of  chromate  be  taken,  or  the  solution  be 
insufficiently  diluted,  it  can  be  easily  recognized.  This  method  is 
applicable  not  only  to  free  chromic  acid  and  soluble  chromates,  but 
also  to  chromates  which  are  insoluble  in  water,  f  It  can  therefore 
be  employed  for  the  indirect  estimation  of  such  bases  as  are  precipi- 
table  by  chromic  acid,  out  of  neutral,  ammoniacal,  or  acetic  acid 
solutions,  as  for  instance  lead,  bismuth,  and  barium. 

Finally,  the  method  may  be  employed  for  the  estimation  of  chromic 
oxids.  The  solution  of  the  latter  is  treated  with  an  excess  of  sodium 
carbonate,  bromine  water  added,  and  heat  applied  until  a  clear  solu- 
tion results.  This  solution,  which  contains  all  of  the  chromium  in 
the  form  of  sodium  chromate,  is  evaporated,  the  residue  dissolved  in 
dilute  acetic  acid  and  the  chromium  completely  precipitated  by  means 
of  lead  acetate.  The  precipitated  lead  chromate  is  then  treated  as 
above. 


*  This  flask  should  be  provided  with  a  stopper  having  two  perforations  through 
which  glass  tubes  are  passed,  one  of  these,  which  serves  to  convey  carbon  dioxid 
gas,  should  reach  close  to  the  surface  of  the  liquid,  the  other  tube  should  end 
just  below  the  stopper,  and  serve  as  the  outlet  tube.  See  Fig.  57. 

t  In  the  case  of  insoluble  chromates  the  salt  is  shaken  directly  with  the  ferrous 
solution,  and  the  mixture  more  highly  diluted,  and  more  strongly  heated,  than 
in  the  case  of  soluble  salts. 


176  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  calculation  is  made  with  reference  to  the  equation,  ki  which 
it  is  shown  that  one  molecule  99.34  of  chromic  oxid  (CrOs)  is  equiva- 
lent to  three  molecules  (166.5)  °f  metallic  iron.  The  quantity  of  iron 

oxidized,  multiplied  by      '       gives  the  weight  of  chromic  oxid  present, 

100.  ^ 

and  from  this,  its  equivalent  in  potassium,  sodium,  lead,  bismuth,  or 
barium  chromate  is  calculated. 

In  the  case  of  potassium  dichromate  (K2Cr2C>7)  one  molecule 
(292.28)  is  equivalent  to  six  atoms  (333)  of  metallic  iron.  The  quantity 

of  iron  oxidized  is  multiplied  by  -^— : — . 

333 

Example.  To  1.5  gms.  of  ammonio-ferrous  sulphate  (containing 
0.2142  gm.  Fe)  add  0.1241  gm.  of  K2Cr2O7  (molecular  weight  292.28), 

N 
and  after  complete  oxidation,  titrate  the  solution  with  —  KMnC>4  to 

determine  the  quantity  of  unchanged  ferrous  salt.  13  cc.  are  required. 
Each  cc.  represents  0.00555  gm-  °f  Fe. 

Thus,  13X0.00555  gm.  =0.07215  gm.,  the  quantity  of  iron  which 
was  not  oxidized  by  the  dichromate.  This,  deducted  from  the  quan- 
tity of  iron  originally  added  (0.2142—0.07215=0.14205  gm.),  gives 
the  quantity  which  was  oxidized. 

Then, 


166.5 
or 

0.14205X292.28 
333 


=  0.1247  gm.  of  K2Cr2O7. 


Example.  To  1.5  gms.  of  ammonio-ferrous  sulphate  (containing 
0.2142  gm.  of  Fe)  add  the  precipitate  of  barium  chromate  obtained 
from  0.2491  gm.  of  BaCl2+2H2O  (molecular  weight  242.52)  and 

N 

after  complete  oxidation,  titrate  with  —  permanganate.     7.8  cc.  are 

10 

consumed,  thus  7.8X0.00555=0.04329  gm.  the  quantity  of  unoxidized 
iron  present.  Then  0.2142—0.04329=0.17091  gm.  of  iron  oxidized  by 
the  barium  chromate. 

o.I7o9IX  242.5^  BaCl2+2H20. 

166.5 


ESTIMATION   OF  COPPER  177 

METHODS  INVOLVING  THE  OXIDATION  OF  THE  SUBSTANCE 
ANALYZED  BY  MEANS  OF  A  FERRIC  SALT,  AND  TITRATION  OF 
THE  RESULTANT  FERROUS  SALT. 

Estimation  of  Tin  (Lowenthal).  When  stannous  chlorid  is 
brought  in  contact  with  ferric  chlorid,  it  acts  as  a  reducing  agent,  the 
ferric  chlorid  being  reduced  to  ferrous,  and  the  stannous  chlorid  oxi- 
dized to  stannic.  This  reaction,  which  is  an  exact  quantitative  one, 
takes  place  according  to  the  following  equation: 

SnCl2  +  Fe2Cl6=  SnCl4+  2FeCl2. 
2)118.1     2)111 
59-°5         55-5 

Every  55.5  parts  of  iron  reduced  to  the  ferrous  state  represents 
59.05  parts  of  tin.  The  quantity  of  ferrous  iron  produced  is  deter- 

N 
mined  by  titration  with  —  permanganate.     Metallic  tin,  or  any  proto- 

salt  of  tin,  will  dissolve  in  ferric  chlorid  solution,  in  the  presence  of  a 
little  hydrochloric  acid  and  act  in  the  manner  described. 

About  0.5  gm.  of  tin  or  an  equivalent  of  stannous  salt  is  introduced 
into  a  flask  graduated  at  250  cc.  5  cc.  of  tolerably  concentrated  ferric 
chlorid  solution  and  a  little  hydrochloric  acid  are  then  added.  It 
is  well  to  drop  into  the  flask  a  crystal  of  sodium  carbonate,  in  order 
to  produce  carbon  dioxid  to  replace  the  air  in  the  flask  and  thus  pre- 
vent oxidation  by  the  oxygen  of  the  air.  The  mixture  is  gently  warmed 
until  the  tin  is  wholly  dissolved,  and  then  the  solution  diluted  with 
cold,  recently  boiled  water,  to  250  cc.  Of  this  solution  50  cc.  is  with- 

N 
drawn  by  means  of  a  pipette  and  titrated  with  —  permanganate.    Each 

N 
cc.  of  —  permanganate  =0.005 5 5  gm.  Fe  =0.005905  gm.  Sn. 

It  is  always  advisable  when  an  exact  assay  is  to  be  made,  to  make 
a  blank  experiment,  using  a  like  quantity  of  water  and  ferric  chlorid 
solution,  and  deducting  the  quantity  of  permanganate  solution  used 
from  the  quantity  required  in  the  assay.  The  difference  is  calculated 
as  tin. 

Estimation  of  Copper  (Fleitmann).  The  copper  salt  is  first 
reduced  either  to  cuprous  oxid  by  means  of  glucose  or  to  the  metallic 
state  by  means  of  pure  zinc.  The  reduced  product  is  then  dissolved 


178  A    MANUAL   OF    VOLUMETRIC  ANALYSIS 


in  a  mixture  of  ferric  chlorid  and  hydrochloric  acid,  a  little  sodium 

N 
carbonate  added  to  expel  the  air,  and  the  titration  with  —  perman- 

ganate begun  as  in  the  preceding  assay.     The  reaction  is 


or 

Cu  +  Fe2Cl6  =  CuCl2  +  2FeCl2 

2)63.1         2)ll£_ 

3i-55      55-5 

N 
Each  cc.  of  —  KMnC>4  =0.00555  gm-  Fe  =0.003155  gm.  Cu. 

VOLUMETRIC    ANALYSIS  BY  MEANS    OF  POTASSIUM   BICHROMATE 

In  some  respects  the  dichromate  possesses  advantages  over  per- 
manganate : 

1.  It  may  be  obtained  in  a  pure  state. 

2.  Its  solution  does  not  deteriorate  upon  standing  as  does  that  of 
permanganate. 

3.  It  is  not  decomposed  by  contact  with  rubber  as  the  permanganate 
is,  and  may  therefore  be  used  in  Mohr's  burette.     Its  great  disdvantage, 
however,  is  that  when  used  in  the  estimation  of  ferrous  salts  the  end 
reaction  can  only  be  found  by  using  an  external  indicator.     The  indi- 
cator which  must  be  used  is   freshly  prepared  potassium  ferricyanid 
T.  S.,  a  drop  of  which  is  brought  in  contact  with  a  drop  of  the  solution 
being  tested,  on  a  white  slab,  at  intervals  during  the  titration,  the  end 
of  the  reaction  being  the  cessation  of  the  production  of  the  blue  color, 
when  the  two  liquids  are  brought  together.     Thus  the  estimation  by 
potassium  dichromate  is  cumbersome,  and  very  exact  results  are  not 
as  easily  obtained  as  with  permaganate. 

Besides  ferrous  salts,  a  great  many  other  substances  may  be  esti- 
mated by  oxidation  analysis  with  dichromate.  Among  them  nitrates, 
sulphates,  arsenous  acid,  barium,  lead,  ferric  salts  after  reduction  by 
stannous  chlorid  or  an  alkaline  sulphite,  but  not  after  reduction  by 
means  of  metallic  zinc.  The  presence  of  the  dissolved  zinc  salt  inter- 
feres with  the  reaction  of  the  ferricyanid  indicator.  Ferrous  salts 
may  be  estimated  in  the  presence  of  hydrochloric  acid,  by  means  of 
dichromate,  without  the  precautions  that  apply  in  the  case  of  per- 
manganate. Chromium  as  chromate  may  be  indirectly  estimated;  an 
excess  of  a  solution  of  a  ferrous  salt  being  added  and  then  the  excess 


DECINORMAL  POTASSIUM  BICHROMATE  179 

determined  by  dichromate.     lodids,   thiosulphates,  and  alkalies  may 
also  be  estimated  by  means  of  potassium  dichromate. 

Jf 

Preparation    of    Decinormal    —     Potassium     Dichromate 

N 
(K2Cr2O7=292.28;    —  V.  S.  =4.8713  gms.  in  1000  cc.). 

4.8713  gms.  of  pure  potassium  dichromate  *  which  has  been  pul- 
verized and  dried  at  120°  C.  is  dissolved  in  sufficient  water  to  make 
1000  cc.  of  solution. 

It  will  be  noticed  that  -fa  of  the  molecular  weight  of  the  dichro- 
mate (expressed  in  grams)  is  taken  in  the  preparation  of  1000  cc.  of 
this  solution.  The  reason  for  this  is  that  one  molecule  of  potassium 
dichromate  when  treated  with  an  acid  yields  three  atoms  of  nascent 
oxygen  which  are  available  for  oxidizing  purposes,  thus 


and  since  each  atom  of  oxyen  is  equivalent  to  two  atoms  of  hydrogen, 
one  molecule  of  the  dichromate  must  be  equivalent  to  six  atoms  of 
hydrogen.  Hence  a  normal  solution  of  potassium  dichromate,  when 
used  as  an  oxidizing  agent,  should  contain  one  sixth  of  the  molecular 
weight,  expressed  in  grams,  in  1000  cc.  (see  definition  for  normal  solu- 
tion) and  its  decinormal  solution  -fa. 

If  a  standard  solution  of  potassium  dichromate  is  to  be  made  for 
use  as  a  precipitant,  as  in  the  titration  of  barium,  one  fourth  of  the 
molecular  weight  is  to  be  taken  for  1000  cc.  of  the  normal  solution,  as 
explained  in  Chapter  III. 

Standard  solution  of  potassium  dichromate  is  sometimes  used  as 
a  neutralizing  solution  for  estimating  alkalies,  phenolphthalein  being 
used  as  indicator. 

When  used  for  this  purpose  the  normal  solution  contains  146.14 
gms.  in  i  liter  (one  half  the  molecular  weight  in  grams).  It  is  then 
the  exact  equivalent  of  any  normal  acid  V.  S. 

2KOH+K2Cr207=2K2Cr04+H2O. 

2)112        2)292.28 

56  gms.    146.14  gms.,  or  1000  cc.  normal  V.  S. 

Decinormal  potassium  dichromate  may  also  be  used  in  con- 
junction with  potassium  iodid  and  sulphuric  acid  for  standardizing 

*  Potassium  dichromate  for  use  in  volumetric  analysis  should  respond  to  all 
the  tests  for  purity  given  in  the  U.  S.  P.,  or  it  should  be  recrystallized  several 
times  and  then  dried. 


180  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

sodium  thiosulphate.       lodin   is   liberated  from    potassium  iodid  in 
this  reaction.     The  reaction  is  expressed  by  the  equation 


Thus  one  molecule  of  the  dichromate  will  liberate  six  atoms  of 
iodin,  therefore,  a  normal  solution  should  contain  one  sixth  of  the 
molecular  weight,  and  a  decinormal  solution  -fa  in  1000  cc.  The  solu- 
tion is  hence  of  the  same  strength  as  that  which  is  used  for  oxidizing 
purposes.  If  the  decinormal  solution  containing  14.614  gms.  in  i  liter 

sN 
is  used,  it  has  the  effect  of  a  —  solution. 

10 

The  decinormal  solution  which  is  used  as  an  oxidizing  agent  is 
chemically  equivalent  to  decinormal  potassium  permanganate.  When 
used  for  the  purpose  of  liberating  iodin  from  potassium  iodid,  it  is 
the  equivalent  of  an  equal  volume  of  decinormal  sodium  thiosulphate. 

For  titrating  ferrous  salts  the  decinormal  solution  of  dichromate  is 
used  in  the  following  manner: 

Make  an  aqueous  solution  of  the  ferrous  salt,  introduce  it  into 
a  flask,  and  acidulate  it  with  sulphuric  or  hydrochloric  acid.  Now 
add  gradually  from  a  burette  the  decinormal  potassium  dichromate 
until  a  drop  taken  out  upon  a  white  slab  no  longer  shows  a  blue 
color  with  a  drop  of  freshly  prepared  potassium  ferricyanide  T.  S. 
Note  the  number  of  cc.  of  the  standard  solution  used,  multiply  this 
number  by  the  factor,  and  thus  obtain  the  quantity  of  pure  salt  in  the 
sample  taken. 

Ferrous  salts  strike  a  blue  color  with  potassium  ferricyanide  T.  S.; 
but  as  the  quantity  of  ferrous  salt  gradually  diminishes  during  the 
titration,  the  blue  becomes  somewhat  turbid,  acquiring  first  a  green, 
then  a  gray,  and  lastly  a  brown  shade.  The  process  is  finished  when 
the  greenish-blue  tint  has  entirely  disappeared. 

The  reaction  of  potassium  dichromate  with  ferrous  salts  always 
takes  place  in  the  presence  of  free  sulphuric  or  hydrochloric  acid  at 
ordinary  temperatures.  Nitric  acid  should  not  be  used. 

If  it  is  desired  to  estimate  ferric  salts  by  this  standard  solution 
it  is  necessary  to  first  reduce  them. 

This  may  be  done  by  metallic  magnesium,  sulphurous  acid,  the 
alkali  sulphites,  or  by  stannous  chlorid. 

Standard  potassium  dichromate  may  be  checked  in  the  same  way 
as  standard  permanganate,  with  pure  metallic  iron. 


FERROUS  SALTS   WITH  POTASSIUM  DICHROMATE       181 
ESTIMATION   OF   FERROUS    SALTS   WITH    POTASSIUM   DICHROMATE. 

One  molecule  of  potassium  dichromate  yields,  under  favorable 
circumstances,  three  atoms  of  oxygen.  This  is  shown  by  the  follow- 
ing equation: 

K2Cr2O7  =Cr2O3  4-K2O  +O3. 

Here  it  is  seen  that  the  three  liberated  atoms  of  oxygen  combine 
at  once  with  the  ferrous  oxid,  converting  it  into  ferric  oxid: 

6FeO+O3=Fe6O9     or    3Fe2O3. 

In  the  oxidation  of  a  ferrous  salt,  the  reaction  takes  place  only 
in  the  presence  of  an  acid. 

The  dichromate  then  gives  up  its  oxygen.  Four  of  its  oxygen 
atoms  combine  at  once  with  the  replaceable  hydrogen  of  the  accom- 
panying acid,  the  other  three  being  liberated.  The  three  oxygen 
atoms  thus  set  free  are  available  either  for  direct  oxidation  or  for  com- 
bination with  the  hydrogen  of  more  acid.  In  the  latter  case  a  corre- 
sponding quantity  of  acidulous  radicals  is  set  free. 


In  this  case  four  of  the  liberated  atoms  of  oxygen  combine  with 
eight  of  the  atoms  of  hydrogen  of  sulphuric  acid  and  liberate  four  SO4 
radicals,  which  at  once  combine  with  the  K2  and  Cr2  of  the  dichromate. 
The  other  three  atoms  are  set  free.  If  seven  sulphuric  acid  mole- 
cules are  used  instead  of  four  molecules,  the  three  free  atoms  of  oxygen 
will  liberate  3  (SO4): 


If  this  liberation  of  3(SO4)  takes  place  in  the  presence  of  a  ferrous 
salt,  the  3(SO4)  Adll  combine  with  six  molecules  of  the  ferrous  salt, 
converting  it  into  a  ferric  salt: 

6FeS04+3S04=Fe6(S04)9=3Fe2(S04)3; 
6FeSO4+K2Cr2O7+7H2SO4=K2SO4+Cr2(SO4)3+7H2O+(3Fe2(SO4)3). 


182  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

If  in  the  above  case  hydrochloric  acid  is  used  instead  of  sulphuric, 
fourteen  molecules  of  the  former  must  be  taken  to  supply  the  necessary 
hydrogen. 

The  seven  liberated  atoms  of  oxygen  must  have  fourteen  atoms  of 
hydrogen  to  combine  with. 

Three  of  these  atoms  of  oxygen  liberate  six  univalent  or  three 
bivalent  acidulous  radicals. 

Therefore,  since  one  molecule  of  K2Cr2C>7  will  give  up  for  oxidizing 
purposes  three  atoms  of  oxygen,  which  are  equivalent  chemically  to 
six  atoms  of  hydrogen,  one  sixth  of  the  molecular  weight  in  grams  of 
the  dichromate,  dissolved  in  sufficient  water  to  make  one  liter,  con- 
stitutes a  normal  solution,  and  one  tenth  of  this  quantity  of  K2Cr2C>7 
in  a  liter,  a  decinormal  solution. 

Thus  the  estimation  of  ferrous  salts  is  effected  by  oxidizing  them 
to  ferric  with  an  oxidizing  agent  of  known  power,  the  strength  of  the 
ferrous  salt  being  determined  by  the  quantity  of  the  oxidizing  agent 
required  to  convert  it  to  ferric. 

Saccharated  Ferrous  Carbonate  (FeCO3  =  115.05).  1.1505 
gms.  of  saccharated  ferrous  carbonate  are  dissolved  in  10  cc.  of 
diluted  sulphuric  acid  and  the  solution  diluted  with  water  to  about 
100  cc.  The  decinormal  potassium  dichromate  is  carefully  added, 
until  a  drop  of  the  solution  taken  out  and  brought  in  contact  with  a 
drop  of  freshly  prepared  solution  of  potassium  ferricyanid  ceases  to 
give  a  blue  color. 

The  number  of  cc.  of  the  dichromate  solution  is  read  off  and  the 
following  equations  applied: 

6FeCO3  +6H2SO4  =6FeSO4  +6H2O  +6CO2  ; 

690.3  9°5-10 

then 


6FeCO3   or   6FeSO4 

6)690.3  6)905.10       6)292.28 

10)115-05  10)150.85        I0)48.7i3_  N 

ii.  505  gms.        15.085  gms.    4.8713  gms.,  or  1000  cc.  —  K2Cr2O7  V.  S. 


N 

Thus  each  cc.  of  —  K2Cr2O7  represents  0.011505  gm.  of  pure 
10 

ferrous  carbonate  or  0.00555  gm.  of  metallic  iron. 


SACCHARATED   FERROUS   CARBONATE 


183 


The  U.  S.  P.  saccharated  ferrous  carbonate  requires  about  15  cc. 


_N 
10 
15  per  cent. 


of  —  K2Cr2Oy  V.  S.  for  complete  oxidation,  corresponding  to  about 
10 


0.011505X15=0.172575  gm. 
0.172575X100 


1-1505 


=  15  per  cent. 


If  strong  sulphuric  acid  is  added  to  saccharated  ferrous  carbonate 
it  will  char  the  sugar,  and  a  black  mass  of  burnt  sugar  is  obtained. 
This  may  be  prevented  by  adding  water  first  and  then,  slowly,  the 
sulphuric  acid. 

Instead  of  sulphuric  acid,  hydrochloric  acid  may  be  used.  This 
will  not  char  the  sugar;  but  the  ferrous  chlorid  which  is  then  formed 
is  too  readly  oxidized  by  the  air. 

It  has  also  been  suggested  that 
as  hydrochloric  acid  so  rapidly 
converts  ordinary  sugar  into  invert 
sugar  as  to  render  it  easily  attacked 
by  the  dichromate,  it  should  be 
cautiously  used,  if  at  all.  Phos- 
phoric acid  has  none  of  these  dis- 
advantages, and  may  be  employed 
with  good  results. 

In  making  estimations  of  fer- 
rous salts  with  potassium  dichro- 
mate, care  should  be  taken  to 
avoid  atmospheric  oxidation.  It 
is  good  practice  to  calculate  ap- 
proximately how  much  of  the 
standard  solution  will  probably  be 
required  to  complete  the  oxidation, 

and  then  add  almost  enough  of  the  standard  solution  at  once,  instead 
of  adding  it  slowly. 

A  white  porcelain  slab  is  then  got  ready,  and  placed  alongside  of 
the  flask  in  which  the  titration  is  to  be  performed.  Upon  this  slab  is 
placed  a  number  of  drops  of  the  freshly  prepared  solution  of  potassium 
ferricyanid,  and  at  intervals  during  the  titration  a  drop  is  taken  from 
the  flask  on  a  glass  rod  and  brought  in  contact  with  one  of  the  drops 


184  A   MANUAL   OF    VOLUMETRIC  ANALYSIS 

on  the  slab.  The  glass  rod  should  always  be  dipped  in  clean  water 
after  having  been  brought  in  contact  with  a  drop  of  the  indicator. 
See  Fig.  58. 

When  a  drop  of  the  solution  ceases  to  give  a  blue  color  on  contact 
with  the  indicator,  the  reaction  is  complete. 

Ferrous  Sulphate  (FeSO4-f  7H2O  =277.42).  Dissolve  about  one 
gram  of  crystallized  ferrous  sulphate  in  a  little  water,  add  a  good 
excess  of  sulphuric  or  hydrochloric  acid,  titrate  with  the  decinormal 
potassium  dichromate,  as  directed  under  Ferrous  Carbonate,  and 
apply  the  following  equation : 

6(FeSO4  .  7H2O)+K2Cr2O7  +  7H2SO4  = 

6)1656.06  6)292.28 

10)276.01  IO)l^:Zl3_  XT 

27.601  gms.  4-8713  gms.,  or  1000  cc.  —  K2Cr2O7  V.  S. 

10 

3Fe2(S04)3+K2S04+Cr2(S04)3+49H20. 

N 
Thus  each  cc.  of  the  —  K2Cr2C>7  V.  S.  represents  0.027601  gm.  of 

crystallized  ferrous  sulphate  or  0.015085  anhydrous.  If  i  gm.  of  the 
salt  is  taken  and  dissolved  as  above,  it  should  require  about  37  cc.  of 
the  standard  solution,  equivalent  to  about  100  per  cent. 


Anhydrous  Ferrous  Sulphate 


6FeSO4 

6)905.10     6)292.28 
10)150.85     10)48.713 

15.085  gms.  4.8713  gms.,  or  1000  cc.  —  K2Cr2O7  V.  S. 

10 

Each  cc.  of  the  standard  solution  represents  0.015085  gm.  of  real 
ferrous  sulphate  or  0.00555  gm-  °f  metallic  iron. 

Dried  (Exsiccated)  Ferrous  Sulphate  of  the  U.  S.  P.  has  the 
approximate  composition  FeSO4+3H2O. 

It  is  tested  in  the  same  manner  as  the  anhydrous  ferrous 
sulphate. 

Granulated  Ferrous  Sulphate  (FeSO4  +  7H2O)  is  tested  in  the 
same  manner  as  crystallized  ferrous  sulphate,  with  which  it  should 
correspond  in  strength. 


ANALYSIS   BY   INDIRECT   OXIDATION 


185 


TABLE  OF  SUBSTANCES  WHICH  MAY  BE  ESTIMATED  BY  MEANS  OF  POTASSIUM 
PERMANGANATE  OR  POTASSIUM  BICHROMATE 


Name. 

Formula. 

Molecular 
Weight. 

—  Factor. 

10 

Acid  chromic  ....        

Cr03 
HPH202 
HNO3 
HNO2 
H2C204 
H2C204+  2H20 
BaO2 
CaCl2 
Ca(PH202)2 
FeCl3 
Fe(PH202)3 
Fe2(S04)3 
FeC03 
FeO 
FeSO, 
FeSO4+7H2O 
Fe2 
H202 
Mn02 
KPH202 
NaPH202 
NaNO, 

99-34 
65.53 
62.57 
46.69 

89.34 
125.1 
168.16 
110.16 
168.86 
161  .04 
249.09 
397.05 
US-OS 
71.38 
150-85 
276.01 

III.O 

33  -76 
86.36 

I03-39 
87.41 
68.57 

0.0033113 
0.0016382 
0.002085 
0.0023345 
0.004467 
0.006255 
0.008408 
0.005508 
0.0021107 
0.016104 
0.024909 
0.0198525 
0.011505 
0.007138 
0.015085 
0.027601 
0-00555 
0.001688 
0.004318 
0.00258475 
0.00218525 
0.0034285 

'  '     hypophosphorous    .              ....... 

'  '     nitrous                                  

'  '     oxalic  (anhydrous) 

"         "      (crystallized)  

Barium  dioxid  

Calcium  chlorid 

'  '        hypophosphite  

Ferric  chlorid 

'  '      hypophosphite  .  .                 ... 

'  *      sulphate  

Ferrous  carbonate 

<  '       oxid  

'  '        sulphate  (anhydrous) 

««             "        (crystallize^)    

Ferrum  (metallic) 

Hydrogen  dioxid    

^Manganese  dioxid 

Potassium  hypophosphite  

Sodium  hypophosphite 

'  '        nitrite               

ANALYSIS   BY   INDIRECT    OXIDATION 

This  method  of  analysis  is  based  upon  the  oxidizing  power  of 
iodin. 

lodin  acts  upon  the  elements  of  water,  forming  hydriodic  acid 
with  the  hydrogen,  and  liberating  oxygen  in  a  nascent  state. 

Nascent  oxygen  is  a  very  active  agent,  and  readily  combines  with 
and  oxidizes  many  substances,  such  as  arsenous  oxid,  sulphurous  acid, 
sulphites,  thiosulphates,  hydrosulphuric  acid,  the  lower  oxids  of  anti- 
mony, and  their  salts. 


H2S03  +H20  +I2  =2HI  +H2S04. 

Therefore  iodin  is  said  to  be  an  indirect  oxidizer,  and  may  be 
used  for  the  estimation  of  a  great  variety  of  substances  with  extreme 
accuracy. 


186  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

When  iodin  is  brought  in  contact  with  certain  oxidizable  sub- 
stances it  is  decolorized.  This  decolorization  occurs  as  long  as  some 
of  the  oxidizable  substance  is  present,  and  ceases  when  oxidation  is 
complete.  Hence  when  the  yellow  color  of  iodin  shows  itself  in  the 
solution  being  analyzed  the  reaction  is  known  to  be  at  an  end.  In 
most  cases  a  more  delicate  end-reaction  is  obtained  by  using  starch 
solution  as  an  indicator.  This  gives  a  distinct  and  unmistakable 
blue  color  with  the  slightest  excess  of  iodin. 

In  making  an  analysis  with  standard  iodin  solution,  the  substance 
under  examination  is  brought  into  dilute  solution  (usually  alkaline), 
the  starch  solution  added,  and  then  the  iodin,  in  the  form  of  a  stand- 
ard solution,  is  delivered  in  from  a  burette,  stirring  or  shaking  con- 
stantly, until  a  final  drop  colors  the  solution  blue;  a  sign  that  a  slight 
excess  of  iodin  has  been  added. 

N 
Preparation    of    Decinormal    Iodin    (1  =  125.9;     —    V.  S.  = 

12.59  gms-  Per  liter).  Dissolve  12.59  gms-  o^pure*  iodin  in  300  cc. 
of  distilled  water  containing  18  gms.  of  pure  potassium  iodid.  Then 
add  enough  water-  to  make  the  solution  measure  at  15°  C.  (59°  F.) 
exactly  1000  cc. 

The  solution  should  be  kept  in  small  glass-stoppered  vials,  in  a 
dark  place. 

The  potassium  iodid  used  in  this  solution  acts  merely  as  a  solvent 
for  the  iodin. 

If  pure  iodin  is  used  in  making  this  solution,  there  is  no  necessity 
for  checking  (standardizing)  it. 

But  if  desired,  the  solution  may  be  checked  against  pure  arsenous 
acid  or  sodium  thiosulphate.  If  there  is  any  doubt  as  to  the  purity 
of  the  iodin,  it  is  best  to  take  a  larger  quantity,  say  14  gms.  instead  of 
the  12.59  gms-  directed  above,  and  then  dilute  the  resulting  solution 
to  the  proper  strength  after  standardizing. 

*  If  pure  iodin  be  not  at  hand,  it  may  be  prepared  from  the  commercial  article 
as  follows: 

Powder  the  iodin  and  heat  it  in  a  porcelain  dish  placed  over  a  water-bath, 
stirring  constantly  with  a  glass  rod  for  20  minutes.  Any  adhering  moisture, 
together  with  any  cyanogen  iodid,  and  most  of  the  iodin  bromid  and  iodin  chlorid, 
is  thus  vaporized. 

Then  triturate  the  iodin  with  about  5  per  cent  of  its  weight  of  pure,  dry  potas- 
sium iodid.  The  iodin  bromid  and  chlorid  are  thereby  decomposed,  potassium 
bromid  and  chlorid  being  formed  and  iodin  liberated  from  the  potassium  iodid. 

The  mixture  is  then  returned  to  the  porcelain  dish,  covered  with  a  clean  glass 
funnel,  and  heated  on  a  sand-bath.  A  pure  resublimed  iodin  is  then  obtained. 


DEC1  NORMAL   SODIUM   THIOSULPHATE  SOLUTION      187 

Standardization  of  lodin  V.  S.  by  Means  of  a  Decinormal 
Sodium  Thiosulphate  Solution.  25  cc.  of  the  iodin  solution  are 
accurately  measured  off  into  a  beaker,  and  then  from  a  burette  the 

N 
—  thiosulphate  is  delivered  until  the  solution  is  of  a  pale  yellow  color, 

i  o 

two  or  three  drops  of  starch  solution  are  then  added,  and  the  titration 
with  the  thiosulphate  solution  continued  until  the  blue  color  of  starch 
iodid  is  discharged. 

If  the  iodin  solution  is  exactly  decinormal,  the  25  cc.  will  require  25 
cc.  of  decinormal  sodium  thiosulphate  to  exactly  complete  the  reaction. 

If  on  the  other  hand  more  than  25  cc.  of  thiosulphate  solution  is 
required,  it  indicates  that  the  iodin  solution  is  too  concentrated,  and 
must  be  diluted  so  as  to  correspond  with  the  decinormal  thiosulphate 
solution,  volume  for  volume. 

Example.  Assuming  that  in  the  above  titration  27  cc.  of  the  thio- 
sulphate solution  were  used,  then  each  25  cc.  of  the  iodin  solution 
must  be  diluted  with  water  to  make  27  cc.  in  order  to  convert  the  iodin 
solution  into  a  strictly  decinormal  solution.  If,  however,  the  iodin 
solution  is  found  to  be  weaker,  as  evidenced  by  its  using  up  less  than 

N 
its  own  volume  of  —   thiosulphate,   its  relative  strength  should  be 

noted  on  the  label  of  the  container. 

Thus  if  only  24.8  cc.  of  the  thiosulphate  solution  are  used  up,  then 

2  C 

i  cc.  of  the  latter  equals  — —  cc.  or  1.008  cc.  of  the  iodin  solution. 
24.8 

N 

i  cc.  of  this  iodin  solution  is  equivalent  to  0.992  cc.  of   —  thio- 

10 

N 
sulphate,  which  is  the  same  as  saying  i  cc.  =0.992  cc.  of   —   iodin, 

or  expressed  in  another  way,  i  cc.  of  this  iodin  solution  contains 
0.01249  -f-gm.  of  iodin. 

Such  an  iodin  solution  may  be  used  as  an  empirical  solution,  and 
in  any  assay  the  quantity  of  it  (in  cubic  centimeters)  which  is  con- 
sumed is  divided  by  1.008  or  multiplied  by  -  -  or  by  0.992,  and  then 

multiplied  by  the  decinormal  factor  for  the  substance  analyzed.  Another 
way  is  to  multiply  the  cc.  of  this  iodin  solution  used  by  the  weight  of 
iodin  contained  in  each  cc.  and  then  by  a  fraction  in  which  the  numer- 
ator represents  the  quantity  of  the  substance  analyzed  equal  to  an 
atom  of  iodin  and  the  denominator  is  the  atomic  weight  of  iodin. 


188  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

Example,    o.i  gm.  of  arsenous  acid  consumes  20  cc.  of  this  empirical 

N 
solution.    How  much  absolute  As2Os  does  it  contain?    The —  factor 

.      .    A    •  I0 

for  As2Oa  is  0.004911  gm. 

20X0.004011 
Method  (a)    —       — r-^ —  =0.974  gm. 

I.OOo 

"        (b)        —  Xo.oo49ii=o.974  gm. 

"         (c)     20X0.992X0.004911  =0.974  gm. 

"         (d)     2oXo.oi249X  —     -  =0.974  gm. 

It  is  a  good  plan  to  have  the  factors  marked  on  the  labels.  In  the 
above  case  the  label  may  be  marked 

24.8 
X or     Xo.992     or     i  cc.  =0.01249  gm.  loam. 

Standardization  of  lodin  V.  S.  by  Means  of  Arsenous  Oxid. 

0.2  gm.  of  pure  resublimed  vitreous  arsenous  oxid  is  weighed  off 
very  carefully  into  a  flask. 

50  cc.  of  water  are  added  and  then  after  the  addition  of  2  gms.  or 
more  of  sodium  bicarbonate,  the  mixture  is  gently  warmed  and  shaken 
until  the  arsenous  oxid  is  completely  dissolved.* 

To  this  solution  a  few  drops  of  starch  indicator  are  added,  and 
then  the  iodiri  solution  delivered  carefully  from  a  burette  until  a 
blue  color  marks  the  end  of  the  reaction. 

As2O3  +   I4   +  2H2O  =  As2O5  +  4HI. 
196.44    4X125.9 

49.11    gms.  of  As2C>3=i25.9    gms.  of  iodin; 
4.911    "     "  As2O3=  12.59    "     "     "      or  looo  cc.  —  V.  S. 
0.2  gm.  of  As2O3  will  require 

1000X0.2  .  N    .   ..    _,  c 

=40.72  cc.  of  a  true  —  iodin  V.  S. 

4.911  10 

*  Arsenous  oxid  is  much  more  readily  soluble  in  alkali  hydroxid,  than  in  car- 
bonated alkalies,  therefore  the  following  method  of  making  the  solution  is  pre- 
ferred: 0.2  gm.  of  arsenous  oxid  are  dissolved  in  a  small  quantity  of  boiling 
water  with  the  aid  of  potassium  hydroxid  (free  from  sulphur),  the  solution  is  then 
acidified  with  hydrochloric  acid,  and  then  again  made  alkaline  by  the  addition 
of  sodium  bicarbonate.  The  latter  must  be  added  in  considerable  excess  being 
careful,  however,  to  avoid  loss  of  solution  during  effervescence. 


THE  STARCH  SOLUTION  189 

Assuming  that  in  the  above  titration  37.4  cc.  of  the  iodin  solution 
were  used,  then  the  iodin  solution  is  too  concentrated  and  must  be 
diluted  so  that  each  37.4  cc.  will  be  made  up  to  40.72  cc. 

After  diluting  in  this  way  a  new  trial  should  be  made. 

It  is  a  good  plan  to  make  a  decinormal  solution  of  the  arsenous 
oxid  by  dissolving  4.911  gms.  of  the  pure  oxid  and  30  gms.  of  sodium 
bicarbonate  in  sufficient  water  to  make  1000  cc.  at  15°  C.  and  to  titrate 
this  with  the  iodin  solution.  25  cc.  of  this  solution  should  require  for 
complete  oxidation,  exactly  25  cc.  of  the  iodin  solution,  if  the  latter 
is  strictly  of  decinormal  strength. 

The  Starch  Solution.  This  solution,  which  is  used  as  an  indi- 
cator in  iodometric  determinations,  is  made  as  follows: 

i  gm.  of  starch  (potato,  arrowroot,  or  corn  starch),  is  triturated 
with  10  cc.  of  cold  water,  until  a  smooth  mixture  is  obtained,  then 
sufficient  boiling  water  is  added,  with  constant  stirring,  to  make  200  cc. 
of  a  thin,  translucent  fluid.  If  the  solution  is  not  translucent  it  should 
be  boiled  for  about  three  minutes,  then  allowed  to  cool,  and  filtered. 
This  solution  does  not  keep  very  long,  in  fact,  it  becomes  useless 
after  standing  one  day,  therefore  it  should  be  freshly  prepared  when 
required. 

This  indicator  is  very  sensitive  to  iodin — it  will  detect  one  part  of 
iodin  in  3,500,000.  If  the  solution  is  not  clear,  or  contains  flocks  of 
insoluble  starch,  and  the  characteristic  beautiful  blue  color  is  not 
obtained  with  iodin ;  instead,  a  greenish  or  brownish  color  is  produced, 
and  the  insoluble  particles  are  even  colored  black  and  are  decolorized 
with  difficulty. 

The  blue  color  which  starch  gives  with  iodin  constitutes  a  very 
delicate  indication  of  the  slightest  excess  of  iodin.  This  color  is 
usually  regarded  as  being  due  to  the  formation  of  a  compound  of 
starch  and  iodin,  called  iodid  of  starch.  It  is  a  compound  of  very 
unstable  character  and  of  doubtful  composition. 

Sodium  thiosulphate  behaves  towards  iodid  of  starch  exactly  as  it 
does  towards  free  iodin — it  takes  up  the  iodin  and  thus  discharges  the 
blue  color. 

Iodid  of  starch  dissociates  upon  heating,  but  reunites  upon  cooling, 
hence  it  is  advisable  to  avoid  heat  in  estimations  where  starch  is  used 
as  an  indicator. 

In  order  to  prevent  the  deterioration  of  this  solution  a  few  drops  of 
chloroform  may  be  added;  this  will  preserve  it  for  a  long  time.  Oil 
of  cassia  is  also  recommended  as  a  preservative.  Moerk  adds  2  cc. 


190  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

of  the  oil  to  a  liter  of  the  cooled  starch  solution.  Zinc  chlorid  or 
iodid  added  to  the  boiling  starch  solution  will  prevent  its  decompo- 
sition for  a  long  time.  A  starch  solution  so  made,  however,  should 
not  be  used  in  titrations  of  sulphids,  because  zinc  reacts  with  sulphids. 

In  the  case  of  solutions  containing  carbonates,  the  precipitate  of 
zinc  carbonate  is  so  small  in  amount  that  it  does  not  interfere  in  the 
least  with  the  recognition  of  the  end-reaction  tint.  Mercuric  iodid  is 
also  a  very  valuable  preservative. 

o.oi  gm.  of  mercuric  iodid  in  a  liter  of  the  starch  solution  is  quite 
sufficient.  A  very  satisfactory  indicator  is  the  commercial  soluble  starch 
which  is  made  by  heating  potato  starch  with  glycerin  and  precipitating 
the  starch  by  repeated  treatment  with  alcohol.  This  starch  dissolves 
readily  in  hot  water,  forming  a  clear  solution,  which  gives  a  very  deli- 
cate reaction  with  iodin.  It  is  best  preserved  under  alcohol,  the 
latter  being  removed  by  nitration  and  evaporation,  when  the  starch  is 
wanted  for  making  a  solution. 

In  making  starch  solution  for  use  as  an  indicator,  long  continued 
boiling  should  be  avoided,  as  this  converts  some  of  the  starch  into 
dextrin. 

On  the  Use  of  Sodium  Bicarbonate  in  Titrations  with  Iodin. 
In  these  titrations  an  excess  of  alkali  is  necessary  in  order  to  neutra- 
lize the  hydriodic  acid  formed.  See  equations  on  page  185. 

Alkali  hydroxids  or  carbonates  cannot  be  used  for  this  purpose, 
because  they  react  with  free  iodin  or  even  with  starch  iodid.  Bicar- 
bonates  ordinarily  have  no  such  action,  and  therefore  sodium  bicar- 
bonate is  usually  directed  to  be  added  in  excess  to  the  solution  to  be 
titrated  with  iodin. 

It  is  well  known  that  sodium  hydroxid  solution  reacts  with  free 
iodin,  with  formation  of  hypoiodite  and  iodid. 

2NaOH  +I2  =NaIO  +NaI  +H2O, 
the  hypoiodite  quickly  forming  iodate. 

3NaIO=2NaI+NaIO3. 

It  is  also  now  a  recognized  fact  that  sodium  carbonate  is  partly 
hydrolyzed  when  in  solution,  with  formation  of  some  sodium  hydroxid 
as  per  equation, 

Na2C03  +H20  =NaOH  +NaHCO3.     . 


SODIUM  BICARBONATE  IN   TITRATIONS  WITH  IODIN     191 

It  therefore  reacts  in  much  the  same  way  with  iodin  as  the  hydroxid, 
though  to  a  less  extent. 

On  the  other  hand,  it  is  generally  supposed  that  bicarbonate  of 
soda  is  without  effect  on  iodin,  and  when,  in  iodometric  estimations, 
addition  of  sodium  bicarbonate  is  indicated,  little  attention  is  given 
to  amount  added  as  long  as  it  be  in  excess. 

The  experiments  of  W.  A.  Puckner,  Proc.  A.  Ph.  A.,  1904,  408, 
prove  that  we  are  entirely  wrong  in  the  supposition  that  sodium  bicar- 
bonate has  no  effect  upon  iodin.  He  showed  that  when  using  i  to 
2  gms.  of  the  bicarbonate,  an  error  of  1.5  to  4.5  cc.  of  decinormal 
iodin  may  be  introduced  even  when  the  sodium  bicarbonate  used  is  of 
exceptional  purity,  and  especially  proven  to  be  free  from  carbonate, 
sulphite  or  thiosulphate.  He  shows  that  when  sodium  bicarbonate  is 
added  to  a  decinormal  iodin  solution,  residual  titration  with  sodium 
thiosulphate  will  show  a  considerable  loss  of  free  iodin,  which  went 
into  combination  in  some  form  or  other  (probably  iodid)  and  that  the 
quantity  so  lost  is  proportional  to  (i)  the  mass  of  sodium  bicarbonate; 
(2)  the  time  of  the  interaction  (the  reaction  is  slow);  (3)  the  concen- 
tration of  the  solution;  (4)  the  temperature,  and  (5)  the  size  of  the 
flask  in  which  reaction  occurs.  These  phenomena  are  due  to  the  fact 
that  sodium  bicarbonate  when  dissolved  in  water  undergoes  hydroly- 
sis, thus 

2NaHCO3=Na2CO3+H2CO3    or     (H2O+CO2). 

This  breaking  up  of  the  NaHCO3  into  Na2CO3  and  H2CO3,  and 
the  latter  into  H2O  and  CO2,  continues  until  the  pressure  of  the  CO2 
above  is  equal  to  the  pressure  of  the  gas  in  the  solution,  i.e.,  until 
equilibrium  has  been  reached.  In  concentrated  solutions  of  NaHCO3 
the  amount  hydrolyzed  is  much  greater  than  in  dilute  solutions.  An 
elevation  of  temperature  materially  increases  the  absorption  of  iodin. 

Less  iodin  is  lost  when  smaller  flasks  are  used,  provided  the  glass 
stopper  completely  shuts  off  communication  with  the  atmosphere. 
The  CO2  will  escape  from  the  solution  until  its  pressure  in  the  solution 
is  equal  to  that  of  the  gas  above.  Thus,  since  a  larger  volume  of  air 
is  contained  in  a  larger  flask,  more  CO2  passes  from  the  liquid  before 
equilibrium  is  established,  hence  more  NaHCO3  is  decomposed,  and 
more  iodin  in  consequence  absorbed.* 

*  For  further  study  of  equilibrium,  see  the  work  of  Dr.  H.  N.  McCoy,  Am. 
Ch.  J.,  vol.  XXIV,  437- 


192  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

Reasoning  from  the  above  observations  it  may  be  said  that: 
i.  Though  sufficient  sodium  bicarbonate  be  used  to  more  than  neu- 
tralize the  hydriodic  acid  formed,  the  solution  titrated  should  be  well 
diluted.  2.  That  the  titration  should  be  done  cold.  3.  That  the 
titration  should  be  done  in  small  stoppered  flasks,  and  4.  It  should  be 
done  quickly. 

ESTIMATION  OF  ARSENOUS  COMPOUNDS 

These  compounds  are  estimated  by  means  of  iodin  in  a  manner 
similar  to  that  described  under  standardization  of  iodin  solution  by 
means  of  arsenous  oxid.  The  method  is  as  follows: 

Arsenous  Oxid,  Arsenous  Acid,  Arsenous  Anhydrid,  Arsenic 
Trioxid  ^8203  =  196.44).  When  arsenous  acid  is  brought  in  contact 
with  iodin  in  the  presence  of  water  and  an  alkali,  it  is  oxidized 
into  arsenic  acid  and  the  iodin  is  decolorized.  The  reaction  is: 


NaHCO3  +HI  =NaI  +H2O  +CO2. 

The  alkali  should  be  in  sufficient  quantity  to  combine  with  the 
hydriodic  acid  formed,  and  must  be  in  the  form  of  potassium  or  sodium 
bicarbonate. 

The  hydroxids  or  carbonates  should  not  be  used.  Starch  solution 
is  used  as  the  indicator,  a  blue  color  being  formed  as  soon  as  the  arsenous 
acid  is  entirely  oxidized  into  arsenic  acid. 

o.i  gm.  of  arsenous  acid  is  accurately  weighed  and  dissolved, 
together  with  about  i  gm.  of  sodium  bicarbonate,  in  20  cc.  of  water 
heated  to  boiling.  Allow  the  liquid  to  cool,  add  a  few  drops  of  starch 
T.  S.,  and  allow  the  decinormal  iodin  to  flow  in,  shaking  or 
stirring  the  mixture  constantly,  until  a  permanent  blue  color  is  pro- 
duced. The  following  equation  illustrates  the  reaction  : 

As2O3  +2H2O  +2l2  =4HI  +  As2O5. 

4)196.44  4)503-6 

io)49-11  10)125.9 

4.911  gms.  i2.59gms.  or  1000  cc.  —  I  V.  S. 

10 

N 
Thus  each  cc.  of  -  -  1  V.  S.  represents  0.004911  gm.  of  pure  As2O3. 


LIQUOR  POTASS11   ARSENIT1S  193 

If  20  cc.  are  consumed  in  the  test,  then 

0.004911  X  20  =0.09822  gm. 

0.09822X100 

=98.22  per  cent. 

O.I 

The  U.  S.  P.  requirement  is  99.8  of  As2O3.  The  starch  T.  S.  is 
not  used  in  the  U.  S.  P.  process,  and  the  end  of  the  reaction  is  known 
by  the  iodin  being  no  longer  decolorized.  But  with  starch  the  indi- 
cation is  exceedingly  delicate,  and  it  should  always  be  used. 

Liquor  Acidi  Arsenosi,  U.  S.  P.  Measure  accurately  10  cc.  of 
the  solution,  add  to  it  i  gm.  of  sodium  bicarbonate,  and  boil  for  a  few 
minutes.  Then  allow  the  liquid  to  cool,  and  dilute  it  to  50  cc.  with 
water.  A  little  starch  T.  S.  is  then  added  and  the  decinormal  iodin 
run  in  from  a  burette,  until  a  final  drop  produces  the  blue  color  of 
starch  iodid. 

N 
Each  cc.  of  —  I  V.  S.  represents  0.004911  gm.  of  As2O3.     (See 

Estimation  of  Arsenous  Acid.) 

The  U.  S.  P.  requirement  is  that  24.6  gms.  of  the  liquor  acidi  arsenosi, 
when  treated  as  above,  will  consume  50  cc.  of  decinormal  iodin. 
Two  grams  of  the  bicarbonate  are  used. 

0.004911X50=0.24555  gm. 

o 

-  =  i  percent. 
24.6 

Liquor  Potassii  Arsenitis,  U.  S.  P.  (Fowler's  Solution).  The 
process  is  similar  to  the  foregoing. 

24.6  gms.  of  the  solution  are  diluted  with  water  to  100  cc.;  the 
mixture  is  very  slightly  acidified  with  diluted  hydrochloric  acid,  and 
then  made  alkaline  with  2  gms.  of  sodium  bicarbonate.  It  should 

N 
require  not  less  than  50  cc.  of  —  IV.  S.,  corresponding  to  i  per  cent 

of  As2Os.  The  use  of  hydrochloric  acid,  in  this  process,  is  to  neu- 
tralize any  potassium  hydroxid  which  may  have  been  formed  through 
hydrolysis  of  the  potassium  bicarbonate  contained  in  the  preparation, 
and  at  the  same  time  to  decolorize  the  solution  in  order  to  better  see 
the  yellow  color  of  the  end  reaction.  No  starch  is  used  in  the  official 
process. 


194  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

The    Direct    Percentage    Assay    of    Arsenous     Compounds. 

A  quantity  of  arsenous  acid  is  taken,  which  is  equal  to  the  weight 
of  pure  AsaOs,  oxidized  by  100  cc.  of  decinormal  iodin,  i.e., 
0.4911  gm. 

N 
If  0.4911  gm.  of  the  sample  be  taken  then  each  cc.  of  —  IV.  S. 

10 


will  represent  y^  of  this  quantity  or  i  per  cent  of  pure  As2C>3.  In 
the  case  of  weak  solutions  of  arsenic,  as  liquor  acidi  arsenosi,  liquor 
potassi  arsenitis,  etc.,  which  contain  only  i  per  cent  of  arsenous  acid. 
A  much  larger  quantity  should  be  taken  for  analysis,  otherwise  the 
quantity  of  standard  iodin  solution  used  will  be  so  small  as  to  diminish 
the  accuracy  of  the  test. 

Thus,  if  only  0.4911  gm.  of  either  of  the  above  solutions  be  taken, 
no  more  than  i  cc.  of  the  standard  solution  would  be  required.  It  is 
better  to  take  enough  of  the  preparation  to  use  up  30  to  50  cc.  of 
standard  solution.  As  above  seen  the  U.  S.  P.  directs  24.6  gms.  to 

N 

be  taken;  this  should  use  up  about  50  cc.  of  —  iodin  V.  S. 

10 

Arsenous  lodid  (Asia  =  452.10). 


4)904.20  4)503.6 

10)226.05  10)125.9 

22.605  gms.  1  2.  59  gms.  =  1000  cc.  —  V.  S. 

10 

N 

Thus  each  cc.  of  —  iodin  V.  S.  =0.022605  gm.  of  arsenous  iodid 
10 

or  0.00372  gm.  of  metallic  arsenic. 

The  U.  S.  P.  directs  to  take  0.5  gm.  of  arsenous  iodid  and  2  gms. 
of  sodium  bicarbonate  and  dissolve  them  in  50  cc.  of  water.  The 
titration  is  conducted  without  the  use  of  starch,  the  end  point  being 

N 
the  production  of  a  slight  yellow  tint.     21.9  cc.  of  —   iodin    should 

be  consumed. 

0.022605X21.9=0.4950495  gm. 

0.4950495  X  ioo 

-  =99  per  cent, 


of  which  82.7  per  cent  should  be  iodin  and  16.3  per  cent  metallic 
arsenic. 


ESTIMATION  OF  ANTIMONY   COMPOUNDS  195 

ESTIMATION     OF     ANTIMONY     COMPOUNDS 

Antimonous  oxid  (Sb2O3)  or  any  of  its  compounds  may  be 
accurately  estimated  by  means-  of  iodin,  in  a  manner  similar  to  that 
described  for  the  estimation  of  arsenous  oxid.  The  antimonous  oxid 
being  oxidized  to  antimonic  oxid,  as  per  equation. 

Sb2O3  +2H2O  +2I2  =4HI  +Sb2O5. 

The  antimonous  oxid  is  dissolved  and  kept  in  solution  by  the  aid 
of  tartaric  acid,  and  then  after  the  addition  of  an  excess  of  sodium- 

N 
bicarbonate,  the    solution  is  titrated  with  —  iodin,   using  starch  as 

an  indicator.  Accurate  results  can  only  be  obtained  if  the  solution 
is  sufficiently  alkaline  to  neutralize  the  hydriodic  acid  formed  during 
the  reaction.  The  titration  should  be  conducted  without  delay  after 
the  addition  of  the  bicarbonate,  otherwise  a  precipitate  of  antimonous 
hydrate  will  be  formed,  upon  which  iodin  has  little  effect.  The 
antimony  must  be  in  solution  to  be  properly  attacked  by  the 
iodin. 

To  o.i  gm.  of  antimonous  oxid  20  cc.  of  water  are  added  and 
the  mixture  heated  to  boiling;  to  this  tartaric  acid  is  added  in  small 
portions  at  a  time  until  the  oxid  is  completely  dissolved.  The  solu- 
tion is  then  neutralized  by  means  of  sodium  carbonate,  and  sufficient 
of  a  saturated  solution  of  sodium  bicarbonate  is  added  to  make  the 
solution  distinctly  alkaline  (about  10  cc.  is  required  for  o.i  gm.  of 
the  antimonous  oxid).  The  mixture  is  now  ready  for  titration  with 
standard  iodin  solution.  This  should  be  done  immediately.  The 
appearance  of  a  permanent  blue  color  marks  the  end  point,  starch 
being  used  as  indicator. 

Sb2O3  +  2H2O-t-2l2=4HI+Sb2O5. 

4)286.24  4)503.6 

10)71.548          10)125.9 

N 

7.1548  gms.        12.59  gms.  or  1000  cc.  —  V.  S. 

10 

N 
i  cc.  of  —  iodin  represent  0.0071548  gm.  of  Sb2O3. 

The  solution  of  the  oxid  may  be  made  by  means  of  hydrochloric 
acid,  and  after  adding  a  portion  of  tartaric  and  diluting  with 
water,  sodium  bicarbonate  is  added  and  the  titration  conducted 
as  above. 


196  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Other  compounds  of  antimony  may  be  estimated  in  the  same 
way.  Antimonic  compounds  are  reduced  to  antimonous  sulphid 
($0283)  by  precipitating  with  hydrogen  sulphid,  and  after  thoroughly 
washing  the  precipitate,  dissolving  it  in  hydrochloric  acid;  thus  a 
solution  of  antimonous  chlorid  is  obtained  from  which  all  traces  of 
hydrogen  sulphid  are  expelled  by  boiling.  This  solution  is  diluted 
with  water,  tartaric  acid  added,  and  finally,  after  making  alkaline 
with  sodium  bicarbonate,  titrated  with  the  standard  iodin  solution 
as  above  described. 

Antimony  and  Potassium  Tartrate  •  (Tartar  Emetic) 
(2  (K[SbO]C4H4O6)+H2O  =659.80).  This  is  the  only  antimonial 
compound  for  which  the  U.  S.  P.  gives  direction  for  a  volumetric 
assay. 

The  U.  S.  P.  directions  are:  i  gm.  of  the  salt  is  dissolved  in  suffi- 
cient water  to  make  100  cc.  32.99  cc.  of  this  solution,  represent- 
ing 0.3299  gm.  of  the  salt,  are  taken  for  assay.  20  cc.  of  a  cold 
saturated  solution  of  sodium  bicarbonate  are  added,  then  a  little 

N 

starch  T.  S.,  and  the  mixture  titrated  with   —  •  iodin  until  a  perma- 

10 

nent  blue  color  appears.     Not  less  than  19.9  cc.  of  the  iodin  solution 
should  be  required. 

The  calculation  is  as  follows  : 


=4HI  +2KHC4H4O6  +2HSbO3 


4)659.80  4)503-6 

10)164.95  10)125.9 


16.495  gms.  I2-59  gms.=  1000  cc.  —  V.  S. 

10 

N 
i  cc.  of    --  iodin  represents    0.016495   gm.  of  2K(SbO)C4H4O6 

+H2O  (crystallized  tartar  emetic). 

K(SbO)C4H4O6  (anhydrous  tartar  emetic)  =320.96. 
2)320.96 
10)160.48 

16.048  gms.  =  12.59  8ms-  of  iodin  or  1000  cc.  —  V.  S. 

N 
Thus   i  cc.  of  —    iodin   represents    0.016048    gm.   of  anhydrous 

tartar  emetic. 


ESTIMATION   OF  SULPHUROUS   ACID   AND  SULPHITES     197 

N 

If  19.9  cc.  of  —  iodin  are  consumed,  then 
10 

0.016495  gm.X  19.9  =0.3282405  gm. 

0.3282405X100 

—  =99.5  per  cent  of  the  crystallized  salt; 
0.3299 

0.016048X19.9=0.3153552  gm.  or  95.6  per  cent  of  the  anhydrous  salt. 


ESTIMATION    OF    SULPHUROUS    ACID    AND    SULPHITES 

These  substances  may  be  accurately  estimated  by  means  of  a  stand- 
ard solution  of  iodin.  When  sulphurous  acid  or  one  of  its  salts  is 
brought  in  contact  with  iodin,  a  complete  oxidation  takes  place.  The 
sulphurous  acid  is  oxidized  to  sulphuric  acid  and  the  sulphite  to  a 
sulphate,  as  the  equations  show. 

H2S03  +H20  +I2 =2HI  +H2S04, 

Na2SO3  +H2O  + 12 =2HI  +  Na2SO4, 

NaHS03  +H20  +I2 =2HI  +NaHSO4. 

There  are  two  methods  which  may  be  employed.  In  one 
method  the  substance  is  brought  into  solution  in  water,  an  excess  of 
sodium  bicarbonate  is  added,  and  then  the  standard  iodin  solution 
is  run  in  until  a  faint  yellow  color  of  free  iodin  marks  the  end-reaction. 
If  starch  solution  is  used  as  indicator  the  end-point  is  the  production 
of  a  blue  color.  The  other  method  is  that  of  Giles  and  Shearer,  who, 
in  a  very  valuable  series  of  experiments  detailed  in  the  J.  S.  C.  I., 
Ill,  197,  and  IV,  303,  suggest  the  following  modification: 

The  weighed  sulphurous  acid  or  the  sulphite  (in  fine  powder)  is 

N 
added  to  an  accurately  measured  excess  of   -  -  iodin,  without  diluting 

with  water.     After  the  mixture  has  been  allowed  to  stand   for  about 
one  hour,  with  frequent  shaking,   the  oxidation  is  complete,  and  the 

N 
excess  of  iodin  is  ascertained  by  titrating  back  with   —  sodium  thio- 

sulphate. 


198  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

N 
The  quantity  of  the  latter  deducted  from  the  quantity  of  —  iodin 

solution  added,  will  give  the  quantity  of  the  latter,  which  reacted  with 
the  sulphite. 

The  neutral  and  acid  sulphites  of  the  alkalies,  alkali  earths,  and 
even  zinc  and  aluminum,  may  be  accurately  estimated  in  this  manner, 
The  less  soluble  salts  requiring,  of  course,  more  time  and  shaking,  to 
insure  their  complete  oxidation.  The  latter  is  the  U.  S.  P.  method. 

Sulphurous  Acid  (Acidum  Sulphurosum,  U.  S.  P.) — This  is  an 
aqueous  solution  of  sulphur  dioxid  (802=63.59)  containing  not  less 
than  6  per  cent,  by  weight,  of  the  gas. 

Sulphurous  acid  when  brought  in  contact  with  iodin  is  oxidized 
into  sulphuric,  the  iodin  being  decolorized  because  of  its  union  with 
the  hydrogen  of  the  accompanying  water,  forming  hydriodic  acid. 

Two  grams  of  sulphurous  acid  are  taken  and  diluted  with  distilled 
water  (recently  boiled  and  cooled  *)  to  about  25  cc.  Two  grams  of 
sodium  bicarbonate  are  added,  and  then  the  decinormal  iodin  V.  S. 
is  delivered  into  the  solution  (to  which  a  little  starch  T.  S.  had  been 
previously  added)  until  a  permanent  blue  color  is  produced.  At  least 
40  cc.  of  the  standard  iodin  solution  should  be  consumed  before  this 
color  appears. 

The  following  equations,  etc.,  show  the  reaction  that  takes  place: 

H2SO3  +H2O  + 12  =  2HI +H2SO4. 

2)81.47  2)251.8 

10)40.735          10)125.9 


4.0735  gms.        12.59  gins,  or  1000  cc.  —  V.  S. 

10 

N 
Thus  each  cc.  of  the  —  V.  S.  represents   0.0040735  gm.  of  pure 

H2S03. 

Sulphurous  acid  being,  however,  looked  upon  as  a  solution  of  SO2 
in  water,  the  quantity  of  this  gas  is  generally  estimated  in  analyses. 

H2O,SO2  +H2O  +I2  =2HI  +H2SO4. 
2)63.59  2)251.8 

10)31.795          10)125.9 

3.1795  gms.         12.59  gms. 


*  "  Recently  boiled "  insures  absence  of  air,  the  oxygen  of  which  would  par- 
tially oxidize  the  sulphurous  acid,  and  "cooled"  is  directed  to  avoid  loss  9f  SO2, 
which  would  occur  if  hot  water  were  used. 


SULPHUROUS  ACID 


199 


N 
Thus  each  cc.  of  :  -  V.  S.  consumed  before  the  blue  color  appears 


10 


represents  0.0031795  gm.  of  SC>2. 

If  40  cc.  are  consumed  in  the  above  analysis,  the  2  gms.  contain 


then 


0.0031795X40=0.12718; 
0.12718X100 


=6.35  per  cent  of  SC>2. 


A  better  method,  however,  is  to  measure  2  cc.  into  a  stoppered 
weighing  bottle,  and  weigh  accurately.  Then  pour  this  into  a  solu, 
tion  of  sodium  bicarbonate,  and  after  the  addition  of  starch  solution- 

N 

titrate  with  —  iodin. 
10 

The  U.  S.  P.  directs  to  introduce  into  a  stoppered 
weighing  bottle,  2  cc.  of  sulphurous  acid  and  weigh 

N 
accurately.     To  this  add  50  cc.  of  —    I  V.  S.,  and 

allow  it  to  stand  for  five  minutes,  then  titrate  with 

N 

—   sodium  thiosulphate  until    the    mixture   is    just 

decolorized.      Subtract  the    number  of   cc.  of  thio- 

N 
sulphate  used  from   the    50  cc.  of  —  I  V.  S.  added, 

and  multiply  the  difference  by  0.318,  and  divide 
this  product  by  the  weight  of  the  acid  taken.  The 
quotient  represents  the  percentage  of  absolute  SO2 
in  the  acid. 

When  a  solution  containing  sulphur  dioxid  is  to 
be  measured  by  means  of  a  rjipette,  it  is  never  ad- 
visable to  fill  the  instrument  by  suction  in  the  usual 
manner,  as  this  would  cause  a  loss  of  the  gas.  A 
better  plan  is  to  fill  the  pipette  by  pressure,  by  the 
use  of  an  arrangement  similar  to  that  shown  in 
Fig.  59- 

The  solution  containing  sulphur  dioxid  or  other  volatile  substance 
is  poured  into  a  flask  which  is  provided  with  a  stopper  through  which 
two  glass  tubes  pass;  one  of  these  tubes  reaches  to  near  the  bottom 
of  the  flask  and  the  other  projects  about  one  half  an  inch  below  the 


FIG.  59. 


200  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

stopper  and  is  bent  outward  above.  To  the  upper  end  of  the  former 
the  pipette  is  attached  by  means  of  a  piece  of  rubber  tubing.  By 
blowing  into  the  flask  through  the  shorter  tube  the  liquid  is  caused 
to  rise  and  fill  the  pipette,  which  may  then  be  easily  pulled  out  of  the 
rubber  tube  connection. 

Sodium  Sulphite  (Na2SO3+  71120=250.39).  i  gm.  of  the  salt  is 
dissolved  in  25  cc.  of  distilled  water  recently  boiled  to  expel  air,  and 
after  the  addition  of  an  excess  of  sodium  bicarbonate  a  little  starch 
T.  S.  is  added,  and  then  the  decinormal  iodin  V.  S.  delivered  in  from 
a  burette,  until  the  blue  color  of  starch  iodid  appears,  which  does  not 
disappear  upon  shaking  or  stirring. 

The  reaction  is  expressed  as  follows: 

=2HI  +Na2SO4  +6H2O. 


2)250.39         2)251.8 
10)125.19       10)125.9 

12.519  gms.     12.59  gms.  or  1000  cc.  —  iodin  V.  S. 

10 

Thus  each  cc.  of  the  standard  solution  represents  0.012519  gm.  of 
crystallized  sodium  sulphite. 

If  i  gm.  of  the  salt  is  taken,  to  find  the  percentage  multiply  the 
factor  by  the  number  of  cc.  of  standard  solution  consumed,  and  the 
result  by  100. 

The  U.  S.  P.  directs  that  0.5  gm.  of  the  finely  powdered  crystals  be 

N 
added  to  50  cc.  of  —    iodin,   contained  in  a  100  cc.  glass-stoppered 

flask,  and  allowed  to  stand  for  one  hour  (shaking  frequently);  the 

N 
solution  is  then  titrated  with  —  sodium  thiosulphate  until  the  color 

•      1-11  IO 

is  discharged. 

Potassium  Sulphite  (K2SO3+  21^0=192.95).  Operate  upon 
0.5  gm.  in  the  same  manner  as  for  sodium  sulphite. 


2)192.95       2)251.8 
10)96.47       10)125.9 

9.647  gms.    12.59  gms.  or  1000  cc.  of  standard  V.  S. 

N 
Each  cc.  of  the  —  iodin  represents  0.009647  gm.  of  crystallized 

potassium  sulphite. 


SODIUM   THIOSULPHATE 


201 


Sodium  Bisulphite  (NaHSO4=  103.35).  Operate  upon  about 
0.25  gm.  in  the  same  manner  as  for  sodium  sulphite,  and  apply  the 
following  equation: 


NaHSO3 

210 


I2 
2)251.8 


HO     =     2HI 


NaHSO4. 


5.1675  gms.   12.59  gms«  or  1000  cc.  —  V.  S. 

10 


N 
Thus  each  cc.  of  —    iodin   represents   0.0051675  gm.  of  sodium 

bisulphite. 

Sodium  Thiosulphate  (Sodium  Hyposulphite)  (Na2S2O3+5H2O 
=  246.46).  This  salt,  when  brought  in  contact  with  iodin,  is  converted 
into  sodium  iodid  and  sodium  tetrathionate.  The  reaction  is  expressed 
by  the  equation 

2Na2S2O3  +I2  =2NaI  +Na2S4O6. 


TABLE  OF  SUBSTANCES  WHICH  MAY  BE  ESTIMATED  BY  MEANS  OF  STANDARD 

IODIN  SOLUTION 


Name. 

Formula. 

Molecular 
Weight. 

N 
—  Factor. 

10 

Acid   sulphurous 

H2SO4 

8l    47 

Antimonous  oxid  .  . 

Sb2O3 

286    24 

O    OO7  1^4.8 

Antimony  et  potassium  tartrate. 
Arse  nous  iodid 

2(K(SbO)C4H406)  +  H20 
AsI3 

659.80 
d.^1     I 

0.016495 

oxid  

As,O» 

1  06    44. 

o  0049  i  i 

Cyanogen 

CN 

2f    Sd. 

Hydrogen  sulphid 

HoS 

77    87 

Iron  (metallic) 

Fe, 

66  '°6 
III    O 

HgCl, 

268  86 

«J.U(J^//^ 

o  026886 

M^ercurous  chlorid 

HeCl 

277    68 

Potassium  cyanid  

KCN 

•*oo-uo 
64.    7O 

O    OO  7  2  7  £ 

"          sulphite  (anhydrous) 
1  '           "        (crystallized)  .  . 
Sodium  bisulphite 

K2S03 
K2S03+2H20 
NaHSO3 

157.19 
192.95 

IO7     7  < 

0.0078595 
0.009648 

'  '        rilphite  (anhydrous)  
(crystallized).. 
'  '       thiosulphate  

Na2S03 
Na2S03+7H20 
Na,S,O,+  <;H,O 

AWO-OJ 
125.23 

250.39 
24.6    4.6 

0.0062615 
O.OI25I9 

Sulphur  dioxid 

SO2 

67   ^o 

Tin  in  stannous  compounds  .  .  . 
Zinc             

Sn2 
Zn, 

UJ'  DV 
236.2 

1  29  8 

O.OO5905 

202  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

• 
It  is  estimated  as  follows:    i  gm.  of  the  salt  is  dissolved  in  20  cc. 

N 
of  water,  a  few  drops  of  starch  T.  S.  are  added,  and  then  the  —  iodin 

is  delivered  in  from  a  burette,  until  the  appearance  of  blue  starch  iodid 
indicates  an  excess  of  iodin. 

N 
Each  cc.  of  —  iodin  represents  0.015706  gm.  of  the  anhydrous 

salt,  or  0.024646  gm.  of  the  crystalline  salt. 

Many  other  substances  besides  those  mentioned  in  the  foregoing 
pages  may  be  estimated  by  titration  with  standard  iodin  solution. 
Among  them  are  cyanids,  hydrogen  sulphid,  stannous  compounds, 
mercurous  compounds,  metallic  zinc,  and  aluminum. 

ESTIMATION   OF   SUBSTANCES   READILY   REDUCED 

Any  substance  which  readily  yields  oxygen  in  a  definite  quantity, 
or  is  susceptible  of  an  equivalent  action,  which  involves  its  reduction 
to  a  lower  quant ivalence,  may  be  quantitatively  tested  by  ascertaining 
how  much  of  a  reducing  agent  of  known  power  is  required  by  a  given 
quantity  of  the  substance  for  its  complete  reduction. 

The  principal  reducing  agents  which  may  be  employed  in  volu- 
metric analysis  are  sodium  thiosulphate,  sulphurous  acid,  arsenous 
acid,  oxalic  acid,  metallic  zinc,  and  magnesium. 

The  sodium  thiosulphate  is  the  only  one  which  is  employed  offi- 
cially in  the  U.  S.  P.  in  the  form  of  a  volumetric  solution.  It  is  used 
in  the  estimation  of  free  iodin,  and  indirectly  of  other  free  halogens, 
or  compounds  in  which  the  halogen  is  easily  liberated,  as  in  the  hypo- 
chlorites,  etc. 

ESTIMATIONS    INVOLVING     THE    USE    OF     SODIUM    THIOSULPHATE    V.    S. 

(lodometry) 

When  sodium  thiosulphate  acts  upon  iodin,  sodium  tetrathionate 
and  sodium  iodid  are  formed,  and  the  solution  is  decolorized. 

This  reaction  takes  place  in  definite  proportions:  one  molecular 
weight  of  the  thiosulphate  absorbs  one  atomic  weight  of  iodin. 

2Na2S2O3  +I2  =2NaI  +Na2S4O6. 

Chlorin  cannot  be  directly  titrated  with  the  thiosulphate,  but  by 
adding  to  the  solution  containing  free  chlorin  an  excess  of  potassium 


DECINORMAL  SODIUM   TH1OSULPHATE  203 

iodid,  the   iodin    is   liberated  in  exact  proportion  to  the   quantity  of 
chlorin  present,  atom  for  atom. 


Then  by  estimating  the  iodin,  the  quantity  of  chlorin  is  ascertained. 
All  bodies  which  contain  available  chlorin,  or  which  when  treated  with 
hydrochloric  acid  evolve  chlorin,  may  be  estimated  by  this  method. 

Also,  bodies  which  contain  available  oxygen,  and  which  when 
boiled  with  hydrochloric  acid  evolve  chlorin,  such  as  manganates, 
chromates,  peroxids,  etc.,  may  be  estimated  in  this  way. 

Solutions  of  ferric  salts,  when  acidulated  and  boiled  with  an  excess 
of  potassium  iodid,  liberate  iodin  in  exact  proportion  to  the  quantity 
of  ferric  iron  present. 

Thus  sodium  thiosulphate  may  be  used  in  the  estimation  of  a 
great  variety  of  substances  with  extreme  accuracy. 

Preparation  of  Decinormal  Sodium  Thiosulphate  (Hypo- 
sulphite) (Na2S2O3  +  5H2O=  246.46  contains  24.646  gms.  in  i  liter). 
Sodium  thiosulphate  is  a  salt  of  thiosulphuric  acid  in  which  two  atoms 
of  hydrogen  have  been  replaced  by  sodium;  it  therefore  seems  that  a 
normal  solution  of  this  salt  should  contain  one  half  the  molecular 
weight  in  grams  in  one  liter. 

But  this  salt  is  used  chiefly  for  the  estimation  of  iodin,  and,  as 
stated  before,  one  full  molecular  weight  reacts  with  and  decolorizes 
one  atomic  weight  of  iodin,  and  since  one  atom  of  iodin  is  chemically 
equivalent  to  one  atom  of  hydrogen,  a  full  molecular  weight  of  sodium 
thiosulphate  must  be  contained  in  a  liter  of  its  normal  solution. 

Sodium  thiosulphate  is  easily  obtained  in  a  pure  state,  and  there- 
fore the  proper  weight  of  the  salt,  reduced  to  powder  and  dried  between 
sheets  of  blotting-paper,  may  be  dissolved  directly  in  water,  and  made 
up  to  one  liter. 

The  U.  S.  P.  directs  that  a  stronger  solution  than  necessary  be 
made,  its  titer  found,  and  then  the  solution  diluted  to  the  proper  measure. 

30  gms.  of  selected  crystals  of  the  salt  are  dissolved  in  enough 
water  to  make,  at  or  near  15°  C.  (59°  C.),  1000  cc. 

This  concentrated  solution  is  then  standardized  by  one  of  the  follow- 
ing methods: 

N 
a.  Standardization  by  Means  of  —  Iodin.     Transfer  10  cc.  of  this 

solution   into  a   flask  or  beaker,  add  a   few  drops   of  starch  T.  S., 


204  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

and  then  gradually  deliver  into  it  from  a  burette  decinormal  iodin 
solution,  in  small  portions  at  a  time,  shaking  the  flask  after  each  addi- 
tion, and  regulating  the  flow  to  drops  toward  the  end  of  the  operation. 
As  soon  as  a  blue  color  is  produced  which  does  not  disappear  upon 
shaking,  but  is  not  deeper  than  pale  blue,  the  reaction  is  completed. 
Note  the  number  of  cc.  of  iodin  solution  used,  and  then  dilute  the 
thiosulphate  solution  so  that  equal  volumes  of  it  and  the  decinormal 
iodin  will  exactly  correspond  to  each  other,  under  the  above-mentioned 
conditions. 

Example.  The  10  cc.  of  sodium  thiosulphate,  we  will  assume, 
require  10.7  cc.  of  decinormal  iodin. 

The  sodium-thiosulphate  solution  must  then  be  diluted  in  the 
proportion  of  10  cc.  to  10.7  cc.,  or  1000  cc.  to  1070  cc. 

After  the  solution  is  thus  diluted  a  new  trial  should  be  made,  in 
the  manner  above  described,  in  which  50  cc.  of  the  thiosulphate  solu- 
tion should  require  exactly  50  cc.  of  the  decinormal  iodin  to  pro- 
duce a  faint  blue  color. 

The  solution  should  be  kept  in  small  dark  amber-colored, 
glass-stoppered  bottles,  carefully  protected  from  dust,  air,  and 
light. 

One  cc.  of  this  solution  is  the  equivalent  of: 

Iodin 0.01259  gram. 

Bromin 0.007936      ' ' 

Chlorin 0.003518      " 

Iron  in  ferric  salts 0.00555        " 

b.  Standardization  by  Means  of  Potassium  Dichromate.  The 
potassium  dichromate  should  be  pure,  if  not  it  should  be  purified 
by  triple  recrystallization  and  then  heating  in  a  porcelain  crucible 
until  the  entire  mass  is  just  fused:  Then  setting  aside  to  cool  in  a 
desiccator  over  calcium  chlorid  or  sulphuric  acid.  The  mass  falls  to  a 
crystalline  powder.  Of  this  purified  potassium  dichromate,  weigh 
off  a  definite  quantity,  say  0.2  gm.,  dissolve  it  in  a  small  quantity  of 
water,  and  pour  the  solution  into  a  beaker  containing  2  gms.  of  pure 
potassium  iodid  (free  from  iodate)  and  100  cc.  of  water.  Acidulate 
the  solution  with  5  cc.  of  concentrated  hydrochloric  or  sulphuric  acid, 
cover  the  beaker,  and  let  stand  for  about  five  minutes,  then  titrate 
with  the  thiosulphate  solution  to  be  standardized  (using  starch  as  an 


DECI NORMAL  SODIUM   THIOSULPHATE  205 

indicator)  until  the  blue  color  is  just  discharged.  The  calculation 
is  then  made  as  follows: 

K2Cr2O7  +6KI  +  I4HC1 =2CrCl3  +8KCH-yH2O  +3X2. 
292.28    6X164.76 

Thus  292.28  gms.  of  potassium  dichromate  oxidizes  6X164.76 
=988.56  gms.  of  potassium  iodid  and  liberates  therefrom  6X125.9 
=  755.4  gms.  of  iodin. 

The  potassium  iodid  must  be  in  excess,  in  fact  for  each  atom  of 
iodin  liberated  one  molecule  of  potassium  iodid  must  be  present  at. 
the  completion  of  the  reaction  in  order  to  keep  the  iodin  in  solution, 
and  thus  prevent  loss  by  volatilization. 

If  292.28  gms.  of  potassium  dichromate  liberate  755.4  gms.  of 
iodin,  0.2  gms.  will  liberate 

755.4X0.2  = 

N 
0.01259  gm.  of  iodin  =  i  cc.  of  —  thiosulphate. 

N 

0.5168  =41.05  cc.  of  —  thiosulphate. 

10 

Therefore,  if  in  the  above  assay  37.60  cc.  of  the  thiosulphate  V.  S. 
were  consumed,  it  must  be  diluted  so  that  each  37.60  cc.  will  measure 
41.05  cc.  in  order  to  convert  the  thiosulphate  solution  into  a  true  deci- 
normal  solution.  A  new  trial  should  then  be  made  with  the  diluted 
solution  to  see  if  its  strength  is  correct. 

The  pharmacopoeial  method  is  a  modification  of  this ;  it  is  as  follows : 

To  a  solution  of  about  i  gm.  of  potassium  iodid  in  10  cc.  of  diluted 

sulphuric  acid  contained  in  a  500  cc.  flask,  add  slowly  from  a  burette 

N 
20  cc.  of  —  potassium  dichromate,  shaking  after  each  addition.     Cover 

the  mouth  of  the  flask  with  a  watch-glass  and  let  stand  for  five  minutes, 
then  dilute  to  about  250  cc.  with  distilled  water,  add  some  starch  T.  S., 
and  then  from  a  burette  the  trial  solution  of  sodium  thiosulphate,  in 
small  portions  at  a  time,  shaking  after  each  addition,  and  towards  the 
end  of  the  operation  reducing  the  flow  to  drops,  until  the  blue  color 
of  the  mixture  changes  to  green.  Note  the  number  of  cc.  of  the  thio- 


206  A    MANUAL   OF    VOLUMETRIC  ANALYSIS 

sulphate  solution  consumed.     Then  dilute  it  with  distilled  water,  so 

N 
that  equal  volumes  of  it  and  —  dichromate  will  exactly  correspond  to 

each  other  under  the  above  conditions  at  25°  C.  (77°  F.) 

Example.  Assuming  that  16  cc.  of  the  thiosulphate  solution  were 
consumed,  then  each  16  cc.  must  be  diluted  to  measure  exactly  20  cc. 
c.  Standardization  by  Means  of  Potassium  Bi-iodate.  This 
method  depends  upon  the  fact  that  when  potassium  iodid  and  bi- 
iodate  are  brought  together  in  the  presence  of  a  small  quantity  of  an 
acid,  an  equivalent  amount  of  iodin  is  set  free.  The  reaction  is  illus- 
trated by  the  equation: 

KH(I03)2-|-ioKI+iiHCl  =  i2l-f-iiKCH-6H2O. 

12)386.94  12)1510.8 

10)32.245  10)125.9  N 

3.2245  gms.  12.59  gms.  or  1000  cc.  —  V.  S. 

10 

Thus  it  is  seen  that  one  molecule  of  the  bi-iodate  causes  the  libera- 
tion of  12  atoms  of  iodin,  against  which  the  thiosulphate  solution  is 
standardized. 

In  order  not  to  use  up  too  large  a  quantity  of  the  thiosulphate 
solution  in  the  titration,  a  very  small  quantity  of  the  bi-iodate  must 
be  taken,  and  since  a  small  error  in  weighing  this  would  entail  a  rela- 
tively large  error  in  the  results  it  is  best  to  use  the  bi-iodate  in  the 
form  of  a  solution  of  known  strength  and  thus  obviate  the  difficulty. 

A  decinormal  solution  of  the  bi-iodate,  i.e.,  one  containing  in 
1000  cc.  3.2245  gms.  of  the  salt  may  be  used  to  advantage.  Such  a 
solution  will  keep  unchanged  for  years,  and  may  be  employed  as 
follows  for  the  standardization  of  sodium  thiosulphate  solution: 

Into  a  glass-stoppered  flask  of  250  cc.  capacity  introduce  10  cc. 
of  a  5  per  cent  solution  of  potassium  iodid,*  one  cc.  of  diluted  hydro- 
chloric acid  and  exactly  25  cc.  of  the  above  potassium  bi-iodate  solu- 
tion. 

This  brownish-yellow  solution  is  now  titrated  with  the  sodium 
thiosulphate  solution  which  is  slowly  delivered  from  a  burette  until 
the  solution  becomes  pale  yellow  in  color;  a  few  drops  of  starch  solution 
are  now  added,  and  the  titration  continued  (the  flow  being  reduced 

*  The  potassium  iodid  must  be  in  sufficient  quantity  not  only  to  react  with  the 
bi-iodate  quantitatively,  as  shown  in  the  equation,  but  also  to  dissolve  the  iodin 
which  is  liberated. 


ESTIMATION  OF  FREE  IODIN  207 

to  drops)  until  the  blue  color  is  just  discharged.  During  the  titration, 
the  flask  should  be  frequently  stoppered  and  vigorously  shaken. 

When  the  blue  color  is  discharged,  note  the  number  of  cc.'s  used. 
If  an  exactly  decinormal  thiosulphate  solution  is  taken,  25  cc.  will  be 
required  to  react  with  the  25  cc.  of  potassium  bi-iodate  solution,  under 
the  above  conditions.  If,  on  the  other  hand,  only  22  cc.  of  the  thio- 
sulphate solution  are  used  in  the  titration,  then  the  latter  is  too  strong, 
and  must  be  diluted  so  that  each  22  cc.  will  measure  25  cc.  in  order 
to  make  it  strictly  decinormal. 

d.  Standardization  by  Means  of  Potassium  Permanganate.  — 
Sodium  thiosulphate  solution  may  be  accurately  standardized  by 
means  of  potassium  permanganate,  if  the  other  substances  used  for 
this  purpose  are  not  at  hand.  The  method  is  easily  understood  by 
referring  to  the  iodometric  standardization  of  potassium  permanga- 
nate solution,  see  page  146. 

Estimation  of  Free  lodin  (1  =  125.9).  I°dm  in  the  dry  state  or 
in  the  form  of  a  tincture  is  brought  into  aqueous  solution  by  means 
of  pure  potassium  iodid  and  then  titrated  with  standard  sodium  thio- 
sulphate. The  potassium  iodid  is  used  here  to  dissolve  the  iodin; 
it  must  be  free  from  iodate  (KIOs)  because  the  presence  of  this  salt 
would  cause  a  liberation  of  iodin  from  the  potassium  iodid. 

Dry  iodin  is  assayed  as  follows  : 

About  o.5gm.of  iodin  are  placed  in  a  tightly  stoppered  weighing  bottle 
and  accurately  weighed.  One  gram  of  potassium  iodid  and  50  cc.  of 

N 

water  are  added,  and  when  the  iodin  is  dissolved,  —  sodium  thio- 

10 

sulphate  is  delivered  from  a  burette  in  small  portions  at  a  time,  shaking 
after  each  addition  until  the  reaction  is  nearly  completed,  and  the  solu- 
tion is  of  a  faint  yellow  color.  A  few  drops  of  starch  indicator  are 
now  added,  and  the  titration  with  the  thiosulphate,  continued  drop  by 
drop  until  a  final  drop  just  discharges  the  blue  color.  The  number 
of  cc.  of  the  thiosulphate  solution  used  is  noted.  This  number  multi- 
plied by  the  decinormal  factor  for  iodin  gives  the  weight  of  the  latter 
present  in  the  sample  assayed. 


I2   + 

2)251.8  2)492.92 

10)125.9  10)246.46 

12.59  gms.  24.646  gms.  or  1000  cc.  —  V.  S. 

10 


208  A    MANUAL   OF    VOLUMETRIC  ANALYSIS 

N 
Each  cc.  of  —  thiosulphate  represents  0.01259  gm.  of  iodin. 

N 

If  in  the  above  assay  39  cc.  of  —  sodium  thiosulphate  were  con- 
sumed, then 

0.01259X39=0.491  gm. 

0.491+100 

; — —  =98.2  per  cent. 

Liquor  lodi  Compositus  (Lugol's  Solution).  This  is  an  aqueous 
solution  of  iodin  and  potassium  iodid. 

It  is  estimated  for  iodine  in  the  same  way  as  the  foregoing.  The 
potassium  iodid  acts  merely  as  a  solvent  for  free  iodin,  and  does  not 
enter  into  the  reaction. 

Ten  or  twelve  grams  of  the  solution  is  a  convenient  quantity  to 
operate  upon.  Starch  T.  S.  is  the  indicator. 

Tinctura  lodi  (Tincture  of  Iodin).  This  is  an  alcoholic  solution 
of  free  iodin,  and  must  be  diluted  with  a  solution  of  potassium  iodid, 
before  titration,  in  order  to  provide  sufficient  liquid  to  keep  the  resulting 
salts  in  solution  and  to  prevent  the  precipitation  of  iodin,  which  would 
result  upon  the  addition  of  the  aqueous  standard  solution. 

Indirect  lodometric  Estimations.  The  titration  methods  pre- 
viously described,  in  which  iodin  is  used  as  a  standard  solution  and 
the  estimation  of  free  iodin  by  means  of  sodium  thiosulphate,  are 
classed  as  direct  iodometric  methods.  The  following  methods,  in  which 
the  strength  of  the  substance  under  analysis  is  determined  by  the 
quantity  of  iodin  which  it  liberates  from  an  iodid,  are  known  as  indi- 
rect iodometric  methods.  Potassium  iodid  is  added  in  excess  *  to  an 
acidulated  solution  of  the  substance,  and  the  liberated  iodin  estimated 
by  means  of  standard  thiosulphate.  These  methods  are  among  the 
most  accurate  of  all  volumetric  analyses,  and  take  in  a  very  large 
class  of  substances.  Among  the  substances  which  may  be  analyzed 
by  this  method  are  chlorin  and  brornin,  and  all  substances  which 
readily  liberate  these  elements:  ferric  salts,  manganates,  chromates, 
metallic  peroxids,  and  other  substances  from  which  oxygen  can  be 
easily  liberated. 

*  The  iodid  should  be  in  sufficient  excess  to  keep  the  liberated  iodin  in  solution 
as  KI.I. 


AQUA   CHLORI  209 

Free  Chlorin  or  Bromin.  Free  chlorin  acts  upon  potassium 
iodid,  liberating  iodin  as  per  the  equation 

C12+2KI=2KC1+I2. 

Thus  it  is  seen  that  each  atom  of  chlorin  will  liberate  one  atom 
of  iodine,  hence  by  determining  the  quantity  of  iodin  by  means  of  a 
standard  thiosulphate  solution  the  quantity  of  chlorin  present  is 
easily  ascertained.  (125.9  ^ms-  °f  I=35-IO>  gms.  of  Cl).  The  same 
applies  to  free  bromin,  one  atom  of  bromin  (79.36)  will  liberate  one 

N 

atom  of  iodin  (125.9)  Br2+2KI=2KBr  +  I2.     1000  cc.  of  —  sodium 

10 

thiosulphate  is  equivalent  to  12.59  gms.  of  iodin,  and  hence  to  3.518 
gms.  of  chlorin  or  7.936  gms.  of  bromin. 

N 
Thus  i  cc.  of  —  sodium  thiosulphate  is  equivalent  to  0.01259  gm- 

of  iodin;  0.003518  gm.  of  chlorin;  0.007936  gm.  of  bromin. 

Chlorin  cannot  be  directly  titrated  with  sodium  thiosulphate 
because  instead  of  the  tetrathionate  being  formed  as  with  iodin,  sul- 
phuric acid  is  produced,  furthermore  there  is  no  readily  observable 
end  point  as  there  is  with  iodin. 

Aqua  Chlori  (Chlorin  Water).  This  is  an  aqueous  solution  of 
chlorin,  Cl  =35.18,  containing  at  least  0.4  per  cent  of  the  gas. 

The  estimation  of  chlorin  is  effected  in  an  indirect  way,  namely, 
by  determining  the  quantity  of  iodin  which  it  liberates  from  potassium 
iodid. 

A  definite  quantity  of  chlorin  will  liberate  a  definite  quantity  of 
iodin  from  an  iodid;  these  quantities  are  in  exact  proportion  to  their 
atomic  weights,  as  the  equation  shows. 

C12   +  2KI  =  2KC1   +   I2. 

2)70-36  2)251.8 

10)35-18  10)125.9 

3.518  gms.  1 2. 59  gms. 

Thus  it  is  seen  that  by  estimating  the  liberated  iodin  the  quantity 
of  chlorin  may  be  determined  with  accuracy. 

Ten  grams  is  a  convenient  quantity  to  operate  upon.  To  this  about 
half  a  gram  of  potassium  iodid  is  added.  A  little  starch  T.  S.  is  then 
introduced,  and  the  titration  is  begun  with  decinormal  sodium  thio- 
sulphate. 


210  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

When  the  blue  color  of  starch  iodid  has  entirely  disappeared  the 
reaction  is  finished. 

The  reaction  between  iodin  and  sodium  thiosulphate  is  illustrated 
by  the  following  equation: 


I2   + 

2)251.8  2)492.92 

10)125^  10)246.46 

12.59  gms.  24.646  gms  or  1000  cc.  —  V.  S. 

10 

N 
Thus  we  see  that  1000  cc.  of  —  Na2S2O3.5H2O  represent  12.59  gms. 

of  iodin,  which  are  equivalent  to  3.518  gms.  of  chlorin. 

Each  cc.  therefore  is  equivalent  to  0.003518  gm.  of  chlorin.  This 
number  is  the  factor  which,  wThen  multiplied  by  the  number  of  cc.  of 

N 

—   thiosulphate   used,   gives   the   weight    in  grams  of   chlorin  con- 

10 

tained  in  the  quantity  of  chlorin  water  acted  upon. 

Chlorinated  Lime  (Calx  Chlorinata,  Chlorid  of  Lime,  Bleaching- 
powder).  This  substance  was  formerly  supposed  to  be  a  compound  of 
lime  and  chlorin,  CaOC^,  and  hence  the  name  chlorid  of  lime. 
It  is  now  generally  considered  to  be  a  mixture  principally  of  calcium 
chlorid  and  calcium  hypochlorite,  CaCl2  +  Ca(ClO)2  or  Ca(OCl)Cl. 
The  hypochlorite  is  the  active  constituent.  This  is  a  very  unstable 
salt,  and  is  readily  decomposed  even  by  carbonic  acid.  When  treated 
with  hydrochloric  acid  it  gives  off  chlorin. 

The  value  of  chlorinated  lime  as  a  bleaching  or  disinfecting  agent 
depends  upon  its  available  chlorin;  that  is,  the  chlorin  which  the 
hypochlorite  yields  when  treated  with  an  acid. 

In  estimating  the  available  chlorin,  the  latter  is  liberated  with 
hydrochloric  acid.  This  liberated  gas,  then,  acting  upon  potassium 
iodid,  sets  free  an  equivalent  amount  of  iodin.  The  quantity  of 
iodin  is  then  determined,  and  thus  the  amount  of  available  chlorin 
found.  0.2  to  0.4  gms.  are  convenient  quantities  to  operate  upon. 

The  U.  S.  P.  directs  to  introduce  into  a  stoppered  weighing  bottle 
between  3  and  4  gms.  of  chlorinated  lime  and  weigh  accurately.  (In 
order  to  make  the  description  simpler  we  will  assume  that  3.5  gms.  is 
the  weight  taken).  This  is  then  triturated  thoroughly  with  50  cc.  of 
water,  and  the  mixture  transferred  to  a  graduated  vessel,  together 
with  the  rinsings  and  made  up  to  1000  cc.  with  water.  This  is  thor- 


CHLORINATED  LIME  211 

oughly  shaken,  100  cc.  of  it  (representing  0.35  gm.  of  the  sample)  are 
removed  by  means  of  a  pipette  and  treated  with  i  gm.  of  potassium 
iodid  *  arid  5  cc.  of  diluted  hydrochloric  acid,  and  into  the  resulting 

N 
reddish-brown  liquid,  the  —  sodium  thiosulphate  is  delivered   from 

a  burette.  Towards  the  end  of  the  titration,  when  the  brownish 
color  of  the  liquid  is  very  faint,  a  few  drops  of  starch  T.  S.  are  added 
and  the  titration  continued  until  the  bluish  or  greenish  color  pro- 
duced by  the  starch  has  entirely  disappeared.  Not  less  than  30  cc. 
of  the  volumetric  solution  should  be  required  to  produce  this  result. 

The  reactions  which  take  place  in  this  process  are  illustrated  by 
the  following  equations : 

Ca  (OCl)Cl  +  2HC1  = CaCl2  +H2O  +  C12 
or  Ca(OCl)Cl  +H2SO4=CaSO4+H2O  +C12, 

C12+2KI=2KC1+I2. 

2)70.36  2)251.8 

io)3S.i8_  10)125.9 

3.5i8gms.=  12.59  gms. 


I2  + 

2)251.8  2)492.92 

10)125.9  10)246.46 

12.59  gms-  24.646  gms.=  1000  cc.  —  V.  S 

10 

It  is  thus  seen  that  i  cc.  of  the  decinormal  sodium  thiosulphate 
represents  0.01259  gm.  of  iodin,  which  in  turn  is  equivalent  to  0.003518 
gm.  of  chlorin. 

Then 

0.003518X30=0.10554  gm. 

0.10554X100  ..     .        .    . 

— — =30.15  per  cent  available  chlorin. 

This  is  a  very  rapid  method  for  estimating  chlorin;  but  when 
calcium  chlorate  is  present  in  the  bleaching-powder  (and  it  often  is, 
through  imperfect  manufacture)  the  chlorin  from  it  is  recorded,  as 

*  In  order  to  assume  a  sufficient  excess  of  potassium  iodid,  take  twice  as  much 
of  it  as  of  the  bleaching  powder. 


212 


A    MANUAL  OF    VOLUMETRIC  ANALYSIS 


well  as  that  from  the  hypochlorite,  the  chlorate  being  decomposed  into 
chlorin,  etc.,  by  hydrochloric  acid.  The  chlorate,  however,  is  of  no 
value  in  bleaching;  its  chlorin  is  not  available.  Hence,  unless  the 
powder  is  known  to  be  free  from  chlorate,  the  analysis  should  be 
made  by  means  of  arsenous-acid  solution,  or  by  using  acetic  acid  instead 
of  hydrochloric,  and  thus  avoid  liberating  chlorin  from  chlorate  which 
may  be  present. 

The  strength  of  bleaching  powder  is  expressed  in  per  cent  of  avail- 
able chlorin  or  in  degrees  (Gay-Lussac).  The  latter  represents  the 
number  of  liters  of  chlorine,  at  o°  C.  and  760  mm.  pressure,  available 
from  one  kilogram  of  the  bleaching  powder.  The  relation  between 
these  two  ways  of  expressing  the  value  is  shown  in  the  table  following: 


Degrees. 
Gay-Lussac. 

Per  Cent 
Chlorin. 

Gay-Lussac. 

Per  Cent 
Chlorin. 

Degrees. 
Gay-Lussac. 

Per  Cent 
Chlorir. 

65 

20.65 

80 

25.42 

95 

30.19 

66 

20.97 

81 

25-74 

96 

30-51 

67 

21.29 

82 

26.06 

97 

30.82 

68 

21.  6l 

83 

26.37 

98 

3!-!4 

69 

21.93 

84 

26.69 

99 

31.46 

70 

22.24 

85 

27.01 

100 

31-78 

7i 

22.56 

86 

27-33 

IOI 

32.09 

72 

22.88 

87 

27.65 

IO2 

32-4I 

73 

23.20 

88 

27.96 

I03 

32-73 

74 

23-51 

89 

28.28 

104 

33  -°S 

75 

23-83 

90 

28.60 

105 

33.36 

76 

24.15 

91 

28.92 

1  06 

33-68 

77 

24.47 

P2 

29-33 

107 

34.00 

78 

24.79 

93 

29-55 

108 

34-32 

79 

25.10 

94 

29-87 

109 

34-64 

The  various  bleaching  preparations  of  the  market  which  depend 
upon  their  available  chlorin  are  all  salts  of  hypochlorous  acid  (HC1O) 
or  solutions  of  such  salts. 

Eau  de  Javelle  (Javelle's  Water)  is  a  solution  of  potassium  hypo- 
chlorite and  potassium  chlorid.  A  solution  of  magnesium  hypo- 
chlorite is  known  in  commerce  as  Ramsay's  or  Grouvelle's  Bleaching 
Fluid.  The  solution  known  as  Wilson's  Bleaching  Fluid  contains 
aluminum  hypochlorite. 

Liquor  Sodae  Chlorinatse  (Solution  of  Chlorinated  Soda;  La- 
barraque's  Solution).  This  is  an  aqueous  solution  of  several  chlorin 
compounds  of  sodium,  principally  sodium  chlorid  and  hypochlorite, 
containing  at  least  2.4  per  cent  by  weight  of  available  chlorin. 


ASSAY   OF  HYDROGEN  DIOXID  213 

In  this  solution,  as  in  chlorinated  lime,  it  is  the  available  chlorin 
which  is  estimated.  The  chlorin  is  first  liberated  with  hydrochloric 
or  sulphuric  acid;  this  then  liberates  iodin  from  potassium  iodid, 
and  the  free  iodin  is  then  determined  by  standard  sodium  thio- 
sulphate. 

Seven  grams  of  chlorinated  soda  solution  are  mixed  with  50  cc.  of 
water,  2  gms.  of  potassium  iodid,  and  10  cc.  of  hydrochloric  acid  are 
then  added,  together  with  a  few  drops  of  starch  T.  S.  Into  this  mixture 
the  decinormal  sodium  thiosulphate  is  delivered  from  a  burette  until 
the  blue  or  greenish  tint  of  the  liquid  is  just  discharged.  Each  cc. 

N 
of  —   thiosulphate  used  up  represents    0.003518    gm.   of  available 

chlorin.  The  potassium  iodid  should  always  be  added  before  the 
hydrochloric  acid,  so  that  the  chlorin  has  potassium  iodid  to  act  upon 
as  it  is  liberated,  and  thus  loss  of  chlorin  is  obviated. 

Bromin  Water,  or  any  substance  containing  free  bromin,  may 
be  assayed  in  exactly  the  same  manner  as  that  described  for  chlorin 
water.  Free  chlorin  must,  however,  be  absent. 

N 
Each  cc.  of  —  thiosulphate  solution  represents  0.007936  gm.  of 

bromin. 

Assay  of  Hydrogen  Dioxid  (H2O2=33.76).  The  iodometric 
method,  which  originated  with  Kingzett,*  is  based  upon  the  fact  that 
iodin  is  liberated  from  potassium  iodid  by  hydrogen  dioxid,  in  the 
presence  of  sulphuric  acid,  and  that  this  liberation  of  iodin  is  in 
direct  proportion  to  the  available  oxygen  contained  in  the  dioxid. 

Then  by  determining  the  amount  of  iodin  liberated,  the  available 
oxygen  is  readily  found. 

H2O2  +  H2SO4  +  2KI  =  K2SO4  +  2H2O  +  I2. 

2)33-76  2)15.88  2)251.8 

1 6.88=  i  available  O==       7.94  I25-9 

This  shows  that  125.9  gms-  °f  iodin  are  liberated  by  16.88  gms. 
of  absolute  dioxid,  which  are  equivalent  to  7.94  gms.  of  available 
oxygen. 

N 
Thus  looo  cc.  of  --  sodium  thiosulphate  V.  S.,  which  absorb  and 

TO 


*  J.  Chem.  Soc.,  1880,  Vol.  37,  p.  792. 


214  A   MANUAL   OF    VOLUMETRIC   ANALYSIS 

consequently  represent   12.59  gms.  of  iodin,  are  equivalent  to  1.688 
gms.  of  H2O2  or  0.794  gm.  of  available  oxygen. 

N 
Each  cc.  of  this  —  V.  S.,  then,  represents,  0.001688  gm.  of  H2O2, 

and  0.000794  gm.  of  available  oxygen. 

The  coefficients  for  weight  of  H2O2  and  of  oxygen,  it  is  seen,  are 
identical  with  those  used  in  the  permanganate  process.  Therefore  the 
coefficient  for  volume  is  also  the  same  in  this  method  as  in  the  other 
if  i  cc.  be  taken  for  assay. 

The  process  is  carried  out  as  follows:  Take  2  or  3  cc.  of  sulphuric 
acid,  dilute  it  with  about  30  cc.  of  water,  add  an  excess  of  potassium 
iodid  (about  i  gm.),  and  then  i  cc.  of  hydrogen  oxid.  After  the 
mixture  has  been  allowed  to  stand  five  minutes  starch  T.  S.*  is  added, 

N 
and  the  titration  with  —  sodium  thiosulphate  begun. 

Note  the  number  of  cc.  required  to  discharge  the  blue  color,  and 
multiply  this  number:  by  0.001688  gm.  to  find  the  quantity,  by  weight, 
of  H2O2;  by  0.000794  gm.  to  find  the  weight  of  available  oxygen;  by 
0.556  cc.  to  find  the  volume  of  available  oxygen. 

If  18  cc.  are  required,  the  solution  is  of  0.556X18  =  10.008  volume 
strength. 

0.001688X18=0.030384  or  3.0384  per  cent  H2C>2. 
0.000794X18=0.014292  or  1.4292  per  cent  of  oxygen. 

With  this  method  the  author  has  always  obtained  satisfactory 
results.  The  lack  of  uniformity  in  the  reaction,  which  is  frequently 
reported,  is  doubtless  due  to  the  use  of  insufficient  acid  or  to  taking  a 
too  concentrated  solution  of  the  dioxid. 

The  best  results  are  obtained  if  the  solution  is  not  more  than 
two  volumes  strength. 

The  sulphuric  acid  used  in  this  assay  must  be  free  from  sulphurous 
acid,  arsenous  acid,  and  nitric  acid,  and  the  potassium  iodid  must 
contain  no  iodate. 

Distillation  Methods.  Manganates,  chromates,  metallic  peroxids, 
and  a  great  variety  of  substances  containing  oxygen,  will,  when  heated 


*  Starch  T.  S.  may  be  omitted,  as  the  decolorization  of  the  iodin  is  distinctly 
seen  if  the  beaker  is  placed  upon  a  white  surface. 


DISTILLATION   METHODS  215 

with   concentrated    hydrochloric,    liberate   an   equivalent    amount   Of 
chlorin.     This  is  illustrated  by  the  following  equation: 

MnO2  +  4HC1  =  MnCl2  +  2H2O  +  C12. 

The  chlorin  which  is  evolved,  is  passed  into  a  solution  of  potassium 
iodid  and  liberates  an  equivalent  of  iodin,  which  latter  substance  is 
then  estimated  by  titration  with  sodium  thiosulphate  solution.  The 
quantity  so  found  is  therefore  a  measure  of  the  original  substance 
and  of  its  oxygen  content.  The  process  may  be  carried  out  by  means 
of  the  apparatus  devised  by  Bunsen,  Fig.  60,  or  by  that  of  Fresenius, 
Fig.  6 1,  or  Mohr,  Fig.  62. 

An  accurately  weighed  quantity  of  the  substance  to  be  analyzed  is 
introduced  into  the  round-bottomed  flask  a,  Fig.  60.  The  flask  is 


FIG.  60. 

then  filled  to  about  two-thirds  its  capacity  with  concentrated  hydro- 
chloric acid,  and  quickly  connected  by  means  of  a  short  rubber  tube 
with  a  long-bulbed  delivery  tube,  b,  which  is  introduced  into  and 
extends  to  the  bottom  of  an  inverted  bulbed  retort,  c.  The  larger 
bulb  of  the  retort  is  filled  to  two  thirds  of  its  capacity  with  a  10  per 
cent  solution  of  potassium  iodid.  Heat  is  applied  to  the  flask,  and 
the  chlorin  distils  over  into  the  potassium  iodid  solution,  which 
becomes  brownish-red  through  liberation  of  iodin.  The  distillation 
is  continued  until  about  one  third  of  the  acid  fluid  has  passed  over 
or  until  a  peculiar  cracking  sound  indicates  the  absorption  of  hot 
hydrochloric  acid  vapor. 

The  flask,  together  with  its  delivery  tube,  is  then  slowly  removed, 
the  heating  being,  however,  continued  until  the  tube  is  entirely  with- 
drawn, in  order  to  prevent  the  iodid  solution  being  drawn  over 
into  the  flask.  The  retort  is  then  shaken  so  that  any  traces  of  chlorin 


216 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


which  may  have  escaped  absorption,  are  taken  up  and  the  contents 
of  the  retort  poured  into  a  beaker;  the  retort  and  the  delivery  tube  are 
then  rinsed  with  water  and  the  rinsings  added  to  the  fluid  in  the  beaker, 
and  titration  with  standard  sodium  thiosulphate  begun  immediately. 

It  is  important  that  the  quantity  of  potassium  iodid  be  sufficient  to 
keep  the  liberated  iodin  in  solution,  and  that  the  potassium  iodid  be 
free  from  iodate,  also  that  the  titration  be  started  without  delay  to 
avoid  liberation  of  iodin  through  action  of  the  air  upon  the  strongly 
acid  potassium  iodid  solution.  When  all  of  the  chlorin  has  passed 


FIG.  61. 

over  and  hydrochloric  acid  gas  begins  to  distil,  the  liquid  in  the  retort 
is  apt  to  be  drawn  back  into  the  flask  because  of  the  great  affinity 
which  hydrochloric  acid  gas  has  for  water,  and  the  resultant  condensa- 
tion in  the  flask.  This  regurgitation  may  be  avoided  by  introducing 
into  the  generating  flask  a  small  piece  of  magnesite,  which  slowly  dis- 
solves in  the  acid  solution  and  so  keeps  up  a  constant  flow  of  carbon 
dioxid,  which  by  its  pressure  prevents  back  flow  of  the  fluid.  The 
bulbs  in  the  retort  and  delivery  tube  are  also  calculated  to  prevent  this 
regurgitation. 

The  Fresenius  apparatus   is   illustrated  in   Fig.   61.     In  this  the 
potassium  iodid  solution  is  contained  in  two  joined  U-shaped  tubes. 


DISTILLATION  METHODS 


217 


The  delivery  tube  from  the  distilling  flask  enters  one  of  the  U-tubes 
through  a  paraffin -soaked  cork  (which  fits  tightly),  and  terminates  just 
above  the  potassium  iodid  solution.  In  operation,  the  U-tubes  should 
be  kept  in  ice  water,  and  all  the  fittings  should  be  air-tight.  Paraffin- 
covered  cork  stoppers  only  should  be  used. 

After  all  the  chlorin  has  passed  over  or  when  about  one-third 
of  the  acid  has  distilled  over,  the  apparatus  is  allowed  to  stand  for  a 
few  minutes,  to  permit  all  traces  of  chlorin  to  become  absorbed;  the 


FIG.  62. 


application  of  a  suction  pump  to  the  rear  outlet  tube  will  help  to 
bring  about  this  result. 

Mohr's  apparatus,  shown  in  Fig.  62,  is  of  very  simple  construction 
and  easy  to  use. 

The  distilling  flask  is  fitted  with  a  paraffin-soaked  cork,  through 
which  a  delivery  tube  containing  one  bulb  passes;  this  delivery  tube 
again  passes  through  a  common  cork  which  loosely  fits  a  stout,  large 
test  tube,  containing  the  potassium  iodid  solution.  The  delivery  tube  is 
drawn  out  to  a  fine  point  and  reaches  to  near  the  bottom  of  the  test 


218  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

tube.     The  latter  is  placed,  when  in  operation  in  a  hydrometer  jar 
containing  cold  water. 

Estimation  of  Manganese  Dioxid  (MnC>2  =86.36).  0.4  gm.  of 
pulverized  manganese  dioxid  is  placed  into  the  distilling  flask  (a, 
Fig.  60  or  Fig.  61)  and  the  latter  filled  to  two-thirds  of  its  capacity 
with  concentrated  hydrochloric  acid  and  connected  without  delay  with 
the  vessel  containing  the  potassium  iodid  solution.  The  flask  is  then 
gradually  heated  so  that  a  steady  current  of  chlorin  passes  over  into 
the  potassium  iodid  solution.  When  the  evolution  of  chlorin  gas 
begins  to  diminish,  the  heat  is  slowly  raised  to  boiling,  and  continued 
at  this  point  until  about  one  third  of  the  acid  liquid  has  distilled  over. 
The  delivery  tube  is  then  removed  and  rinsed  as  previously  described, 

N 
and  the  liberated  iodin  titrated  by  means  of  —  sodium  thiosulphate 

solution,  of  which  we  will  assume  60  cc.  were  consumed. 
The  reactions  are  as  follows  : 

(a)  MnO2  +2KI  =MnCl2  +  2H2O  +C12. 

86.36  70.36 


(b)  C 

70.36  251.8 

(c)  I2  +  2Na2S2O3  .  5H2O=2NaI+Na2S4O6  +  ioH2O. 
2)251.8  2)492.92 

10)125.9          10)246.46 

12.59  gms.         24.646  gms.=  1000  cc.  —  V.  S. 

10 

N 
o.oi259gm.  =        ice.  —  V.  S. 

These  equations  show  that  492.92  gms.  of  sodium  thiosulphate  will 
decolorize  251.9  gms.  of  iodin,  which  quantity  is  liberated  by  70.36  gms. 
of  chlorin,  which  is  itself  liberated  from  hydrochloric  acid  by  86.36 
gms.  of  manganese  dioxid. 

Therefore  i  cc.  of  a  decinormal  solution  of  sodium  thiosulphate 
(containing  24.646  gms.  in  1000  cc.)  is  equivalent  to  0.01259  gm-  °f  I; 
0.003518  gm.  of  Cl;  0.004318  gm.  of  MnO2;  0.000794  gm.  of  O 
(available). 

The  60  cc.  of  the  thiosulphate  solution  used  in  this  assay  will 
therefore  represent  0.004318X60=0.25908  gm.  of  pure  MnO2,  or 
64.77  per  cent. 

0.25908X100 

-  =64.77  per  cent. 


POTASSIUM  DICHROMATE  219 

This  is  the  method  which  should  be  used  for  the  assay  of  native 
manganese  dioxid.  The  freshly  precipitated  manganese  dioxid  of  the 
Pharmacopoeia  may  be  assayed  by  the  more  easily  performed  digestion 
method  described  on  page  222. 

Estimation  of  Chromic  Acid  and  Chromates.  Chromic 
Anhydrid,  Chromium  Trioxid  (CrO3  =99.34),  when  heated  with  con- 
centrated hydrochloric  acid,  liberates  chlorin  as  per  the  equation, 

Cr034-6HCl=CrCl3+3H20+   C13. 
99-34 


99.34  parts  of  CrO3  liberates  3  X35-i8  parts  of  Cl,  hence  one  atomic 
weight  of  chlorin,  35.18  parts,  represents  33.113+  parts  of  CrO3. 

N 
Or  i  cc.  of  —  sodium  thiosulphate  represents  0.0033113  gm.  of  CrO3. 

Potassium  Dichromate  (K2Cr2O7=292.28).  This  salt,  as  ex- 
plained  in  a  previous  chapter,  has  three  atoms  of  oxygen  available  for 
oxidation.  A  molecule  of  this  salt  is  therefore  equivalent  to  six  atoms 
of  chlorin,  and  when  boiled  with  hydrochloric  acid  will  liberate  six 
atoms  of  chlorin,  as  the  equation  shows. 


292.28  6X35.18 

Thus  one  atom  of  liberated  chlorin  will  represent  one  sixth  of  292.28, 
which  is  48.713+  parts  of  potassium  dichromate.  Then  i  cc.  of 

N 
—  sodium  thiosulphate  will  represent  0.0048713  gm.  of  K2Cr2O7. 

In  the  same  way  all  other  chromates  may  be  treated,  but  these  com- 
pounds will,  when  treated  with  hydrochloric  acid,  liberate  chlorin  at 
once  and  without  the  application  of  much  heat,  hence  some  chlorin  is 
apt  to  be  lost  before  the  distillation  flask  can  be  connected  with  the 
apparatus,  and  therefore  it  is  more  convenient  to  employ  the  digestion 
method  later  described. 

The  reaction  in  the  case  of  neutral  potassium  chromate  is  as  follows  : 

K2Cr04+8HCl  =  2KCl+4H20+CrCl3+Cl3. 
192.94 

Lead  peroxid,  PbO2;  cobalt  ic  oxid,  Co2O3;  nickel  oxid,  Ni2O3, 
as  well  as  many  other  substances,  may  be  assayed  by  this  distillation 
method. 


220  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Estimation    of   Alkali    lodids    by    the    Distillation    Method. 

This  method  is  based  upon  the  fact  that  metallic  iodids  when  treated 
with  ferric  salts  in  acidulated  solution  yield  up  all  of  their  iodin. 
As  shown  in  the  equation 


The  iodin  thus  set  free  is  distilled  into  a  solution  of  potassium 
iodid  and  its  quantity  determined  by  titration  with  sodium  thiosulphate 
in  the  usual  manner. 

For  the  reaction,  ferric  sulphate  or  ammonio-femc  alum  may  be 
used;  the  latter  is,  however,  preferred  because  of  its  paler  color.  Ferric 
sulphate  and  ferric  chlorid  are  so  dark  in  color  that  the  determina- 


FIG.  63. 

lion  of  the  end-reaction  is  quite  a  difficult  matter;  furthermore,  these 
salts  frequently  contain  traces  of  nitrates  which  if  present  liberate 
chlorin  from  the  chlorids  or  distil  over  and  liberate  iodin  from  the 
potassium  iodid  solution  in  the  receiving  vessel.  Ferric  chlorid  is 
particularly  objectionable  because  of  the  tenacity  with  which  it  holds 
the  last  portions  of  iodin. 

The  distillation  may  be  done  in  the  Fresenius  apparatus,  Fig.  61, 
or  better  in  that  shown  in  Fig.  63.  The  latter  consists  of  a  100  cc. 
distilling  flask  (ci),  connected  by  means  of  a  glass  tube  with  a  nitrogen 
flask  (ft),  which  contains  a  10  per  cent  potassium  iodid  solution,  and 
which  is  kept  in  a  vessel  of  ice  water  when  in  use.  The  stoppers  used 
are  cork,  well  soaked  in  paraffin.  The  construction  of  the  flask  is 
particularly  suitable,  because  it  presents  a  large  surface  to  the  vapor 
of  iodin  which  distils  over,  and  because  the  titration  can  be  done 


ALKALI   IODIDS  BY   DISTILLATION  METHOD  221 

directly  in  it,  thus  avoiding  the  necessity  of  transferring  its  contents 
to  a  beaker  or  other  vessel, 

The  glass  tube  which  conveys  the  iodin  vapor  must  not  be  carried 
into  the  solution  of  potassium  iodid  and  must  not  be  drawn  to  a  fine 
point.  The  reason  for  this  is  that  the  iodin  condensing  at  the  point 
would  soon  choke  up  the  tube  and  so  prevent  the  further  passage  of 
iodin  vapor.  Any  iodin  which  condenses  in  the  tube  is  washed 
down  into  the  potassium  iodid  solution  by  the  steam  during  the 
distillation. 

The  Process.  Into  the  flask  (a)  is  introduced  about  5  gm.  of 
ammonio  -ferric  alum,  50  cc.  of  water,  20  cc.  of  diluted  sulphuric  acid 
(i  :  10),  and  the  iodid  to  be  examined,  accurately  weighed.  Take  about 
0.5  gm.  The  flask  is  then  connected  with  the  receiving  vessel  (b\ 
which  is  about  half  filled  with  a  10  per  cent  potassium  iodid  solution, 
and  after  connections  are  made  tight,  heat  is  gradually  applied  to  the 
distilling  flask.  After  most  of  the  iodin  has  passed  over,  the  heat  is 
raised  to  boiling,  and  continued  at  this  temperature  until  about  one 
fourth  of  the  liquid  has  passed  over,  and  the  solution  in  the  distilling 
flask  is  no  longer  of  a  brown  color. 

When  the  receiving  vessel  has  sufficiently  cooled,  it  is  disconnected 
and  its  contents  titrated  with  decinormal  sodium  thiosulphate,  using 
starch  as  indicator.  Before  beginning  titration,  however,  it  is  neces- 
sary to  rinse  the  lower  extremity  of  the  tube  and  the  stopper  into  the 
solution  in  the  receiving  vessel,  in  order  that  every  trace  of  iodin  be 
collected. 

The  calculation  is  then  made  as  follows  : 


(a)  Fe2(SO4)3  . 

329.52  251.8 

(b)  I2    +    2Na2S2O3  +  5H2O  =  2NaI  +  Na2S4O6  +  ioH2O. 
2)251.8  2)492.92 

10)125.9          10)246.46 

12.59  gms-        24.646  gms  =  1000  cc.  —  V.  S. 

10 

N 
0.01259  gm-  —       i  cc.  —  V.  S. 


Referring  to  the  above  equations  we  see  that  251.8  gms.  of  iodin 
are  liberated  from  329.52  gms.  of  potassium  iodid,  thus  125.9  gms.  of 
iodin  represents  164.76  gms.  of  potassium  iodid,  and  therefore  i  cc. 


222 


A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


of  decinormal  sodium  thiosulphate  solution  representing  0.01259  gm- 
of  iodin  will  at  the  same  time  represent  0.016476  gm.  of  KI. 

N 

If  in  the  above  assay  29  cc.  of  —  sodium  thiosulphate  are  con- 
sumed, we  multiply  the  factor  for  KI  =0.016476  gm.  by  29,  this  gives 
0.477604  gm.,  the  quantity  of  pure  KI  in  the  .5  gms.  taken,  which  is 
about  95  per  cent. 

Digestion  Methods.  The  distillation  methods  above  described 
may  be  avoided  in  many  cases  and  the  more  easily  performed  digestion 
process  used.  For  instance,  freshly  precipitated  manganese  dioxid, 
lead  peroxid,  chromic  acid,  chlorates,  bro- 
mates,  iodates,  ferric  salts,  and  a  great  many 
other  substances,  may  be  assayed  by  mere 
digestion  with  hydrochloric  acid  at  a  slightly 
elevated  temperature. 

The  digestion  is  performed  in  a  strong  glass 
bottle,  pro\ided  with  an  accurately  fitting  ground- 
glass  stopper  which  is  tied  down  by  means 
of  wire  or  secured  by  a  clamp.  See  Fig.  64. 

Before  using  the  bottle  for  this   operation 
it  should  be  tested  by  securely  tying  down  the 
FlG-  64-  stopper  and   immersing  the   bottle   entirely   in 

hot  water  to  see  if  the  stopper  fits  sufficiently  tight.  If  it  does  not, 
bubbles  of  air  will  escape  from  inside,  and  the  bottle  is  useless  for  the 
purpose  intended.  In  that  event  the  stopper  must  be  reground  into 
the  neck  of  the  bottle  with  a  little  very  fine  emery  and  water.  The 
capacity  of  the  bottle  may  vary  from  50  to  150  cc. 

The  Process.  The  substance  is  accurately  weighed  and  intro- 
duced into  the  bottle  together  with  a  small  quantity  of  coarsely  pow- 
dered glass  or  small  pure  flint  pebbles  (to  prevent  caking,  especially  in 
the  case  of  insoluble  powders).  A  sufficient  excess  of  potassium  iodid 
solution  is  then  added,  followed  by  some  pure  concentrated  hydro- 
chloric acid.  The  stopper  is  then  quickly  inserted,  firmly  secured  by 
wire  or  a  clamp,  and  the  bottle  placed  in  a  water  bath,  and  the  water 
gradually  heated  to  boiling;  this  temperature  being  continued  until 
decomposition  is  complete,  which  is  usually  in  about  half  an  hour. 
The  bottle  is  then  allowed  to  cool  slowly  and  its  contents  emptied 
into  a  beaker.  Then  after  washing  the  bottle  and  adding  the  wash- 
ings to  the  contents  of  the  beaker  the  liberated  iodin  is  estimated  by 
titration  with  sodium  thiosulphate. 


ESTIMATION   OF  CHLORATES,  BRO  MATES,    AND   IODATES  223 

The  potassium  iodid  used  in  this  process  must  be  absolutely  free 
from  iodate. 

N 
i  cc.  of  —  sodium  thiosulphate  is  equivalent  to 

KC1O3  ..............  0.002028  gm. 

NaClO3  .............  0.0017616  " 

KBrO3  .....  .  ........  0.0027643  " 

KI08  ...............  0.003540  " 

MnO2  ...............  0.004318  " 

PbO2  ...............  0.011855  " 

CrO3  ...............  0.0033113  " 

Estimation  of  Chlorates,  Bromates,  and  lodates.  The  estima- 
tion of  these  salts  is  based  upon  the  fact  that  in  each  case  one  equivalent 
of  the  acid  or  its  monobasic  salt  liberates  six  equivalents  of  chlorin 
and  consequently  six  equivalents  of  iodin  when  decomposed  by  the 
digestion  method. 

This  is  illustrated  by  the  equations: 

(a)  KC1O3+6HC1=3H2O+KC1+C16; 

(b)  KBrO3+6HCl=3H2O+KBr+Cl6; 
(0  KI03  +6HC1  =3 


and 

(d)  C16+6KI=6KC1  +  I6. 

In  the  distillation  process,  however,  bromates  and  iodates  liberate 
only  four  equivalents  of  iodin,  while  bromous  chlorid  and  iodous 
chlorid  remain  in  the  retort,  therefore  in  these  cases  the  digestion  is 
preferable  to  the  distillation  method. 

If  the  bromate  or  iodate  to  be  assayed  contains  any  bromid  or 
iodid,  bromin  or  iodin  respectively  will  be  liberated  upon  the  addi- 
tion of  the  acid,  according  to  the  equations 

(a)  5KBr+KBrO3+6HCl=6KaCl+3H2O+Br6; 

(b)  5KI  +KIO3  +6HC1  = 


therefore  the  method  is   not   applicable  for  the  assay  of  such  mix- 
tures. 


224  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  presence  of  either  of  these  salts  may  be  ascertained  by  moisten- 
ing a  small  quantity  of  the  salt  with  dilute  sulphuric  acid,  when  if  a 
yellow  or  brown  coloration  results,  either  a  bromid  or  an  iodid  res- 
pectively is  present. 

Example.  Estimation  of  Potassium  Chlorate.  0.2  gm.  of  the  salt 
is  introduced  into  the  digestion  bottle  of  about  100  cc.  capacity,  10  cc. 
of  water  added,  and  about  4  gms.  of  potassium  iodid  (or  sufficient  of  its 
saturated  solution).  This  is  followed  by  lo  cc.  of  concentrated  hydro- 
chloric acid,  the  stopper  quickly  inserted,  firmly  secured  by  wiring 
or  a  clamp,  and  the  flask  placed,  stopper  downward,  in  a  water-bath. 
The  water  is  gradually  raised  to  boiling  and  kept  at  this  temperature 
for  about  half  an  hour.  It  is  then  allowed  to  cool  slowly,  and  the 
contents  of  the  bottle  washed  into  a  beaker  and  titrated  with  deci- 
normal  sodium  thiosulphate,  using  starch  as  indicator.  The  number 

N 
of  cubic  centimeters  of  —  thiosulphate  solution  used,  multiplied  by 

0.002028  gm.,  gives  the  weight  of  pure  KClOs  present  in  the 
sample. 

In  the  assay  of  bromates  and  iodates  a  smaller  quantity  of  hydro- 
chloric acid  may  be  used  and  a  lower  temperature,  say  50°  C.,  is  suffi- 
cient for  decomposition. 

Example.  Estimation  of  Potassium  Br  ornate.  0.2  gm  of  the  salt 
are  dissolved  in  15  cc.  of  water,  4  gms.  of  potassium  iodid  are  added, 
followed  by  4  cc.  of  concentrated  hydrochloric  acid.  The  bottle  'is 
securely  closed,  as  in  the  foregoing  assay,  and  heated  for  half  an  hour 

N 
at  50°  C.     Then  after  decomposition,  titration  with  —  sodium  thio- 

sulphate solution  is  begun,  each  cc.  of  wrhich  represents  0.0027643  gm. 
of  pure  KBrO3. 

If  y-^Tj-  of  one  sixth  of  the  molecular  weight  of  either  salt  be  taken 

N 
for  assay,  each  cc.  of  the  —  thiosulphate  solution  used  will  indicate 

i  per  cent  purity. 

Estimation  of  Ferric  Salts.  When  a  ferric  salt  in  an  acidulated 
solution  is  digested  with  an  excess  of  potassium  iodid  the  salt  is  re- 
duced to  the  ferrous  state,  and  iodin  is  set  free. 


One  atom  of  iodin  is  liberated  for  each  atom  of  iron  in  the  ferric 
state.      The  liberated  iodin  is  then  determined  bv  sodium  thiosul- 


FERRIC   CHLORID  225 


phate  in  the  usual  way.  125.9  gms-  °f  i°din  =  55.5  gms.  of  metallic 
iron. 

This  is  the  method  of  the  U.  S.  P.    It  is  given  in  detail  here. 

0.555  gm.  of  the  salt  is  dissolved  in  10  or  15  cc.  of  water  and  2  cc. 
of  hydrochloric  acid  in  a  glass-stoppered  bottle  having  a  capacity  of 
about  100  cc.  i  gm.  of  potassium  iodid  is  then  added,  and  the  mix- 
ture digested  for  half  an  hour  at  a  temperature  of  40°  C.  (104°  F.). 
During  the  digestion  the  stopper  should  be  left  in  the  bottle,  and  the 
heat  not  allowed  to  rise  too  high,  otherwise  the  liberated  iodin  will 
be  volatilized.  When  cool  a  few  drops  of  starch  T.  S.  are  added. 

N 
It  is  now  ready  for  titrating  with  —  sodium  thiosulphate.    Each  cc. 

corresponds  to  i  per  cent  of  metallic  iron. 

When  the  quantity  of  metallic  iron  and  the  chemical  formula  for 
the  ferric  salt  under  estimation  are  known,  the  quantity  of  pure  salt 
is  easily  found  by  calculation. 

In  all  the  estimations  of  ferric  iron  it  is  convenient  to  take  0.555  gm- 
of  the  salt.  Each  cc.  of  the  volumetric  solution  used  will  then  rep- 
resent i  per  cent  of  metallic  iron,  assuming  the  atomic  weight  of  iron 
to  be  55.5. 

Ferric  salts  may  be  tested  in  many  other  ways.     For  instance: 

A  ferric  salt  in  solution  may  be  filtered  through  a  column  of  zinc 
dust,  which  reduces  it  to  the  ferrous  state.  This  is  then  estimated 
with  potassium  permanganate  V.  S.  in  the  usual  manner,  or  the  ferric 
solution  is  treated  with  a  few  small  pieces  of  zinc  or  magnesium  coarsely 
powdered,  until  complete  reduction  is  effected.  When  a  red  color  is 
no  longer  produced  by  sulphocyanate  of  potassium  the  ferric  salt  is 
completely  reduced,  and  may  be  estimated  with  potassium  perman- 
ganate V.  S. 

Stannous  chlorid,  ammonium  bisulphite,  and  other  substances 
may  also  be  used  as  reducing  agents. 

Ferric  Chlorid  (Fe2Cl6  or  FeCl3.)  0.555  gm.  of  the  dry  salt  is 
dissolved  in  a  glass  -stoppered  bottle  (having  a  capacity  of  about  100  cc.) 
in  10  cc.  of  water  and  2  cc.  of  hydrochloric  acid,  and  after  the  addition 
of  i  gm.  of  potassium  iodid  is  kept  for  half  an  hour  at  a  tempera- 
ture of  40°  C.  (104°  F.),  then  cooled  and  titrated  with  decinormal 
sodium  thiosulphate  until  the  color  of  the  liquid  is  discharged.  Each 
cc.  of  the  decinormal  thiosulphate  solution  used  represents  0.00555 
gm.,  or  i  per  cent  of  metallic  iron,  or  0.016104  gm.  of  pure  ferric 
chlorid. 


226  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  following  equations  illustrate  the  reactions: 


2)322.08  2)251.8 

161.04  I2S-9 

Then 

I2    +   2Na2S2O3.5H2O  =  2NaI  +  Na2S4O6  +  ioH2O. 
2)251.8        2)246.46 
10)125-9       10)123.23  ^ 

12.59  gms.     12.323  gms.=  iooo  cc.  —  V.  S. 

10 

N 
Thus  each  cc.  of  —  thiosulphate  represents  0.01259  gm'  °f  iodin 

and  consequently  0.016104  gm.  of  Fe2Cl6. 

The  assay  of  other  official  ferric  salts  is  practically  the  same  as 
that  described  above. 

N 
Each  cc.  of  —  sodium  thiosulphate  is  equivalent  to 

Ferrum,  Fe  ......................................  0.00555  gm- 

Ferric  ammonium  sulphate  Fe2(SO4)3  .  (NH4)2SO4.  ..  0.026413  " 

'  '      chlorid,  Fe2Cl6  .............................  0.016104  '  ' 

"      (cryst.),  Fe2Cl6  +  i2H20  ...............  0.026832  (l 

"      nitrate,  Fe2(NO3)  6  ..........................  0.024021  " 

'  '      oxid,  Fe2O3  ................................  0.007932  '  ' 

41      sulphate,  Fe2(SO4)3  .........................  0.0198525  " 

In  the  case  of  the  official  scale  salts  of  iron,  which  are  mostly  of 
indefinite  and  variable  composition,  it  is  the  quantity  of  metallic  iron 
present  which  is  determined. 

The  official  solutions  of  ferric  salts  are  estimated  in  the  same 
manner.  It  is  the  rule  to  take  i.n  gms.  of  the  solution  for  assay; 
then  each  cc.  of  the  thiosulphate  will  represent  0.5  per  cent  of  metallic 
iron. 

REDUCTION    METHODS    INVOLVING    THE    USE    OF    STANDARD    ARSENOUS 

ACID  SOLUTION   (Chlorometry) 

As  previously  described,  arsenous  oxid  when  brought  in  contact 
with  iodin  in  an  alkaline  solution  results  in  an  oxidation  of  the  former 
to  arsenic  oxid,  and  a  conversion  of  the  iodin  to  hydriodic  acid,  as 
shown  in  the  equation 

As2O3  +  2H2O+I4=As2O5+4HI. 


DECINORMAL    N/io  ARSENOUS   ACID  227 

Advantage  is  taken  of  this  reaction  for  the  estimation,  not  only  of 
arsenous  and  antimonous  compounds,  but  also  of  iodin  and  the  other 
halogens,  chlorin  and  bromin,  as  well  as  of  all  those  bodies  which 
when  heated  with  hydrochloric  acid  evolve  chlorin,  as  for  instance, 
the  peroxids. 

The  reaction  with  chlorin  is  as  follows : 

C14  +  2H2O  +  As2O3  =  4HC1  +  As2O5. 

This  reaction  is  really  an  oxidation,  so  far  as  the  formation  of 
arsenic  oxid  (As2O5)  is  concerned,  but  there  is  no  accompanying 
reduction.  The  conversion  of  the  halogen  to  an  haloid  acid  is  not 
strictly  a  reduction  in  the  accepted  sense  of  the  word.  Nevertheless, 
for  obvious  reasons,  we  speak  of  analyses  done  by  means  of  arsenous 
acid  as  reduction  methods. 

The  chief  value  of  this  method  is  found  in  the  estimation  of 
free  chlorin,  as  in  chlorin  water,  and  the  available  chlorin  existing 
in  hypochlorites  or  that  evolved  from  hydrochloric  acid  by  heating 
with  peroxids.  Hence  the  designation  "chlorometry." 

In  carrying  out  this  method,  free  alkali  must  be  present  to  combine 
with  the  haloid  acid  which  is  formed.  The  alkali  must  be  in  the 
form  of  bicarbonate.  Normal  carbonates  or  hydroxids  are  not  suitable, 
see  page  190. 

The  solutions  required  are : 

Decinormal  iodin,  see  page  186; 

Decinormal  arsenous  acid; 

Starch  solution,  see  page  189;  or  iodized  starch  test  paper. 

Preparation  of  Decinormal  --  Arsenous  Acid  (As2C>3  =  196.44;) 

/N  \  T 

I —  V.  S.  =4.911  gms.  in  i  liter,  j 

4.95  gms.  of  the  purest  sublimed  arsenous  anhydrid  (As2O3)  are 
dissolved  in  about  250  cc.  of  distilled  water  with  the  aid  of  about 
20  gms.  of  pure  sodium  bicarbonate.  The  anhydrid  should  be  in  fine 
powder,  and  the  mixture  heated  to  effect  complete  solution.  This  is 
then  diluted  with  some  water,  cooled,  and  made  up  to  1000  cc.  It  is 
then  standardized  with  decinormal  iodin,  using  starch  as  indicator. 
Decinormal  arsenous  acid  solution  should  correspond,  volume  for 
volume,  with  decinormal  iodin  solution. 

If  this  solution  is  made  from  pure  arsenous  acid  it  will  hold  its 


10 


228  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

titer  for  years,  but  if  any  sulphur  is  present  there  will  be  an  absorption 
of  oxygen  from  the  air  and  a  consequent  oxidation  to  arsenic  oxid. 
If  the  presence  of  sulphur  is  suspected,  the  solution  should  be  tested 
with  silver  nitrate,  when  its  presence  will  be  indicated  by  the  forma- 
tion of  a  reddish  precipitate. 

Iodized  Starch  Test  Paper.  A  portion  of  starch  solution  is 
mixed  with  a  few  drops  of  potassium  iodid  solution  and  in  this  are 
soaked  strips  of  pure  white  filtering  paper.  This  test  paper  is  used 
in  the  damp  state;  it  is  then  far  more  sensitive. 

Estimation  of  Free  Halogens.  The  estimation  of  chlorin,  bro- 
min,  or  iodin  by  the  chlorometric  method  depends,  as  before  stated, 
upon  their  power  of  oxidizing  arsenous  acid.  When  a  free  halogen 
is  brought  in  contact  in  alkaline  solution  with  arsenous  acid,  the  latter 
is  oxidized  to  arsenic  acid,  while  the  halogen  is  transformed  into  a 
haloid  acid,  as  per  equations 

UHC1 
<  4HBr. 

UHI 

The  estimation  may  be  carried  out  in  two  ways :  ist,  by  direct  titra- 
tion  with  a  standard  arsenous  oxid  solution,  using  iodized  starch  test 
paper  as  indicator;  2d,  by  residual  titration,  an  excess  of  the  standard 
arsenous  oxid  being  taken,  and  then  retitrating  with  standard  iodin 
solution,  using  starch  as  indicator.  The  residual  titration  method 
need  not  be  employed  for  free  iodin,  as  this  can  be  titrated  direct 
with  the  arsenous  oxid  solution,  using  starch  as  indicator.  Further- 
more, iodin  need  not  be  brought  into  solution  to  be  titrated  by  this 
method. 

The  estimation  of  free  halogens  by  the  direct  chlorometric  method 
is  as  follows: 

An  accurately  weighed  quantity  of  substance  made  alkaline  by  the 
addition  of  sodium  bicarbonate  is  titrated  with  decinormal  arsenous 
acid  solution,  and  from  time  to  time  during  the  titration  a  drop  of 
the  solution  is  removed  on  the  end  of  a  pointed  glass  rod  and  brought 
in  contact  with  a  piece  of  iodized  starch  test  paper.  So  long  as  free 
chlorin  or  bromin  is  present  the  liquid  will  cause  a  blue  stain  on  the 
test  paper,  but  when  the  halogen  is  all  taken  up  no  blue  color  is  pro- 
duced. 

If  the  exact  point  is  overstepped  the  residual  method  must  be  used. 
A  little  additional  excess  of  the  arsenous  acid  solution  may  be  added, 


AVAILABLE   CHLORIN  IN  BLEACHING  POWDER         229 

together  with  a  few  drops  of  starch  solution,  and  the  excess  then  titrated 
by  means  of  decinormal  iodin  solution  until  the  blue  color  is  pro- 
duced. The  volume  of  decinormal  iodin  solution  so  used,  deducted 
from  the  total  volume  of  arsenous  acid  solution  taken,  gives  the  exact 
quantity  which  was  oxidized  by  the  halogen  and  from  this  the  per- 
centage of  chlorin  or  bromin  may  be  calculated. 

Example  i.  Estimation  of  Chlorin  in  Chlorin  Water.  20  cc.  of 
chlorin  water  (sp.gr.  i.o)  titrated  by  the  direct  method  required  22  cc. 
of  decinormal  arsenous  acid  solution  before  the  iodized  starch  test 
paper  indicated  the  completion  of  the  reaction. 

By  referring  to  the  equation  we  see  that  each  cc.  of  the  arsenous 
acid  solution  represents  0.003518  gm.  of  chlorin.  Therefore,  if  22  cc. 
were  used,  the  20  cc.  of  chlorin  water  must  have  contained  22  Xo.oo35i8 
gm.  of  chlorin,  which  is  0.077396  gm. 

The  20  cc.  of  chlorin  water  (sp.gr.  i.o)  weigh  20  gms.     Hence 

0.077396X100 

-  =0.38  +per  cent. 
20 

Example  2.  The  20  cc.  of  chlorin  water  weighing  20  gms.  were 
treated  with  26  cc.  of  the  arsenous  acid  solution,  starch  solution  was 
then  added,  and  the  excess  of  arsenous  acid  solution  titrated  by 
means  of  decinormal  iodin.  4  cc.  of  the  latter  were  required,  then 

N 
4  from  26  cc.  leaves  22  cc.,  the  quantity  of  the  —  arsenous  acid  solu- 

tion which  reacted  with  the  chlorin.  The  calculation  is  the  same 
as  in  Example  i  . 

The  reaction  is  as  follows: 


C14  +  2H2O  +  As2O3  =  4HC1  +  As2O5. 

4)140.72 


10)49-"  N 


3.518  gms.=       4.911  gms.  or  1000  cc.  —  V.  S. 

TO 

N 
0.003518  gms.  i  cc.  —  V.  S. 

Estimation  of  Available  Chlorin  in  Bleaching  Powder.  3.5  gms. 
of  the  bleaching  powder  (chlorinated  lime)  are  triturated  thoroughly 
with  50  cc.  of  water,  and  the  mixture  transferred  to  a  graduated 
vessel,  together  with  the  rinsings,  and  made  up  to  1000  cc.  with  water. 
This  is  thoroughly  shaken.  100  cc.  of  it  (representing  0.35  gm.  of 


230  .4    MANUAL   OF   VOLUMETRIC   ANALYSIS 

the  sample)  is  removed  by  means  of  a  pipette  and  titrated  with  deci- 
normal  arsenous  acid  solution,  as  described  in  the  foregoing  assay, 
using  either  the  iodized  starch  test  paper  as  indicator  or  retitrating 

N  N 

the  excess  of  —  arsenous  acid  solution,  added  by  means  of  —  iodin 

,    .  10  10 

solution. 

N 
Each  cc.  of  —  As2O3  V.  S.  represents  0.003518  gm.  of  available 

chlorin. 


4)140-72        4)  196-44 

10)49-" 


3.518  gms.=  4.911  gms.  or  1000  cc.  —  V.  S. 

10 

As  seen  by  referring  to  the  above  equation  this  process  deter- 
mines the  value  of  the  chlorinated  lime  by  measuring  the  amount 
of  arsenous  acid  which  the  oxygen  present  in  the  active  constituent 
(Ca(OCl)Cl)  is  capable  of  oxidizing.  In  the  formula  of  this  com- 
pound there  are  two  atoms  of  chlorin  and  one  atom  of  oxygen.  There- 
fore the  quantity  of  bleaching  powder  which  yields  35.18  parts  of 
available  chlorin  will  also  supply  8  parts  of  oxygen;  this  may  there- 
fore be  taken  as  the  measure  of  the  chlorin.  The  same  method  may 
be  employed  for  the  assay  of  all  other  solutions  containing  available 
chlorin. 

Assay  of  Manganese  Dioxid  (Chlorometric).  The  chlorometric 
assay  of  manganese  dioxid,  as  well  as  that  of  all  other  bodies  which 
liberate  chlorin  when  heated  with  hydrochloric  acid,  may  be  made 
in  similar  manner  to  that  described  for  the  iodometric  assays  of  these 
substances,  the  same  apparatus,  etc.,  being  used. 

N 
The  liberated  chlorin  is,  however,  titrated  with  —  arsenous  acid 

solution. 

The  chlorin  may  be  distilled  into  a   solution  of  sodium  carbonate 

N 

and  this  solution  then  titrated  with  —  arsenous  acid  or  the  chlorin 

10 

N 
may  be  distilled  directly  into  a  measured  volume  of  —  arsenous  acid 

N 

solution  and  the  latter  then  titrated  with  —   iodin   solution,   using 

10 

starch  as  indicator,  the  difference  between  the  volume  of  iodin  solu- 
tion used  and  that  of  the  arsenous  acid  solution  taken  is  the  measure 
of  the  latter  which  reacted  with  the  chlorin. 


ESTIMATION  OF  IRON  231 

It  is  a  good  plan  in  each  case  to  divide  the  solution  into  two  or 
three  equal  parts  and  to  titrate  each  separately. 

REDUCTION    METHODS    INVOLVING   THE    USE    OF    STANNOUS    CHLORID 

Stannous  chlorid  (SnCl2)  is  a  very  powerful  reducing  agent. 
Its  action  in  this  respect  depends  upon  its  affinity  for  chlorin 
which  it  readily  abstracts  from  most  other  chlorids.  In  its  action 
upon  mercuric  chlorid,  a  portion  of  the  latter  is  always  reduced  to 
the  metallic  state. 

This  reducing  action  of  stannous  chlorid  is  utilized  in  certain 
volumetric  processes,  especially  in  the  estimation  of  iron.  In  this 
case  it  possesses  an  advantage  over  permanganate,  in  that  the  iron 
must  be  in  the  ferric  state,  in  which  condition  it  is  most  usually  found, 
while  if  permanganate  is  used,  a  preliminary  reduction  to  the  ferrous 
state  is  necessary  before  titrating.  The  great  disadvantage,  how- 
ever, is  in  the  fact  that  even  short  contact  with  air  will  quickly 
oxidize  it,  and  thus  spoil  its  titer.  In  consequence  of  this  it  must 
be  frequently  tested,  and  can  be  used  only  in  the  form  of  empirical 
solutions. 

It  is  particularly  useful  in  the  titration  of  ferric  salts,  which  salts 
can  be  accurately  estimated  by  direct  titration  with  it,  the  end-point 
being  recognized  by  the  disappearance  of  the  yellow  color  of  the  ferric 
solution.  These  salts  may  also  be  estimated  residually  by  adding 
an  excess  of  stannous  chlorid  solution  of  known  strength  and  reti- 
trating  the  excess  by  means  of  standard  iodin,  using  starch  as  indi- 
cator. 

The  reactions  are: 

Fe2Cl6  +  SnCl2  =  2FeCl2  -f  SnCl4 
and 

SnCl4+2HI. 


The  Estimation  of  Iron  by  Means  of  Stannous  Chlorid  Solu- 
tions may  be  accurately  affected  by  the  following  procedure,  as 
suggested  by  Fresenius.  The  solutions  necessary  are: 

(a)  A  solution  of  ferric  chlorid  containing  10  gms.  of  pure  iron 
in  a  liter. 

This  is  made  by  dissolving  10.04  gms.  of  thin  annealed  binding 
wire  (which  contains  99.6  per  cent  of  pure  iron)  in  a  sufficient  quan- 
tity of  pure  hydrochloric  acid.  A  small  quantity  of  potassium  chlorate 
is  then  added  to  effect  complete  oxidation  of  the  iron,  and  the  excess 


232 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


of    chlorin  expelled  by  boiling.     This  solution  is  then  cooled  and 
diluted  to  one  liter. 

(b)  A  solution  of  stannous  chlorid  made  by  dissolving  about  10  gms. 
of  pure  tin  in  200  cc.  of  strong,  pure  hydrochloric  acid.  This  may 
be  done  by  heating  the  tin  in  small  pieces  with  the  acid  in  a  flask, 
and  introducing  a  few  pieces  of  platinum  foil  to  excite  galvanic  action. 
The  solution  so  obtained  is  diluted  to  about  one  liter  with  distilled 

water  and  should  be  preserved  in  a  bottle, 
such  as  shown  in  Fig.  65,  to  which  air 
can  only  gain  access  through  a  strongly 
alkaline  solution  of  pyrogallic  acid. 
When  so  kept  the  strength  of  the  solu- 
tion can  be  preserved  for  several  weeks. 
(c)  A  solution  of  iodin  in  potassium 
iodid.  This  may  be  approximately  or 
exactly  decinormal. 

The  procedure  is  as  follows: 
ist.    The    relation   between    the    tin 
solution  and  the  iodin  solution  is  found. 
2d.  The    relation    between    the    tin 
solution  and  the  iron  solution  is  deter- 
mined. 

3d.  The  assay. 

The  relation  between  the  tin  solu- 
tion and  the  iodin  solution  is  found  as 
follows : 


FIG.  65. 


Two  cubic  centimeters  of  the  tin  solution  are  put  into  a  beaker,  a 
little  starch  solution  added,  and  the  iodin  solution  then  delivered  in 
from  a  burette  until  the  blue  color  occurs.  If  4  cc.  are  used  then 
each  2  cc.  of  iodin  solution  represents  i  cc.  of  tin  solution. 

The  relation  between  the  tin  solution  and  the  iron  solution  is  found 
as  follows: 

Fifty  cubic  centimeters  of  the  iron  solution  (representing  0.5  gm. 
of  iron)  are  put  into  a  small  flask  together  with  a  little  hydrochloric 
acid  and  heated  to  gentle  boiling.  The  tin  solution  is  then  delivered 
from  a  burette  until  the  yellow  color  of  the  iron  solution  is  nearly 
discharged.  It  is  then  added  continuously,  drop  by  drop,  until  the 
color  is  entirely  gone.  Assuming  that  35  cc.  were  required,  then  each 
35  cc.  of  tin  solution  are  equivalent  to  0.5  gm.  of  pure  iron.  If  the 
end-point  is  not  clearly  recognized  and  an  excess  of  the  tin  solution 


ESTIMATION  OF  MERCURIC   SALTS  233 

was  added,  the  solution  should  be  quickly  cooled,  a  few  drops  of 
starch  solution  added,  and  the  excess  estimated  by  titrating  with  the 
iodin  solution,  each  cc.  of  which  represents  0.5  cc.  of  the  tin  solution. 
The  excess  so  found,  deducted  from  the  total  quantity  of  tin  solution 
added,  gives  the  quantity  of  the  latter,  which  corresponds  to  0.5  gm. 
of  iron. 

Having  determined  these  data,  the  analyst  can  readily  estimate 
any  unknown  quantity  of  iron  in  solution  in  the  ferric  state. 

If  the  iron  is  partly  or  wholly  in  the  ferrous  state  it  may  be  oxi- 
dized by  adding  some  potassium  chlorate  and  boiling  to  expel  excess 
of  chlorin. 

The  Assay.  A  solution  of  iron  taken  for  analysis,  required  24  cc. 
of  the  tin  solution.  The  quantity  of  iron  present  is  calculated  by  pro- 
portion as  follows: 

35  cc.  :  0.5  gm.  :  :  24  cc.  :  x\         #=0.34  gm. 

To  secure  accurate  results  the  iron  solution  assayed  must  be  fairly 
concentrated,  because  then  the  end-reaction  is  more  readily  seen,  and 
also  because  the  greater  the  dilution  the  larger  the  amount  of  tin  solu- 
tion will  be  required.  It  is  good  policy  to  use  very  little  excess  of  the 
tin  solution,  so  that  only  a  very  small  quantity  of  iodin  solution  is 
required. 

Estimation  of  Mercuric  Salts  (Laborde).  This  depends  upon 
the  fact  that  stannous  chlorid  solution  added  to  a  solution  of  a  mercuric 
salt  reduces  the  latter  first  to  mercurous  chlorid  (calomel)  and  finally 
the  calomel  to  metallic  mercury.  The  reduction  to  calomel  results 
in  the  formation  of  a  white  precipitate,  and  when  the  mercuric  salt  is 
completely  reduced  the  stannous  chlorid  acts  upon  and  reduces  the 
calomel  to  metallic  mercury,  which  results  in  the  production  of  a  char- 
acteristic brownish  color. 

The  reactions  are  as  follows: 


SnCl2  +2HgCl2  ^SnCl4  +2HgCl; 
SnCl2  +2HgCl  =SnCl4  +Hg2. 

According  to  Laborde  the  tin  solution  is  made  by  dissolving  8  gms. 
of  pure  tinfoil  by  means  of  heat  in  ioo.cc.  of  pure  hydrochloric  acid, 
and  diluting  to  2  liters. 

This  tin  solution  is  checked  against  a  solution  of  mercuric  chlorid 
containing  10  gms.  per  liter.  To  counteract  the  hindering  effect  of 


234  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

the  hydrochloric  acid  the  solution  under  analysis,  containing  o.i  gm.  of 
mercuric  chlorid,  is  mixed  with  5  cc.  of  a  solution  containing  100  gms.  of 
ammonium  acetate  and  100  cc.  of  acetic  acid  to  the  liter.  The  acetic 
acid  promotes  the  disappearance  of  the  brown  color  which  occurs  at 
the  point  where  the  tin  solution  is  in  excess,  but  before  reduction  is 
complete.  The  titration  with  the  tin  solution  is  continued  until  a 
permanent  brown  color  occurs. 

If  the  brown  color  is  too  dark  from  overstepping  of  the  end-point, 
the  addition  of  i  cc.  of  the  mercuric  chlorid  solution  will  render  the 
solution  white  again,  and  the  titration  can  then  be  carried  further. 

This  method,  which  is  convenient,  rapid,  and  very  accurate,  can  be 
employed  in  many  cases.  If  the  mercuric  solution  contains  any  free 
mineral  acids,  the  latter  must  be  neutralized  with  ammonia  (in  the 
presence  of  ammonium  acetate,  to  prevent  formation  of  ammoniated 
mercury.) 

The  presence  of  alkali  and  alkali  earth  salts  or  most  salts  of 
other  metals  (except  iron,  gold  and  platinum),  do  not  in  the  least 
interfere  with  the  accuracy  of  the  results.  The  same  is  true  of  organic 
acids,  either  free  or  in  combination  with  alkali. 


PART   II 

CHAPTER  XIII 
ACETIC  ACID  AND  ACETATES 

Vinegar.  Vinegar  is  impure  diluted  acetic  acid.  Its  strength  may 
be  estimated  in  the  same  manner  as  acetic  acid.*  Phenolphthalein 
must  be  used  as  an  indicator.  Litmus  will  give  only  approximate 
results,  because  potassium  and  sodium  acetate  both  have  a  slightly 
alkaline  reaction  with  litmus,  but  show  no  reaction  with  phenol- 
phthalein.f  The  absence  of  mineral  acids  must  be  assured  before 
the  volumetric  test  is  applied. 

The  strength  of  vinegar  may  also  be  estimated  by  distilling  no  cc. 
until  100  cc.  come  over.  The  100  cc.  will  contain  80  per  cent  of  the 
whole  acetic  acid  present  in  the  no  cc.,  and  may  be  titrated;  or  the 
specific  gravity  of  the  distillate  may  be  taken,  and,  by  consulting 
the  table  on  the  next  page,  the  per  cent  strength  of  the  distillate 
found.  By  adding  20  per  cent  to  this  the  strength  of  the  original 
vinegar  is  obtained. 

Vinegar  usually  contains  from  three  per  cent  to  six  per  cent  of  acetic 
acid. 

ESTIMATION  OF  FREE  MINERAL  ACIDS  IN  VINEGAR 

Mr.  Hehner  has  devised  the  method  given  below,  which  has  the 
merit  of  being  speedy,  scientific,  and  accurate. 

The  method  is  based  upon  the  fact  that  acetates  of  the  alkalies 
are  always  present  in  commercial  vinegar,  and  when  vinegar  is  evap- 
orated to  dryness,  and  the  ash  ignited,  the  acetates  of  the  alkalies  are 
converted  into  carbonates.  If  the  ash  has  an  alkaline  reaction  no 

*  See  page  104. 

f  Even  dark-colored  vinegar  may  be  titrated  in  this  way  when  diluted.  If 
the  color,  however,  is  too  dark,  litmus-paper  or  phenolphthalein  paper  may  be 
used  by  bringing  a  drop  of  the  liquid  in  contact  with  the  paper  from  time  to  time 
during  the  titration. 

235 


236  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

ACETIC  ACID  TABLE 


Per  Cent 
of 
Absolute 
Acetic 
Acid. 

Specific  Gravity 

3B& 

Per  Cent 
of 
Absolute 
Acetic 
Acid. 

Specific  Gravity 

«{#& 

Per  Cent 
of 
Absolute 
Acetic 
Acid. 

Specific  Gravity 

•*{#£ 

I 

.0007 

26 

1.0363 

51 

1.0623 

2 

.0022 

27 

i-°375 

52 

1.0631 

3 

.0037 

28 

.0388 

53 

.0638 

4 

.0052 

29 

.0400 

54 

.0646 

5 

.0067 

3° 

.0412 

55 

•°653 

6 

.0083 

31 

.0424 

56 

.0660 

7 

.0098 

32 

.0436 

57 

.0666 

8 

.0113 

33 

.0447 

58 

.0673 

9 

.0127 

34 

.0459 

59 

.0679 

10 

.0142 

35 

.0470 

60 

.0685 

ii 

•OI57 

36 

.0481 

61 

.0691 

12 

.0171 

37 

.0492 

62 

.0697 

13 

.0185 

38 

.0502 

63 

.0702 

14 

.0200 

39 

-0513 

64 

.0707 

15 

.0214 

40 

-0523 

65 

.0712 

16 

.0228 

4i 

-0533 

66 

.0717 

17 

.0242 

42 

-0543 

67 

.0721 

18 

.0256 

43 

-0552 

68 

.0725 

19 

.0270 

44 

.0562 

69 

.0729 

20 

..0284 

45 

-0571 

70 

-0733 

21 

.0298 

46 

.0580 

71 

-°737 

22 

.0311 

47 

.0589 

72 

.0740 

23 

.0324 

48 

.0598 

73 

.0742 

24 

-0337 

49 

.0607 

74 

.0744 

25 

-°35° 

5° 

.0615 

75 

.0746 

free  mineral  acid  is  present.  If,  however,  the  ash  is  neutral  or  acid 
some  free  mineral  acid  must  be  present. 

The  quantitative  process  in  detail  is  as  follows:   50  cc.  of  vinegar 

N 
are  mixed  with  25  cc.  of  —  soda  or  potash.     The  liquid  is  evaporated 

to  dryness  on  a  water-bath,  and  the  residue  carefully  incinerated 
at  the  lowest  possible  temperature,  to  convert  the  acetates  into  car- 
bonates. When  cooled,  25  cc.  of  —  sulphuric  acid  are  added,  the 

mixture  heated  to  expel  CC>2  and  filtered.  The  filter  is  washed 
with  hot  water,  phenolphthalein  T.  S.  added,  and  the  filtrate  and 

N  N 

washings  carefully  titrated  with  —  alkali.     Each  cc.   of  —   alkali 

10  10 

used  represents  0.0046875  gm.  H2SO4  or  0.003618  gm.  HC1. 


ACETIC  ACID   AND   ACETATES  237 

Mohr's  Method  for  Estimating  Acetic  Acid  in  Vinegar.  Take  20 
gms.  of  the  vinegar,  add  an  excess  of  pure  precipitated  calcium  car- 
bonate (say  3  gms.),  set  aside  until  reaction  is  complete,  shaking  occa- 
sionally, and  then  boil  to  drive  off  the  CO2. 

Now  separate  the  residual  calcium  carbonate  by  nitration,  wash  it 

N 
thoroughly  with  boiling  water,  and  dissolve  in  a  measured  excess  of  — 

N  x 

hydrochloric  acid,  say  35  cc.,  and  titrate  back  with  —  sodium  hy- 

droxid,  using  phenolphthalein  as  indicator.     Assuming  that  4  cc.  were 

N 
used,  then  35—4=31,  the  number  of  cc.  of  —  hydrochloric  acid  which 

reacted  with  the  residual  calcium  carbonate.     Thus  we  have  31 X 
0.049675  gm.  =1.54  gms.  of  residual  calcium  carbonate.     Deduct  this 
from  the  3  gms.  taken,  and  we  arrive  at  the  quantity  which  was  taken 
up  by  the  acetic  acid,  namely,  1.46  gms. 
Therefore  the  20  gms.  of  vinegar  contain 


1.46X119.16 

-  =i-75  gms.  or  8.75  per  cent. 
y  y'o  o 

The  reactions  are  as  follows  : 

CaCO3  +  2HC2H3O2  =Ca(C2H3O2)2  +H2O  +CO2. 
99.35  119.16 

Hence  i  gm.  of  calcium  carbonate  represents  1.2  gms.  of  acetic  acid. 


CaC03  +2HCl  =  CaCl2+H20+C02. 

2)99-35      2)72.36 

49.675        36.18=  1000  cc.  —  HC1  V,  S. 

i 

0.049675  =       i  cc.  "      "       " 


This  process  answers  well  for  dark-colored  liquids  and  is  especially 
useful  for  impure  brown  pyroligneous  acid. 

Pettenkofer's  Method.  A  measured  excess  of  standard  barium 
hydroxid  solution  is  added  to  the  acetic  acid  or  the  vinegar,  and  titrated 
back  with  decinormal  acid,  using  turmeric  paper  as  indicator.  This 
method  is  the  best  for  high-colored  vinegars. 


238  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

ESTIMATION   OF  METALLIC   ACETATES 

Acetates  of  lead,  iron,  etc.,  are  treated  with  an  excess  of  normal 
alkali  carbonate,  which  precipitates  the  metal  as  carbonate  while 
an  alkali  acetate  is  formed  in  the  solution.  The  mixture  is  boiled, 
filtered,  and  the  precipitate  thoroughly  washed  on  the  filter  with  hot 
water.  The  filtrate  and  washings  are  mixed  and  made  up  to  a  definite 

N 
volume.     An  aliquot  portion  is  taken  out  and  titrated  with  —  acid 

solution.  The  difference  between  the  quantity  of  acid  used  and  that 
of  the  alkali  carbonate  originally  added  is  calculated  into  acetate 
by  multiplying  by  0.06  gm.  If  other  salts  than  acetate  are  present, 
proceed  as  follows:  Add  excess  of  alkali  carbonate  solution  to  pre- 
cipitate the  metal,  exactly  neutralize  the  filtrate  with  hydrochloric  acid, 
evaporate  to  dryness,  ignite  the  residue  to  convert  the  acetate  into 
carbonate,  and  then  titrate  with  normal  acid  solution  in  the  usual  way. 

Any  other  organic  acid  present  will  of  course  be  recorded  as  acetic. 

Acetates  of  Sodium  and  Potassium.  These  acetates  may  be 
estimated  by  ignition,  which  reduces  them  to  carbonates,  when  they 
can  be  titrated  with  standard  acid,  as  described  under  Analysis  of 
Organic  Salts  of  the  Alkalies,  page  83. 

Acetate  of  Lime.  The  presence  of  tarry  matters  and  other  impuri- 
ties in  the  commercial  acetate  of  lime  makes  it  necessary  to  adopt 
special  methods  for  its  valuation.  In  the  case  of  very  impure,  dark- 
colored  samples  the  acetic  acid  can  only  be  estimated  by  distillation 
with  phosphoric  acid  and  water  to  nearly  dryness,  and  then  estimating 

N 

the  acetic  acid  direct  with  —  alkali.     The  distillation  should  be  con- 

10 

ducted  in  a  retort,  so  arranged  that  there  can  be  no  vitiation  of  the 
distillate  through  sputtering  of  the  acid  liquid  in  the  retort,  i  gm. 
of  the  sample  is  placed  in  a  retort  and  10  cc.  of  a  40  per  cent  phos- 
phoric acid  added,  together  with  about  40  cc.  of  water,  and  the  dis- 
tillation commenced  over  a  naked  flame  and  continued  to  near  dryness. 
The  retort  is  then  cooled,  50  cc.  of  water  introduced,  and  the  distillation 
again  performed,  and  the  same  repeated  a  third  time.  This  will  be 
sufficient  to  carry  over  all  of  the  acetic  acid.  The  combined  distillates 

N 
are  then  titrated  with  —  alkali,  using  phenolphthalein  as  indicator. 

Weber's  Method  (Z.  A.  C,  xxrv  614)  is  based  on  the  insolubility 
of  silver  acetate  in  alcohol.  10  gms.  of  the  sample  are  powdered  and 
placed  in  a  250  cc.  flask,  a  little  water  added  and  heated  to  extract 
all  soluble  matter;  then  after  cooling,  the  mixture  is  diluted  to  250  cc.; 
the  solution  filtered  and  25  cc.  of  the  filtrate  mixed  in  a  beaker  with 
50  cc.  of  absolute  alcohol,  and  the  acetic  acid  precipitated  by  adding 


ACETATE   OF  LIME  239 

alcoholic  solution  of  silver  nitrate.  The  precipitate,  which  consists 
of  silver  acetate,  together  with  any  chlorid,  sulphate,  etc.,  is  then  washed 
on  a  filter  with  60  per  cent  alcohol,  till  all  free  silver  nitrate  is  removed. 
The  precipitate  is  then  dissolved  in  weak  HNOs  and  the  solution 

N 
titrated  with  —  NaCl.     This  method  gives  very  satisfactory  results. 

K.  R.  Haberland  (Zeitsch  f.  Anal.  Chem.,  1899,  217),  submits  the 
following  method  for  the  estimation  of  commercial  acetates: 

Ten  grams  of  the  finely  powdered  acetate  of  lime  are  introduced 
into  a  300  cc.  flask,  together  with  50  cc.  of  water  and  n  cc.  of  hydro- 
chloric acid  (sp.gr.  1.124),  a  condenser  is  attached  and  the  mixture 
distilled  until  the  fluid  is  of  a  syrupy  consistency,  then  50  or  60  cc. 
of  distilled  water  are  added  and  the  distillation  continued  to  dryness. 
The  distillate  is  received  in  a  250  cc.  flask  and  water  added  to  the  mark. 

The  distillate  which  contains  hydrochloric  acid  as  well  as  acetic 
acid  is  titrated  for  total  acidity  in  one  portion,  and  for  hydrochloric 
acid  in  another,  as  follows : 

Fifty  cubic  centimeters  of  the  distillate  are  titrated  with  normal 
sodium  hydroxid,  using  phenolphthalein  asv  indicator.  The  quantity 
of  normal  alkali  used  multiplied  by  5  gives  total  acidity. 

Then  25  cc.  of  the  distillate  are  placed  into  a  100  cc.  flask,  15  cc. 
of  pure  nitric  acid  are  added,  followed  by  an  excess  of  decinormal 
silver  nitrate  (the  quantity  of  which  is  known),  and  finally  distilled 
water  to  make  100  cc.  After  complete  subsidence  of  the  precipitate, 
50  cc.  of  the  clear  supernatant  liquid  are  removed  and  titrated  with 
decinormal  ammonium  sulphocyanate,  using  ammonio-ferric  sulphate 
as  indicator.  The  amount  of  sulphocyanate  solution  used,  multiplied 
by  2,  is  deducted  from  the  quantity  of  decinormal  silver  nitrate  solution 
added.  This  gives  the  quantity  of  the  latter,  which  represents  the 
hydrochloric  acid  present  in  25  cc.  of  the  distillate. 


CHAPTER  XIV 
BORIC  ACID  AND  BORATES 

Free  Boric  Acid  may  be  estimated  by  means  of  barium  hydroxid, 
as  suggested  by  Will.  The  method  is  said  to  be  fairly  accurate. 

The  boric  acid  solution  is  titrated  with  a  barium  hydroxid  solution 
of  known  strength,  until  the  turbidity  appearing  at  first  is  completely 
and  exactly  removed. 

The  equation  is  as  follows: 

4H3BO3  +Ba(OH)2  =BaB4O7  +  7H2O. 

Thompson's  Method  (Jour.  Soc.  Chem.  Ind.,  xn,  432).  The  addi- 
tion of  glycerin  to  a  boric  acid  solution  to  the  extent  of  30  per  cent 
develops  the  acidity  of  the  acid  to  a  great  degree.  It  may  then  be 
titrated  with  standard  sodium  hydroxid  solution,  using  phenolphthalein 
as  indicator.  (See  page  104.) 

Boric  Acid  in  Borax  may  be  estimated  as  follows : 
Add  methyl  orange  solution  (on  which  H3BO3  has  no  effect)  to 
the  solution  of  borax,  and  then  just  sufficient  standard  sulphuric  acid 
solution  to  acidulate.  Boil  and  exactly  neutralize  with  standard 
sodium  hydroxid.  All  the  boric  acid  is  now  in  a  free  state;  sufficient 
glycerin  is  now  added  so  that  the  solution  contains  at  least  30  per 
cent,  and  the  titration  with  standard  sodium  hydroxid  is  begun,  in  the 
presence  of  phenolphthalein. 

N 
i  cc.  —  NaOH  =0.06154  gm.  H3BO3: 

N 
i  cc.  — NaOH  =0.05013  gm.  ^26407. 

E.  F.  Smith's  Process  (Am.  Chem.  Jour.,  1882).  Take  10  cc.  of 
borax  solution  containing  o.i  gm.;  add  10  cc.  of  solution  of  manganese 
sulphate,  containing  0.06  gm.  of  MnSO4,  and  finally  20  cc.  of  strong 
alcohol.  A  white  flocculent  precipitate  of  MnB4C>7  separates.  Set 
aside  for  half  an  hour  to  settle;  filter,  wash  the  precipitate  with  alcohol, 
and  evaporate  the  filtrate  and  washings  to  dryness.  Then  dissolve  the 

240 


BORIC  ACID   AND   BORAXES  241 

residual  manganese  salt  in  water,  add  some  strong  solution  of  zinc 
sulphate,  heat  to  near  the  boiling  point,  and  titrate  with  potassium 
permanganate  until  a  permanent  pink  is  produced.  Each  cc.  of  the 
permanganate  represents  0.0044985  gm.  of  MnSO4. 

In  the  above  titration  6.4  cc.  were  required  =0.02879  gm- 
This  deducted  from  the  0.06  gm.  added  gives  us  0.031209  gm., 
the  amount  which  combined  with  the  borax. 

149.95  gms.  MnSO4=20o.52  gms.  Na2B4C>7; 
149.95  gms-  MnSO4  =  i38.88  gms.  6203. 
Thus  the  o.i  gm.  of  borax  analyzed  contained 

0.0417  gm.  of  pure  Na2B4O7=4i.7  per  cent 
or  0.0289  gm-  °f  B2C>3  =28.9  per  cent. 

The  reactions  are  as  follows: 

Na2B4O7+MnSO4==MnB4O7+Na2SO4. 

200.52  149-95 


449.85  3J3-96 

With  Ferric  Salicylate  as  Indicator.  Jules  Wollf  (Compt. 
rend.,  130-1128)  suggests  the  use  of  a  solution  of  ferric  salicylate  in 
sodium  salicylate  as  an  indicator  for  the  titration  of  boric  acid  and 
borates. 

In  the  case  of  borax,  for  instance,  the  solution  is  treated  with  a 
known  volume  of  standard  acid  in  excess.  The  ferric  salicylate  indi- 
cator is  then  added,  and  the  free  acid  is  titrated  until  the  violet  tint 
is  replaced  by  a  clear  madder-red  color.  The  end-reaction  is  very 
sharp.  If  ammonia  salts  are  present  an  excess  of  soda  is  added  and 
the  mixture  boiled  to  drive  off  the  ammonia  before  adding  the  acid. 

The  indicator  is  prepared  as  follows:  5  to  6  gms.  of  sodium  sali- 
cylate are  dissolved  in  25  cc.  of  distilled  water,  ferric  chlorid  solution 
added  drop  by  drop  until  a  slight  permanent  turbidity  results,  the 
solution  is  filtered  and  divided  into  two  equal  parts  ;  to  one  a  sufficient 
quantity  of  soda  solution  is  added  to  give  a  deep  orange  tint,  the  other 
is  treated  with  acid  to  the  development  of  a  red  tint.  The  two  are 
then  mixed  and  10  gm.  of  sodium  salicylate  dissolved  in  the  mixture. 


242  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Cladding's  Process  (J.  A.  C.  S.,  1898,  288).  i  gm.  of  the  substance 
to  be  examined  is  placed  in  a  flask  resting  on  a  wire  screen  over  a 
Bunsen  burner,  together  with  a  little  95-per-cent  methyl  alcohol  and 
5  cc.  of  syrupy  phosphoric  acid.  Flask  A  (Fig.  66),  two-thirds  filled  with 
methyl  alcohol,  is  placed  on  the  water-bath  E.  Flask  B  is  connected 
with  condenser  D,  and  flask  C  connected  to  receive  the  distillate.  Heat 
is  now  applied  to  the  water-bath  E,  and  when  the  methyl  alcohol 
boils,  flask  A  is  connected  to  the  tube  which  passes  to  the  bottom  of 
flask  B.  A  current  of  methyl-alcohol  vapor  is  thus  continually  passing 
through  the  liquid  in  flask  B,  and  carries  over  the  boric  acid.  Heat  is 
applied  under  flask  B  and  so  regulated  that  the  liquid  remains  between 
15  and  25°  C. 

The  distillation  is  carried  on  in  this  way  for  about  half  an  hour, 
or  until  about  100  cc.  of  distillate  are  obtained.  A  mixture  of  40  cc. 


FIG.  66. 

glycerin  and  100  cc.  water  is  now  carefully  neutralized,  using  phenol- 
phthalein  as  indicator,  and  then  added  to  the  distillate,  and  the  mixture 
titrated  with  standard  soda  solution.  A  blank  assay  should  be  made, 
using  all  the  reagents,  and  any  acidity  found  deducted  from  the  final 
results.  Gladding  found  36.57  per  cent  of  boric  acid  in  borax,  the 
theoretical  per  cent  being  36.65,  while  boric  acid  itself  gave  99.9  per 
cent. 

In  this  process  the  distillation  must  be  continued  until  all  of  the 
boric  acid  has  passed  over.  This  usually  occupies  about  half  an  hour. 

The  method  depends  upon  the  fact  that  boric  acid  in  alcoholic 
solution  is  remarkably  volatile.  If  a  vessel  containing  such  a  solution 
be  covered  with  a  glass  plate  and  allowed  to  stand  for  about  twenty- 
four  hours  at  the  ordinary  temperature  of  the  atmosphere,  a  distinct 
white  coating  of  boric  acid  will  be  deposited  upon  the  glass.  The 
presence  of  boric  acid  in  a  substance  may  be  determined  by  treating 
it  with  sulphuric  acid  and  alccihol  in  a  test-tube  closed  with  a  stopper 
bearing  a  bent  glass  tube,  boric  acid  being  then  deposited  in  the  tube. 


BORIC   ACID   AND   BORATES  243 

The  quantitative  analysis  of  boric  acid  in  mixtures  or  compounds  may 
be  made  in  this  way.  The  acid  is  first  extracted  with  absolute  alcohol 
in  an  extraction  apparatus  provided  with  a  reflux  condenser  and  the 
solution  so  obtained  distilled. 

Gladding  found  that  a  distillation  of  crystallized  boric  acid  made 
without  the  addition  of  phosphoric  or  other  acid  yielded  all  of  the 
boric  acid  present.  A  similar  distillation  of  borax  he  found  to  yield 
19.50  per  cent  of  boric  acid  out  of  a  total  of  36.65  per  cent  present,  or 
slightly  more  than  50  per  cent  of  the  whole  amount. 


CHAPTER  XV 
CARBONIC  ACID  AND  CARBONATES 

ALKALI  carbonates  may  be  accurately  estimated  by  titration 
with  standard  acid  solutions,  as  described  on  page  177. 

They  may  also  be  estimated  by  precipitation  with  calcium  or 
barium  chlorid  and  the  precipitated  carbonate  then  treated  with  an 
excess  of  standard  acid,  and  retitrated  with  standard  alkali. 

Calcium  chlorid  is  preferred  where  it  can  be  used,  because  the 
physical  characters  of  the  calcium  carbonate  are  such  as  to  render  it 
more  rapidly  and  thoroughly  washed  than  is  the  case  with  barum 
carbonate.  If  caustic  alkalies  are  present,  however,  barium  chlorid 
must  be  used,  as  calcium  hydroxid  is  very  insoluble,  and  is  in  conse- 
quence precipitated  with  the  carbonate. 

If  ammonia  is  present,  the  precipitation  of  calcium  carbonate  or 
barium  carbonate  is  not  complete.  In  this  case  it  is  necessary  to 
heat  the  mixture  for  several  hours. 

Example.  The  carbonate  is  dissolved  in  water,  heated  and  treated 
with  calcium  or  barium  chlorid  in  excess,  and  the  mixture  boiled  for 
a  few  minutes,  filtered,  and  the  precipitate  rapidly  washed  with  several 
portions  of  hot  water.  The  precipitate  together  with  the  filter  is 
placed  in  a  flask  and  a  measured  excess  of  normal  acid  added,  and 
the  mixture  boiled  until  the  precipitate  is  dissolved  and  the  CC>2  ex- 
pelled. 

Phenolphthalein  is  then  added,  and  lastly  normal  alkali  from  a 
burette  until  a  faint  pink  appears. 

The  quantity  of  normal  alkali  used  is  deducted  from  the  acid 
added,  and  the  quantity  of  the  latter  which  went  into  combination 
with  the  precipitate  found.  The  reactions  are  written  thus: 

Na2CO3  +BaCl2  =BaCO3  +  2NaCl; 
then  BaCO3  +  2HCl=BaCl2+H2O  +  CO2. 


The  factors  for  the  alkali  carbonates  are  the  same  as  when  esti- 
maled  by  direct  titration  with  acid,  which  see.  See  also  the  Gasometric 
Method,  page  691. 


244 


CARBONIC   ACID   AND   CARBONATES 


245 


CARBONIC   ACID   IN   INSOLUBLE   CARBONATES 

This  may  be  estimated  by  decomposition  with  an  acid,  and  con- 
ducting the  CC>2  into  strong  ammonia-water  which  absorbs  it  com- 
pletely. The  CO2  is  then  precipitated  by  calcium  chlorid  and  esti- 
mated as  explained  under  alkali  carbonates.  The  ammonia-water 
must  be  free  from  CO2.  If  any  be  present,  it  must  be  removed  by 
means  of  calcium  chlorid.  The  decomposition  is  effected  in  the  appa- 
ratus shown  in  Fig.  67.  The  carbonate 
and  some  water  is  put  in  A,  hydrochloric 
acid  in  b,  ammonia  water  in  B,  and  some 
pieces  of  broken  glass  in  c,  through  which 
the  ammonia  is  poured  into  the  flask. 

The  flask  containing  the  ammonia-water 
is  heated  until  it  is  filled  with  its  fumes. 
Then  the  hydrochloric  acid  is  run  into  the 
carbonate  by  opening  the  pinch-cock,  and 
when  decomposition  of  the  carbonate  is 
complete  the  liquid  is  boiled,  and  finally  a 
slow  current  of  air  free  from  CO2  is  drawn 
through  the  apparatus  to  carry  over  the  last 
traces  of  the  gas. 

The  apparatus  is  then  disconnected,  c 
is  rinsed  into  B,  calcium  chlorid  added 
and  the  solution  boiled  for  some  time, 
and  the  precipitated  carbonate  treated 
as  explained  ir  the  foregoing  process. 
In  either  of  the  foregcing  processes  the  precipitated  barium  or  cal- 
cium carbonate  may  be  dissolved  in  hydrochloric  acid,  evaporated  to 

N 

dryness  and  the  amount  of  chlorin,  as  chlorid,  found  by  means  of  — 

10 

silver  nitrate  solution  in  presence  of  chromate. 


FIG.  67. 


N 
i  cc.  —  acid  or  silver  =0.002  1835  gm.  CO2J 


N 

i  cc.  —  •  acid  or  silver  =0.00526  gm. 
10 


Certain  insoluble  carbonates  may  also  be  estimated,  by  adding 

N 
to  a  weighed  quantity  of  the  salt  a  measured  excess  of  —  sulphuric 

acid,  then  boiling  to  drive  off  CC>2  and  titrating  the  excess  of  acid 


246  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

N       N 
by  means  of  —  or  —  alkali,  as  described  under  Estimation  of  Calcium 

I  10 

Carbonate,  page  91. 

In  alkali  carbonates  containing  bicarbonate,  the  excess  of  carbonic 
acid  over  that  necessary  to  form  neutral  carbonate  may  be  found  by 
adding  barium  chlorid  in  excess  to  the  somewhat  dilute  solution, 
then  standard  baryta-water  in  excess,  and  estimating  the  excess  by 

N 

—  oxalic  acid,  as  above.     In  this  case,  all  the  carbonic  acid  contained 

10 

as  neutral  carbonate  is  precipitated  by  the  barium  chlorid  first  added, 
but  the  remainder  stays  in  solution,  and  is  estimated  by  addition  of 
the  baryta-water.  See  page  82. 

Carbonic  acid  in  solution  in  water  may  be  measured  by  adding 

N 

an  excess  of  standard  baryta-water,  and  estimating  the  excess  by  — 

10 

oxalic  acid.  The  whole  of  the  carbonic  acid  is  precipitated  as  car- 
bonate immediately  upon  addition  of  the  baryta-water,  and  the  excess 

N 
is  found  by  adding  —  oxalic  or  hydrochloric  acid,  until  a  glass  rod 

dipped  in  the  fluid  no  longer  produces  a  brown-red  spot  upon  turmeric 
paper.  The  end  of  the  reaction  is  very  sharply  shown.  When  carbon 
dioxid  is  contained  in  a  gaseous  mixture,  it  may  be  estimated  by  pass- 
ing through  a  measured  volume  of  standard  baryta-water,  and,  with- 
out filtering,  estimating  the  excess  of  baryta. 

N 
cc.  — 

10 
CARBONIC-ACID   GAS  IN  THE  ATMOSPHERE 

This  is  done  by  the  modified  Pettenkofer's  method  as  follows:  A 
glass  globe  or  bottle  holding  from  5  to  10  liters  is  filled  with  the  au- 
to be  tested,  by  means  of  a  bellows;  baryta-water' of  known  strength 
is  then  introduced  in  convenient  quantity. 

The  bottle  is  then  securely  closed  and  set  aside  for  about  one 
hour,  rotating  it  at  intervals,  so  that  the  liquid  is  spread  over  the  entire 
inner  wall  of  the  bottle. 

When  the  time  is  up  the  baryta-water  is  emptied  out  quickly  into 
a  beaker,  covered  carefully  with  a  watch-glass,  and  when  the  barium 
carbonate  has  subsided  a  portion  of  the  clear  liquid  is  withdrawn  and 

N 
titrated  with  —  oxalic  acid  solution.*    The  difference  between  the 

10 

N 
*  —  hydrochloric  acid  may  be  used  with  equally  good  results. 


CARBONIC  ACID  GAS  IN  THE  ATMOSPHERE  247 

quantity  of  oxalic  acid  solution  required  to  neutralize  the  barium 
hydroxid  solution,  before  and  after  contact  with  the  air,  is  the  quan- 
tity equivalent  to  the  carbonic  acid  gas  absorbed. 

The  Baryta-water  is  made  by  dissolving  about  7  gms.  of 
pure  crystallized  barium  hydroxid  in  1000  cc.  of  distilled 
water. 

This  solution,  being  prone  to  absorb  CC>2  out  of  the  air,  must  be 
kept  in  a  special  bottle,  such  as  is  illustrated  in  Fig.  49,  which  prevents 
access  of  CO2  and  admits  of  the  withdrawal  of  any  quantity  of  solution 
without  inverting  the  bottle. 

The  Bottle  which  is  used  to  collect  the  air  should  hold  from  5  to 
10  liters,  and  its  exact  capacity  must  be  known.  This  may  be  found 
by  filling  the  bottle  to  the  bottom  of  the  cork  with  water  and  then 
accurately  measuring  the  water.  Before  using  the  bottle  it  must  be 
absolutely  dry. 

The  Analysis.  Into  the  bottle,  the  capacity  of  which  is  exactly 
known  —  we  will  assume  it  to  be  7100  cc.  —  is  blown  the  air  to  be  tested, 
by  means  of  a  bellows. 

100  cc.  of  the  baryta-water  are  then  introduced,  thus  leaving  7000 
cc.  of  air  in  the  bottle. 

The  bottle  is  now  securely  closed  and  set  aside  for  about  half  an 
hour,  rotating  it  occasionally  so  as  to  spread  the  liquid  over  the  entire 
inner  wall  of  the  bottle.  While  waiting  for  the  half  hour  to  expire, 
a  convenient  quantity  of  baryta-water  is  taken  and  its  strength  com- 
pared with  decinormal  oxalic  acid  solution  by  titrating  with  the  latter, 
using  phenolphthalein  as  indicator. 

50  cc.  of  baryta  water  is  a  convenient  quantity.  This  is  placed 
in  a  beaker,  a  few  drops  of  phenolphthalein  added,  and  then  titrated 

N 

with  the  —  acid  solution  until  the  color  just  disappears. 
10 

Let  us  assume  that  40  cc.  of  the  latter  were  consumed;  80  cc. 
will  then  be  consumed  by  100  cc.  of  baryta-water. 


2)170.16    2)125.1 

io)8s.o8_    io)62.55_  N 

8.508  gms.   6.255  gms-  or  I00°  cc.  —  V.  S. 

10 


Ba(OH)2  +  C02=BaC03+H20. 
2)170.16       2)43.66 
10)85.08     10)21.83 

8.508  gms.  2.183  gms- 


248  A   MANUAL   OF    VOLUMETRIC   ANALYSIS 

These  equations  show  that  2.183  gms-  °f  carbon  dioxid  will  neu- 

N 
tralize  as  much  barium  hydroxid  as  1000  cc.  of  —  oxalic  acid  solution. 

N  I0 

And  thus  each  cc.  of  the  —  oxalic  acid  solution  is  chemically  equiva- 
lent to  0.002183  gm.  of  carbon  dioxid;  therefore  100  cc.  of  the  baryta- 
water  is  capable  of  absorbing  80X0.002183  gm.  =0.17464  gm.  of 
C02. 

The  next  step  is  to  determine  the  quantity  of  CO2  that  was  absorbed 
by  the  100  cc.  of  baryta-water  which  was  introduced  into  the  bottle 
of  air. 

The  liquid  is  poured  out  of  the  bottle  into  a  small  beaker,  carefully 
covered  with  a  watch-glass,  and  the  barium  carbonate  allowed  to  settle. 
Then  50  cc.  of  the  clear  supernatant  liquid  are  drawn  out  of  the  beaker 
by  means  of  a  pipette,  treated  with  a  few  drops  of  phenolphthalein 

N 
T.  S.,  and  titrated  with  the  —  oxalic  acid  until  the  red  color  is  just 

TO 

discharged.  Note  the  number  of  cc.  consumed,  double  it,  and 
deduct  this  number  from  80,  the  quantity  which  100  cc.  of  baryta- 
water  consumed  before  being  brought  in  contact  with  CC>2. 

Example.  Assuming  that  30  cc.  of  the  oxalic  acid  solution  were 
required  by  the  50  cc.  of  the  baryta- water  after  exposure,  the  100  cc. 
then  would  require  60  cc.  There  is  thus  a  loss  of  alkalinity  equivalent 

N 

to  20  cc.  of  —  oxalic  acid.     This  is  due  to  the  absorption  of  carbon 
10 

dioxide,  which  neutralizes  the  hydroxid  by  forming  a  carbonate. 

N 
Now  since  each  cc.  of  —  oxalic  acid  is  chemically  equivalent  to 

0.002183  gm.  of  CC>2,  the  baryta-water  must  have  absorbed 


20X0.002183  gm.  =0.02366  gm.  of  CO2- 


Therefore  the  7000  cc.  of  air  which  the  bottle  held,  contained  0.04366 
gm.  of  CC>2. 

In  stating  the  result  of  an  analysis  the  quantity  of  CC>2  by  volume 
in  10,000  cc.  of  air  is  generally  given. 

In  the  above  case  7000  cc.  of  air  contained  0.04366  gm.  of  CC>2; 
10,000  cc.  of  this  same  air,  then,  contains 


0.04366X10,000             0.04366X10 
or  =0.06237 

7000  7 


CARBONIC   ACID  GAS  IN   THE  ATMOSPHERE 


249 


If  several  bottles  are  in  use  it  is  convenient  to  mark  upon  them 
the  multiplier  and  divisor,  thus: 

10,000  10 
or    — . 

7000  7 

In  calculating  the  volume  of  a  gas,  the  temperature  and  pressure 
must  be  taken  into  acount. 

By  referring  to  the  following  table  the  volume  occupied  by  o.ooi  gm. 
of  CC>2  at  different  temperatures  can  be  seen. 

The  volume  of  0.06237  gm.  of  CC>2  at  16°  C.  is 


0.06237X0.53843 


o.ooi 


cc. 


TABLE  SHOWING  VOLUME  OF  .001  GM.  OF  CARBON  DIOXID  AT  VARIOUS 
TEMPERATURES 


c.° 

F.° 

Cc. 

c.° 

F.° 

Cc. 

0 

32 

0.50863 

J3 

55-4 

0.533U 

i 

33-8 

0.51049 

14 

57-2 

0-53471 

2 

35-6 

°-5I235 

i5 

59 

0.53657 

3 

37-4 

0.51451 

16 

60.8 

0.53843 

4 

39-2 

0.51608 

i7 

62.6 

0.54030 

5 

4i 

0.51794 

18 

64.4 

0.54216 

6 

42.8     - 

0.51980 

19 

66.2 

0.54402 

7 

44-6 

0.52167 

20 

68 

0.54589 

8 

46.4 

°-  52353 

21 

69.8 

0-54775 

9 

48.2 

o-52539 

22 

71.6 

0.54961 

10 

So 

0.52726 

23 

73-4 

o.55i77 

it 

Si-  8 

0.52912 

24 

75-2 

0-55334 

12 

53-6 

0.53098 

If  the  pressure  remains  constant,  the  volume  of  a  gas  increases 
regularly  as  the  temperature  increases,  and  decreases  as  the  tempera- 
ture decreases.  (Charles'  Law.) 

This  expansion  or  contraction  amounts  to  -5-^  of  the  volume  of 
the  gas  for  each  degree  centigrade. 

Thus  by  calculation  the  volume  of  o.ooi  gm.  CC>2  (0.50863  cc.) 
at  any  temperature  may  be  found. 

-gn^  of  0.50863  =0.001863. 

Then  to  find  the  volume  at  any  given  C.  temperature  multiply  the 
degree  of  temperature  by  0.001863,  and  add  the  answer  to  0.50863. 


250  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

The  Pettenkofer  method  has  long  been  the  favorite,  but  it  contains 
certain  inherent  sources  of  error,  which,  however,  can  be  obviated 
by  skilful  manipulation  and  with  the  use  of  improved  apparatus. 
The  principal  errors  of  this  method  and  its  usual  modifications  are 
due  to  absorption  of  carbon  dioxid  from  the  outer  air  or  the  breath 
of  the  operator,  and  the  action  of  the  caustic  alkali  on  the  glass  of 
the  container,  as  well  as  the  presence  of  small  amounts  of  the  pre- 
cipitated barium  carbonate  in  the  fluid  taken  out  for  titration. 

These  errors  can  be  obviated  by  employing  Walker's  method,*  as 
modified  by  Woodman, f  which  is  outlined  briefly,  as  follows: 

To  a  definite  volume  of  air,  usually  i  to  2  liters,  is  added  a  measured 
amount  of  standard  barium  hydroxid,  care  being  taken  to  avoid  con- 
tact of  the  solution  with  the  air.  After  the  absorption  of  the  carbon 
dioxid,  the  solution  is  filtered  under  reduced  pressure,  through  asbestos, 
and  the  clear  barium  hydroxid  received  into  a  known  excess  of  standard 
hydrochloric.  The  absorption  vessel  is  rinsed  out  with  water,  free 
from  carbon  dioxid.  The  excess  of  acid  is  then  determined  by  titra- 
tion with  barium  hydroxid. 

N 
The  standard  solutions  required  are  —  hydrochloric  acid,  and 

N  ^ 

approximately   -     -  barium  hydroxid,   its  exact  strength  relative  to 

acid  being  found  by  titration  when  required. 

The  bottles  in  which  the  air  is  collected  for  analysis  should  be  of 
about  2  liters  capacity  (the  exact  capacity  must  be  known),  and 
coated  on  the  inside  with  paraffin;  this  prevents  action  of  the  alkali 
on  the  glass  during  the  absorption. 

It  also  admits  of  more  complete  draining  and  washing.  The 
bottles  should  be  provided  with  rubber  stoppers,  through  which  pass 
a  long  tube  reaching  to  near  the  bottom  of  the  bottle,  and  a  short 
tube  which  reaches  just  below  the  stopper.  Both  tubes  may  be  pro- 
vided with  stop-cocks  or  with  rubber  tubing  and  pinch-cocks.  The 
bottles  are  best  filled  with  the  air  to  be  analyzed,  by  suction  applied 
at  the  short  tube,  the  air  entering  through  the  longer  tube;  this  reduces 
the  danger  of  contamination  by  the  breath  of  the  opeartor. 

The  absorption  is  carried  on  for  about  thirty  minutes  and  the  solu- 
tion then  filtered  through  an  asbestos  filter  in  a  Soxhlet  filtering  tube 
under  diminished  pressure,  all  air  being  excluded.  The  filtrate  being 
received  in  a  measured  quantity  of  the  standard  hydrochloric  acid, 
the  absorption  bottle  must  be  thoroughly  rinsed  with  100  to  150  cc.  of 
water,  free  from  carbon  dioxid,  or  with  a  prepared  "  wash  water  " 
made  as  suggested  by  Walker  by  adding  i  cc.  of  a  10  per  cent  solu- 

*  J.  Chem.  Soc.,  77,  mo  (1900;.  f  J.  A.  C.  S.,  XXV,  150. 


CARBONIC  ACID  IN  NATURAL  WATERS  251 

tion  of  barium  chlorid  and  3  drops  of  phenolphthalein,  then  titrating 
with  barium  hydroxid  to  a  faint  permanent  pink  tint. 

The  barium  hydroxid  solution  must  be  kept  in  a  bottle  coated  on 
the  inside  with  either  paraffin  or  barium  carbonate  (to  prevent  action 
of  the  alkali  on  the  glass),  and  provided  with  a  device  for  protecting 
the  solution  against  contact  with  carbon  dioxid  of  the  air  (see  Fig.  49). 

All  rubber  stoppers  and  connections  with  which  the  solution  may 
come  in  contact  should  be  cleaned  with  a  five  per  cent  potash  solution, 
washed,  and  boiled  with  a  dilute  dichromate  and  sulphuric  acid  mix- 
ture, and  then  rubbed  and  washed  until  free  from  acid. 

ESTIMATION    OF    CARBONIC    ACID    IN    NATURAL   WATERS 

Carbonic  acid  exists  in  natural  waters  presumably  in  three  different 
forms : 

(1)  Free  carbonic  acid  (H^COs).     It  is  generally  considered  that 
the  carbon  dioxid  (CO2)  which  is  dissolved  in  water  exists  as  a  true 
acid  having  the  formula  H2CC>3. 

(2)  Normal  carbonates  of  calcium  and  magnesium  (CaCOa  and 
MgCO3)  and  of  the  alkalies. 

(3)  Bicarbonates  of  the  same  CaH2(CO3)2  and  MgH2(CO3)2. 
The  carbonic  acid  existing  free  may  be  completely  driven  off  by 

heating  the  water.  Carbonic  acid  existing  in  the  half-bound  state, 
i.e.,  bicarbonate,  may  also  be  driven  off  (though  less  completely)  on 
boiling.  Hence  these  two  forms  are  distinguished  as  volatile,  while 
that  existing  as  normal  carbonate  is  fixed. 

The  estimation  of  carbonic  acid  in  natural  waters  consists  in  deter- 
mining the  amount  of  free  and  half-bound  carbonic  acid  which  may 
be  present. 

The  Pettenhofer  Method.  This  method  is  based  upon  the  action 
which  calcium  or  barium  hydroxid  has  upon  free  and  half-bound 
carbonic  acid,  whereby  insoluble  calcium  and  barium  carbonates  are 
formed  and  precipitate  out  of  solution.  A  measured  excess  of  the 
hydroxid  is  used,  and  the  portion  unacted  upon  is  determined  volu- 
metrically  with  a  standard  acid  solution. 

The  reaction  is  as  follows: 

CaH2(CO3)2+CO2+2Ba(OH)2=2BaCO3+CaCO3+2H2O. 

If  magnesium  bicarbonate  is  present  in  the  water  the  reaction 
between  it  and  the  barium  or  calcium  hydroxid  is  the  same  as  shown 
in  above  equation;  but  magnesium  carbonate  (MgCO3),  being  more 
soluble  than  the  carbonates  of  calcium  and  barium,  does  not  pre- 
cipitate as  readily,  and  instead  reacts  with  the  hydroxid,  forming  mag- 
nesium hydroxid  which  precipitates.  The  presence  therefore  of 
magnesium  carbonate,  or  in  fact  any  magnesium  salt,  causes  the 


252  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

calcium  or  barium  hydroxid  to  be  used  up.     This  precipitation  of 
magnesium  is  prevented  by  the  introduction  of  ammonium  chlorid, 
which,  by  forming  a  soluble  salt  of  ammonium  and  magnesium  chlorid, 
prevents  any  loss  of  calcium  or  barium  hydroxid. 
The  reaction  is  as  follows : 

MgCO3  +4NH4C1  +Ba(OH)2  =MgCl2(NH4Cl)2  -f  BaCO3 +2NH4OH. 

As  here  shown  an  equivalent  amount  of  ammonium  hydroxid  is 
formed  and  hence  the  alkalinity  is  not  changed.  Heat  must  not  be 
applied  in  this  case,  as  this  would  volatilize  the  ammonia. 

If  alkali  carbonate,  sulphate,  or  any  other  alkali  salt  is  present, 
the  acid  of  which  would  be  precipitated  by  the  calcium  or  barium 
hydroxid,  the  addition  of  a  small  quantity  of  a  neutral  solution  of 
barium  or  calcium  chlorid  will  remove  it. 

This  addition,  besides  removing  carbonates  and  other  interfering 
salts,  also  prevents  any  irregularities  which  might  arise  from  the  pres- 
ence of  free  alkali  in  the  calcium  or  barium  hydroxid  or  of  magnesium 
carbonate  in  the  water  itself.  This  irregularity  is  due  to  a  double 
decomposition  which  occurs  between  magnesium  or  alkali  oxalate 
and  calcium  carbonate  (which  is  seldom  entirely  absent  from  the 
titrated  fluid),  forming  calcium  oxalate  and  alkali  or  magnesium  car- 
bonate, which  two  latter  will  of  course  take  up  oxalic  acid. 

The  Process.  100  cc.  of  the  water  are  put  into  a  flask  of  about  200  cc. 
capacity,  3  cc.  of  strong  calcium  or  barium  chlorid  solution,  and  2  cc. 
of  a  saturated  solution  of  ammonium  chlorid  are  added,  followed  by 
45  cc.  of  calcium  or  barium  hydroxid  solution,  the  strength  of  which 

N 
was  previously  ascertained  by  means  of  —  acid.    The  flask  is  then 

closed  with  a  well-fitting  rubber  stopper,  shaken  and  set  aside  for 
about  twelve  hours  or  until  the  precipitate  has  fully  settled.  The 
fluid  contents  of  the  flask  measure  150  cc.  50  cc.  of  this  clear  liquid 

N 
are  removed  by  means  of  a  pipette  and  titrated  with  —  oxalic  acid, 

using  turmeric  paper  as  the  indicator.  The  quantity  used  multiplied 
by  3  gives  the  total  quantity  of  calcium  or  barium  hydroxid  solution 
left.  This  deducted  from  the  original  45  cc.  added,  gives  the  quan- 
tity which  reacted  with  the  carbon  dioxid  present.  This  quantity 
multiplied  by  0.002183  gm.  will  give  the  weight  of  carbon  dioxid  exist- 
ing free  or  in  the  form  of  bicarbonate  in  the  100  cc.  of  water  taken 
for  analysis.  In  making  such  an  analysis  the  first  step  is  to  deter- 
mine the  relation  between  the  calcium  or  barium  hydroxid  solution 

and  the  —  oxalic  acid.     This  is  done  by  titrating  45  cc.,  with  the 
10 


CARBONIC  ACID   IN  NATURAL   WATERS  253 

acid,  shaking  thoroughly  during  the  operation  until  the  alkaline  reaction 
has  just  vanished.  This  end-reaction  is  known  to  be  reached  when 
a  drop  taken  out  on  a  pointed  glass  rod  and  applied  to  turmeric  paper 
produces  no  brown  spot.  Two  trials  should  be  made,  the  first  a 
rough  one,  the  second  should  be  exact. 

If  the  water  contains  only  free  carbonic  acid  it  is  best  to  use  barium 
instead  of  calcium  hydroxid,  because  the  calcium  carbonate  first  formed 
is  amorphous  and  perceptibly  soluble  in  water,  to  which  it  imparts  an 
alkaline  reaction.  Hence  the  unprecipitated  lime  cannot  be  estimated 
until  the  calcium  carbonate  has  separated  in  crystalline  form,  which 
is  insoluble  and  takes  from  eight  to  ten  hours. 

N 

Instead  of  —  oxalic  acid,  standard  sulphuric  or  hydrochloric  acid 
10 

may  be  used,  and  the  turmeric  paper  may  be  replaced  as  indicator 
by  rosolic  acid  or  any  other  indicator  which  is  not  affected  by  ammo- 
nium compounds. 

This  method  is  simple,  expeditious,  and  fairly  satisfactory,  but 
there  are  a  great  many  precautions  which  must  be  observed  in  order 
to  obtain  reliable  results.  They  are  as  follows:* 

(1)  The  avoidance  of  exposure  to  air  of  the  barium  hydroxid  solu- 
tion and  of  the  sample  of  water  to  be  analyzed.     The  former  should 
be  kept  in  a  bottle  whose  outlet  to  the  air  is  provided  with  a  tube  con- 
taining fused  calcium  chlorid  and  stick  potash.    (See  Fig.  48.)    Undue 
exposure  of  the  water  should  likewise  be  avoided,  to  prevent  its  absorb- 
ing carbon  dioxid,  which  it  does  readily. 

(2)  The  standardization  of  the  barium  hydroxid  should  be  carried 
out  in  a  manner  similar  to  the  method  employed  in  the  titration  of 
the  sample.     If  the  titration  is  done  in  the  presence  of  rosolic  acid, 
the  standardization  should  be  done  writh  the  same  indicator  and  in 
the  presence  of  ammonium  chlorid. 

(3)  The  titer  of  the  oxalic  acid  solution   (if  this  is  used),  should 
be  found  just  before  using,  as  it  deteriorates  rapidly. 

(4)  The  use  of  a  syphon  to  introduce  the  water  into  the  bottle  in 
which  the  precipitation  is  done. 

(5)  The  reagents  should  be  added  to  the  sample  in  the  order  given 
in   the   description   of  the  process.     The   barium  hydroxid  solution 
being  added  last,  and  in  order  to  avoid  any  exposure  it  should  be 
introduced  by  means  of  a  long  delivery  tip  on  the  burette,  the  lower 
end  of  which  dips  below  the  surface  of  the  sample  in  the  bottle. 

(6)  The  allowance  of  sufficient  time  for  the  separation  of  the  car- 
bonate in  crystalline  form  before  withdrawing  the  supernatant  liquid 
for  titration. 

*  Ellms  and  Beneker,  J.  A.  C.  S.,  XXIII,  412. 


254  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

(7)  The  bottle  in  which  the  precipitation  is  done  should  be  pro- 
vided with  a  tightly  fitting  stopper,  either  rubber  or,  better  still,  ground 
glass  stoppers,  well  vaselined. 

(8)  The  removal  of  the  50  cc.  of  liquid  for  titration  should  be 
carefully  done  in  order  to  avoid  stirring  up  the  precipitate  in  the 
bottom. 

(9)  The  titration  of  the  withdrawn  portion  of  the  liquid  should 
take  place  immediately,  and  the  acid  should  be  run  in  quickly. 

Ellms  and  Beneker,  J.  A.  C.  S.,  xxin,  405,  detail  numerous  care- 
fully made  experiments  appertaining  to  this  subject. 

Even  with  strict  observance  of  all  of  these  precautions,  there  are 
several  sources  of  error,  as  pointed  out  by  Forbes  and  Pratt,  J.  A.  C.  S., 
xxv,  744:  "They  are  due  to  the  fact  that  working  on  such  a  small 

N 

quantity  of  water  as  100  cc.  and  employing  —  acid,  very  slight  errors 

10 

in  the  titration  of  the  aliquot  portion  of  50  cc.,  in  measuring  out  the 
sample  for  titration  and  in  measuring  the  original  sample,  make  a 
considerable  difference  in  the  final  results." 

In  order  to  lessen  the  effect  of  these  errors  Forbes  and  Pratt  have 
modified  the  method  as  follows: 

"Ground-glass  stoppered  bottles,  holding  approximately  480  cc., 
are  accurately  calibrated  by  weighing  completely  filled  with  water. 
The  bottle  is  filled  with  the  water  to  be  analyzed  by  means  of  a  syphon, 
the  glass  stopper  inserted,  leaving  no  air  bubbles,  and  the  neck  of 
the  bottle  wiped  dry.  The  glass  stopper  is  then  carefully  removed 
and  57  cc.  of  the  water  withdrawn  by  means  of  an  accurately  calibrated 
pipette,  in  order  to  make  room  for  the  reagents.  3  cc.  of  strong  barium 
chlorid  solution  (8  gms.  per  liter),  2  cc.  of  saturated  ammonium  chlorid 
solution,  and  50  cc.  of  standard  barium  hydroxid  are  then  introduced, 
the  bottle  quickly  stoppered,  well  shaken,  and  set  aside  to  settle." 

"There  is  now  in  the  bottle  an  air-space  of  2  cc.  which  is  left  to 
avoid  the  possibility  of  loss  of  liquid  when  the  stopper  is  inserted. 
After  the  precipitated  carbonate  have  completely  settled  out,  several 

N 
portions  of  100  cc.  are  syphoned  off  and  titrated  with  —  sulphuric 

N  5° 
acid.     The  barium  hydroxid  used  is  approximately  — .     The  figure 

obtained  by  averaging  several  results  of  titration  of  portions  of  100  cc. 
is  taken  as  the  true  value." 

The  use  of  this  large  quantity  of  water  and  the  titration  of  100  cc. 
portions  reduce  considerably  the  errors  due  to  the  difficulty  of  obtain- 
ing the  end-point  and  those  due  to  inaccuracies  of  measurement. 

The  decrease  in  the  concentration  of  the  barium  hydroxid  solution 
due  to  the  use  of  the  larger  quantity  of  water  makes  it  necessary  to 


CARBONIC  ACID  IN  NATURAL  WATERS  255 

allow  a  longer  time  for  the  precipitation  of  the  carbonates  Twelve 
to  sixteen  hours,  or  over  night,  is  usually  sufficient. 

Trillich  *  describes  a  radical  modification  of  the  Pettenkoffer 
method,  in  which  the  precipitation  of  magnesium  hydroxid  is  allowed 
to  take  place,  instead  of  being  prevented  by  the  addition  of  ammonium 
chlorid.  Then  from  a  direct  gravimetric  determination  of  the  amount 
of  magnesium  present  in  another  portion  of  the  sample,  the  proper 
correction  is  applied  to  the  result  obtained  volumetrically.  Since 
40  parts  of  MgO  will  react  with  as  much  barium  hydroxid  as  44  parts 
of  CC>2,  the  correction  is  attained  by  multiplying  each  part  of  MgO 
present  by  i.i  and  subtracting  the  product  from  the  apparent  amount 
of  carbonic  acid  found  by  the  volumetric  determination.  This  method 
differs  from  the  Pettenkoffer  method  in  that  5  cc.  of  barium  chlorid 
solution  are  used  instead  of  3  cc.,  omitting  ammonium  chlorid  and 
using  phenolphthalein  as  indicator  for  the  determination  of  free  and 
half-bound  carbonic  acid. 

It  is  recommended  to  use  a  barium  hydroxid  solution  in  which, 
to  each  9  gms.  of  barium  hydroxid  0.5  gm.  of  barium  chlorid  is  added, 
in  order  to  convert  the  sodium  or  potassium  hydroxids  which  are 
common  impurities  of  barium  hydroxid  into  chlorids  and  thus  obviate 
the  disturbing  effect  which  these  impurities  would  occasion. 

In  using  this  method  it  is  quite  evident  as  pointed  out  by  Ellms 
and  Beneker,  that  unless  the  precipitation  of  the  magnesium  by  barium 
hydroxid  is  complete,  an  error  is  introduced  into  the  correction.  A 
large  excess  of  barium  hydroxid  is  necessary  in  order  to  effect  com- 
plete precipitation. 

In  order  to  differentiate  between  free,  half-bound,  and  fixed  carbonic 
acid,  Trillich  uses  that  portion  of  his  solution  which  contains  the 
precipitated  carbonates  and  titrates  it  with  standard  hydrochloric  acid, 
using  cochineal  as  indicator.  This  gives  the  total  carbonic  acid. 

By  subtracting  the  free  and  half-bound  from  this  he  obtains  the 
fixed  carbonic  acid,  and  by  finding  the  difference  between  the  free 
and  half-bound  and  the  fixed,  he  estimates  the  free  carbonic  acid 
(assuming  the  half-bound  to  be  equivalent  to  the  fixed). 

Lunge-Trillich  or  Seyler  Method.f  This  method,  which  is 
looked  upon  as  the  best  volumetric  method  for  the  determination  of 
free  and  half-bound  carbonic  acid,  is  based  upon  the  assumption  that 
in  the  bicarbonates  of  the  alkali  earth  bases  there  is  one  molecule 
of  half-bound  carbonic  acid  for  each  molecule  of  fixed,  and  that  these 
bicarbonates  are  neutral  to  phenolphthalein. 

When  a  solution  containing  free  carbonic  acid  is  titrated  with 

*  Ztschr.  angew.  Chem.,  1889,  June  i5th. 

t  Chem.  News,  70-104  (1894),  Analyst,  22-312  (1897). 


256  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

sodium  carbonate,  using  phenolphthalein  as  indicator,  sodium  bicar- 
bonate is  formed, 

H2C03  +Na2C03  =2NaHC03, 

which  is  neutral  to  phenolphthalein.  As  soon  as  the  free  acid  is  all 
taken  up,  the  further  addition  of  sodium  carbonate  produces  a  pink 
color.  Thus  the  free  carbonic  acid  is  obtained  directly.  The  fixed 
carbonic  acid  may  then  be  determined  by  Hehner's  method,  and  from 
this  the  half-bound  is  obtained,  assuming  this  to  be  equal  to  the  fixed, 
in  waters  which  are  acid  to  phenolphthalein.  With  waters  that  are 
alkaline  to  phenolphthalein  the  determination  of  the  "  phenolphthalein 
alkalinity  "  is  first  made,  and  then  the  total  alkalinity  with  lacmoid 
or  methyl  orange  by  the  Hehner  process.  Twice  the  phenolphthalein 
alkalinity  subtracted  from  the  total  alkalinity  gives  the  half-bound 
carbonic  acid,  no  free  acid  being  present  in  this  case,  and  the  half- 
bound  being  less  than  the  fixed,  i.e.,  some  of  the  normal  carbonates 
are  held  in  solution  without  the  aid  of  any  bicarbonate. 

If  no  free  carbonic  acid  is  present  the  half-bound  may  equal  the 
fixed,  and  the  water  will  be  neutral  to  phenolphthalein. 

Thus  it  is  assumed  that  if  the  water  be  acid  or  neutral  to  phenol- 
phthalein there  is  present  for  each  molecule  of  fixed  carbonic  acid, 
one  molecule  of  half-bound  acid,  and  that  if  a  water  is  alkaline  to 
phenolphthalein  only,  one  half  of  the  carbonic  acid  in  the  form  of 
normal  carbonate  is  found  by  titrating  with  acid  in  the  presence  of 
phenolphthalein,  as  per  equation: 

2Na2C03  +H2S04  =  2NaHC03  +Na2SC>4. 

The  pink  color  produced  by  phenolphthalein  with  sodium  car- 
bonate is  destroyed  when  one  half  of  the  latter  becomes  saturated  with 
the  carbonic  acid  liberated  by  the  mineral  acid. 

The  carbonates  of  alkaline  earth  bases  act  in  a  similar  manner. 

2CaC03+H2S04=CaH2(C03)2+CaSO4. 

The  details  of  the  Seyler  process  are  as  follows : 

For  the  determination  of  free  carbonic  acid,  100  cc.  of  the  water 
are  introduced  into  a  tall  glass  cylinder  by  means  of  a  siphon,  several 
drops  of  a  neutral  alcoholic  solution  of  phenolphthalein  are  added, 

N 
and  the  solution  titrated  with  —  sodium  carbonate,  stirring  carefully 

5° 
and  thoroughly  until  a  faint  permanent  pink  color  is  obtained. 

For  the  determination  of  the  fixed  carbonic  acid  from  which  the 
half-bound  is  estimated,  Hehner's  method  is  used. 


LUNGE-TRILL1CH   OR  SEYLER  METHOD  257 

N 
The  same  or  a  fresh  portion  of  the  water  is  titrated  with  —  sulphuric 

acid,  using  methyl  orange  or  lacmoid  as  indicator.  Seyler  gives  a 
series  of  formulas  for  calculating  the  results  which  greatly  simplifies 
the  work. 

If  100  cc.  of  the  sample  are  operated  upon  and  the  standard  solu- 

N 

tions  are  —  ,  the  following  formulae  give  the  results  in  parts  per  million: 
5° 

I.  For  Waters  which  are  Acid  or  Neutral  to  Phenol  phthalein. 

Free  carbonic  acid  ....................  =4.4  p 

Fixed  or  half  -bound  carbonic  acid  ......  =4.4  m 

Volatile  carbonic  acid  .................  =4.4   (m  +p)    , 

Total  carbonic  acid  ...................  =4-4   (2  m 


N 
p=cc.  of  —  sodium  carbonate  required, 

N 
m=cc.  of  —  sulphuric  acid  required  in  the  presence  of  methyl  orange 

or  lacmoid. 

II.  For  Waters  which  are  Alkaline  to  Phenolphthalein. 
Fixed  carbonic  acid  .................    =4.4  m 

Half-bound  or  volatile  carbonic  acid—   =4.4  (m—2pr) 
Total  carbonic  acid  .................    =4.4  (2m  —  2p') 

N 
m=cc.  --  sulphuric  acid  required   in  the  presence  of  methyl  orange 

O 

or  lacmoid, 

N 
p'  =  cc.  —  sulphuric  acid  required  in  the  presence  of  phenolphthalein. 

Examples.     Waters  which  are  Acid  to  Phenolphthalein. 

N 

For  Free  Carbonic  Acid.     100  cc.  of  the  sample  titrated  with  — 

50 

sodium  carbonate  in  the  presence  of  phenolphthalein  require  4.5  cc.  of 
the  standard  solution,  therefore  4.4X4.5=19.85  parts  per  million. 

N 
For  Fixed  and  Half-bound  Carbonic  Acid.     100  cc.  titrated  with  — 

50 

sulphuric  acid  in  the  presence  of  methyl  orange  required  1.34  cc.  of 

standard  acid,  therefore  4.4X1.34=5.896  parts  per  million. 

The  Volatile  Carbonic  Acid  is  the  sum  of  the  above  two  results,  i.e., 
25.746  parts  per  million. 

The  Total  Carbonic  Acid  present  is  the  sum  of  the  fixed,  half-boundt 
andfree,  i.e.,  19.85+5.896+5.896=31.642  parts  per  million. 


258  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

For  Waters  which  are  Alkaline  to  Phenolphthalein. 

Fixed  Carbonic  Acid.  100  cc.  of  the  sample  titrated  with  — 

5o 

sulphuric  acid,  using  methyl  orange  as  indicator,  required  3.6  cc. 
Therefore  4.4X3.6=15.84  parts  per  million. 

Half-bound  or    Volatile   Carbonic  Acid.     100  cc.   of  the  sample 

N 
titrated  with  —  acid  in  the  presence  of  phenolphthalein  required  1.2  cc. 

Twice  this  amount  subtracted  from  3.6  cc.  and  then  multiplied  by 
4.4  will  give  5.28  parts  per  million. 

[For  further  details  of  this  method  see  Journal  of  the  American  Chemical 
Society,  J.  W.  Ellms,  XXI,  359;  Ellms  and  Beneker,  XXIII,  405;  and  Forbes 
and  Pratt,  XXV,  742.] 

Precautions: 

1.  If  the  water  contains  much  free  carbonic  acid  it  is  better  to  take 
less  than  100  cc. 

2.  The  stirring  should  not  be  too  vigorous,  otherwise  there  will 
be  a  loss  of  CC>2. 

3.  The  titration  should  be  quickly  completed  in  order  to  avoid 
absorption  of  CC>2  from  the  air. 

4.  The  sodium  carbonate  solution  should  be  made  with  water 
which  has  been  thoroughly  boiled  and  cooled  out  of  contact  with  the 
air,  and  the  finished  solution  should  not  be  exposed  any  more  than 
absolutely  necessary. 

Estimation  of  Small  Quantities  of  Carbon  Monoxid  in  the  Air; 
lodometrically.*  This  method  depends  upon  the  reaction  between 
carbon  monoxid  and  iodin  pentoxid  at  150°  C.  or  over,  attention  to 
which  was  first  called  by  Ditte  in  1870,1 


The  process  is  conducted  by  Kinnicut  and  Sanford  as  follows: 
Twenty-five  grams  of  iodin  pentoxid  are  placed  in  a  small  U-tube, 
which  is  suspended  in  an  oil  or  glycerol  bath  and  connected  with  a 
Wolff  blood-absorption  tube  containing  0.5  gm.  potasium  iodid  dis- 
solved in  5  cc.  of  water.  The  tube  containing  the  iodin  pentoxid  is 
also  connected  with  two  U-tubes,  one  containing  sulphuric  acid,  the 
other  small  pieces  of  potassium  hydroxid,  so  as  to  remove  from  the 
air  to  be  analyzed  (before  it  comes  in  contact  with  the  iodin  pentoxid) 
all  unsaturated  hydrocarbons,  hydrogen  sulphid,  sulphur  dioxid,  and 
other  reducing  gases  which  would  react  with  the  iodin  pentoxid. 

*  L.  P.  Kinnicut  and  G.  R.  Sanford.     J.A.C.S.,  XXII,  14. 
tBull.  Soc.  Chim.,  13,  318. 


CARBON  MONOXID  259 

The  oil  bath  in  which  the  U-tube  containing  the  iodin  pentoxid  is 
suspended  is  heated  to  150°  C.  (The  reaction  is  not  quantitative  at 
lower  temperatures.)  A  measured  quantity  of  air  (i  liter  is  a  con- 
venient quantity)  is  passed  through  the  apparatus  at  the  rate  of  about 
i  liter  in  two  hours,  the  rate  being  controlled  by  forcing  the  air  out 
of  the  container  by  means  of  mercury,  the  flow  of  which  is  regulated 
by  means  of  a  stop-cock.  The  temperature  and  barometric  pressure 
must  of  course  be  noted,  and  the  volume  of  air  analyzed  reduced  to 
o°  C.  and  760  mm.  pressure. 

The  titration  of  the  liberated  iodin  is  made  in  the  Wolff  blood- 

N 

absorption  apparatus  employing sodium  thiosulphate.    Nicloux, 

1000 

Compt.  rend.,  126,  746,  determined  the  iodin  set  free  by  the  depth 
of  color  formed  in  a  chloroform  solution.  Gautier,  Compt.  rend., 
126,  931,  determined  the  quantity  of  carbon  dioxid  formed. 

A  modification    of    this   process    is  described  by  Morgan    and 
McWhorter,  J.  A.  C.  S.,  xxix,  1589. 


CHAPTER  XVI 
CHLORIN,  BROMIN,  AND  IODIN 

FREE  chlorin  may  be  estimated  by  the  addition  of  potassium 
iodid,  and  titration  of  the  liberated  iodin  by  means  of  sodium  thio- 
sulphate,  as  described  on  page  209. 

The  same  method  may  be  employed  for  the  estimation  of  free 
bromin.  Free  iodin  is  estimated  by  direct  titration  with  sodium 
thiosulphate.  See  page  207. 

The  available  chlorin,  as  for  instance  that  existing  in  bleaching 
powder,  Javelle  water,  etc.,  is  estimated  by  liberating  the  chlorin  with 
an  acid,  and  then,  after  the  addition  of  potassium  iodid,  estimating  the 
liberated  iodin.  See  page  210. 

The  estimation  of  free  halogens  may  also  be  made  by  titration 
with  standard  arsenous  acid  solution,  as  described  on  page  228,  and 
by  means  of  the  same  standard  solution  the  available  chlorin  in 
bleaching  powder  and  other  hypochlorites  may  be  estimated.  By 
the  same  methods  the  chlorin  which  is  liberated  by  heating  certain 
substances,  as  for  instance  the  metallic  peroxids,  with  hydrochloric 
acid  may  be  accurately  estimated.  See  page  214. 

Gross,  J.  A.  C.  S.,  xxv,  989,  estimates  free  iodin  by  converting 

N 
it  into  zinc  iodid  and  titrating  with  —  silver  nitrate,  using  potassium 

chromate  as  indicator.  2  gms.  of  the  iodin  are  placed  in  a  flask, 
40  cc.  of  water,  and  about  4  gms.  of  shot-zinc  are  added.  The  flask 
is  then  shaken  and  allowed  to  stand  with  stopper  inserted  until  fluid 
is  colorless.  When  all  the  iodin  is  taken  up  by  the  zinc,  the  solution 
is  filtered  into  a  half  liter  flask,  the  residue  well  washed  with  hot  water 
until  free  from  iodid,  and  the  fluid  then  made  up  to  500  cc.  50  cc. 
of  the  solution  are  placed  in  a  porcelain  capsule  and  titrated  with 

—  silver  nitrate,  using  potassium  chromate  as  indicator,  until  end- 

10 

point,  a  slight  brownish  color,  is  reached. 

Chlorids,  Bromids,  and  lodids.  These  salts  are  most  readily 
estimated  by  titration  with  standard  silver  solution,  as  described 
under  analysis  by  precipitation  (page  no).  A  distillation  method  for 
alkali  iodid  is  described  on  page  220. 

260 


CHLORIDS,  BROMIDS,  AND   1ODIDS  261 

The  modified  Personne  method  in  which  alkali  iodids  are  titrated 
with  standard  mercuric  chlorid  solution,  is  described  on  page  134. 

These  salts  may  also  be  estimated  by  means  of  Volhard's  sulpho- 
cyanate  method.  See  page  122. 

In  the  case  of  chlorids,  however,  as  pointed  out  by  M.  A.  Rosanoff 
and  A.  E.  Hill,*  it  is  necessary  to  remove  the  precipitated  silver  chlorid 
by  filtration  before  beginning  titration  with  the  sulphocyanate  solution; 
this  is  because  ferric  sulphocyanate  is  decomposed  by  the  silver  chlorid 
according  to  the  following  equation: 

Fe(CNS)3  +3AgCl  =FeCl3  +3AgCNS. 

In  the  experiments  of  Rosanoff  and  Hill,  it  was  furthermore  demon- 
strated that  this  decomposition  is  a  rapid  one.  Forty-three  per  cent 
of  sulphocyanate  is  destroyed  by  only  an  equivalent  of  silver  chlorid 
in  two  minutes.  It  must  therefore  be  expected  that  a  large  excess 
of  silver  chlorid  (such  as  is  present  during  a  titration  by  Volhard's 
method)  will  destroy  practically  all  of  the  sulphocyanate  in  a  few 
seconds.  This  in  fact  is  the  case.  In  the  estimation  of  bromids  and 
iodids  it  is  unnecessary  to  filter  out  the  silver  precipitate. 

Chlorids,  Bromids,  and  Iodids  may  be  estimated  in  presence  of  each 
other  by  the  method  of  Benedict  and  Snell  (J.  A.  C.  S.,  xxv,  1138). 

This  method  is  based  upon  the  fact  that  potassium  iodate  (KIOs) 
will  liberate  iodin  from  iodids  upon  acidification  with  acetic  acid, 
and  bromin  from  bromids  on  acidification  with  dilute  nitric  acid. 

The  process  which  is  subjoined  includes: 

(1)  A  determination  of  total  halogens  by  titration  with  silver  nitrate 
or  any  of  the  usual  methods. 

(2)  A  determination  of  iodin. 

(3)  A  determination  of  chlorin;    the  bromin  being  estimated  by 
difference. 

For  the  determination  of  iodin  a  quantity  of  the  substance  con- 
taining not  over  0.5  gm.  of  iodin  or  0.15  gm.  of  chlorin,  is  dissolved 
in  water  and  made  up  to  50  cc.  in  a  100  cc.  cylinder  with  a  close  fitting 
glass  stopper.  Neutral  potassium  iodate  is  added  in  about  twice  the 
quantity  necessary  to  react  with  all  the  bromin  and  iodin  believed 
to  be  present.  The  mixed  solution  is  acidified  with  4  or  5  cc.  of  30 
per  cent  acetic  acid  and  shaken  with  30  to  40  cc.  of  carbon  disulphid 
until  all  the  liberated  iodin  has  been  taken  up  by  the  latter.  The 
aqueous  solution  is  now  separated  from  the  carbon  disulphid  layer 
by  filtration  through  a  wet  filter  and  the  carbon  disulphid  is  thoroughly 
washed  with  cold  water  on  the  filter.  The  filtrate  and  washings  are 
reserved  for  the  chlorin  determination.  The  carbon  disulphid  solu- 

*  J.  A.  C.  S.,  XXIX.  269. 


262  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

tion  is  transferred  to  another  beaker  by  puncturing  the  filter,  and  is 
covered  with  20  to  25  cc.  of  75  per  cent  alcohol;  any  carbon  disulphid 
left  adhering  to  the  filter  is  rinsed  into  the  beaker  with  a  portion  of 
the  75  per  cent  alcohol. 

The  iodin  is  now  titrated  with  sodium  thiosulphate  with  constant 
stirring.  No  starch  indicator  is  necessary. 

For  the  determination  of  the  chlorin,  the  aqueous  filtrate  from 
the  carbon  disulphid  is  treated  with  5  cc.  nitric  acid  (sp.gr.  1.18)  to 
liberate  the  bromin,  and  is  boiled  in  a  covered  beaker  until  colorless. 
The  excess  of  iodate  is  next  destroyed  by  adding  a  quantity  of  potas- 
sium iodid  slightly  in  excess  of  the  amount  necessary  to  react  with  it. 
The  solution  is  again  boiled  until  colorless,  2  or  3  cc.  more  of  dilute 
nitric  acid  being  added  if  the  color  is  not  completely  discharged  after 
ten  or  fifteen  minutes'  boiling.  A  minute  or  two  after  the  color  has 
completely  disappeared  the  solution  is  taken  from  the  flame,  cooled, 
and  neutralized  with  sodium  carbonate.  To  secure  exact  neutraliza- 
tion, a  little  calcium  carbonate  may  be  added  at  first  and  then  sodium 
carbonate  solution,  until  a  precipitate  just  forms.  The  chlorin  is 

N 

then  determined  by  titration  with  —  silver  nitrate,  using  potassium 

10 

chromate  as  indicator. 

Chlorates,  Bromates,  and    lodates.      These  may  be  estimated 

N 

by  titrating  with  —  silver  nitrate  solution  after  ignition.     They  are 
10 

reduced  by  heat  to  chlorids,  bromids,  and  iodids  respectively. 

(a)  KClO3+heat=KCl  +  O3. 

(b)  KC1  +  AgNO3=AgCl  +  KNO3. 
10)74.04     10)168.69 

7.404 gms.  16.869=1000  cc.  —  AgNO,  V.  S. 
10 

N 

Thus  each  cc.  of  the  —  AgNO3  V.  S.  represents  0.007404  gm.  of 
10 

KC1  =0.012168  gm.  KC1O3.  The  factor  is  TI^THF  tne  molecular 
weight  in  grams  of  any  univalent  chlorate,  bromate,  or  iodate  and 
wfarff  tnat  °f  bivalent  salts. 

Chlorates,  Bromates,  and  lodates  may  also  be  estimated  by  digestion 
with  excess  of  hydrochloric  acid  in  the  presence  of  potassium  iodid.  In 
each  case  the  liberated  halogen  acts  upon  the  potassium  iodid  and 
sets  free  an  equivalent  of  iodin,  the  amount  of  which  is  then  esti- 

N 

mated  by  means  of  —  sodium  thiosulphate. 
10 

(a)     KC103+6KI+6HC1  =  7KC1+3H20+I6. 

121.68  755-40 


TITRATIONS  WITH  STANDARD  POTASSIUM   IODATE     263 

(b)     I2  +  2(Na2S2O3  .  5H2O)  =  2Na2  +  Na2S4O6  +  ioH2O, 
2)251.8  2)492.92 

10)125-9          10)246.46 

12.59  gms.         24.646  gms.  or  1000  cc.  —  V.  S. 

10 

N 

Each  cc.  of  —  Na2S2O3  =0.01259  gm-  °f  iodin  =0.0020428  gm.  of 
10 

KC103. 

This  method  is  more  fully  described  on  page  222. 

By  Reduction  with  Hydroxylamin  Sulphate.  Fritz  Weber, 
in  Pharm.  Ztg.,  1906,  364,  employs  hydroxylamin  sulphate  as  a 
reducing  agent  for  the  quantitative  estimation  of  chlorates,  bromates, 
and  iodates,  the  resulting  chlorid,  bromid,  and  iodid  being  titrated 
with  standard  silver  nitrate.  For  the  estimation  of  chlorate: 

Dissolve  i  gm.  of  the  chlorate  in  200  cc.  of  water,  add  20  gms.  of 
hydroxylamin  sulphate,  acidulate  with  nitric  acid,  warm,  and  titrate 
with  standard  silver  nitrate. 

For  the  estimation  of  bromate: 

Dissolve  i  gm.  of  the  bromate  in  200  cc.  of  water,  supersaturate 
with  ammonia  water,  add  20  gms.  of  hydroxylamin  sulphate,  acidu- 
late as  above  with  nitric  acid,  and  titrate  with  standard  silver  nitrate. 

For  the  estimation  of  iodate; 

Dissolve  i  gm.  of  the  iodate  in  100  cc.  of  water,  supersaturate  with 
ammonia  water,  add  10  gms.  of  hydroxylamin  sulphate,  and  proceed 
as  above  described  for  bromate.  If  any  iodin  is  liberated,  a  little 
sulphurous  acid  is  added,  and  the  solution  again  acidulated  with  nitric 
acid. 

Titrations  with  Standard  Potassium  Iodate  (KIO3).  According 
to  L.  W.  Andrews,  J  A.  C.  S.,  xxv,  756,  potassium  iodate  may  be 
employed  as  a  standard  for  titrating  iodids,  chlorates,  and  free  iodin, 
as  well  as  arsenous,  antimonous,  and  ferrous  compounds,  in  fact  for 
the  estimation  "of  almost  all  the  substances  to  which  Bunsen's  process 
of  distillation  with  potassium  iodid  and  hydrochloric  acid  is  applica- 
ble, with  at  least  equal  precision  and  far  simpler  apparatus." 

When  potassium  iodate  is  added  to  a  solution  of  an  iodid,  in  the 
presence  of  a  small  quantity  of  acid,  iodin  is  set  free,  as  per  equation: 

5KI +KI03  +6HC1  =6KC1  +3I2  +3H2O. 

If,  however,  a  larger  quantity  of  acid  is  present,  iodin  chlorid  (IC1) 
forms  as 

2KI +KI03  +6HC1  =3KC1  +3IC1 +3H2O. 

If  chloroform  or  carbon  tetrachlorid  is  used  as  an  indicator,  the 
immiscible  solvent  remains  violet  in  the  first  case,  but  in  the  latter 


264  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

becomes  colorless,  while  the  supernatant  solution  becomes  bright 
yellow  from  the  presence  of  the  iodin  chlorid. 

This  behavior  is  explained  as  follows: 

Iodin  chlorid  being  the  salt  of  a  very  weak  base,  undergoes  hydroly- 
sis in  a  neutral  or  feebly  acid  solution  with  the  production  of  hypo- 
iodous  acid  (HIO)  and  hydrochloric  acid. 

IC1+H20=HIO+HC1. 

The  hypoiodous  acid  undergoes  spontaneous  conversion  into  iodic 
acid,  whereas  in  the  presence  of  a  great  excess  of  acid  this  hydrolysis 
is  prevented. 

The  Process.  Titration  of  lodid.  0.2  gm.  of  potassium  iodid  are 
dissolved  in  20  cc.  of  water  in  a  glass  -stoppered  bottle  of  250  cc.  capacity, 
5  cc.  of  chloroform,  and  30  cc.  of  concentrated  hydrochloric  (sp.gr. 
1.2  1  )  are  added  and  the  mixture  titrated  with  standard  potassium 
iodate  (10.62  gms.  in  the  liter)  shaking  briskly  until  the  chloroform 
loses  its  color.  The  end-reaction  is  exceedingly  sharp.  Each  cc.  of 
the  standard  iodate  =0.016476  gm.  of  potassium  iodid. 

2KI     +    KI03  +  6HC1  =  3KC1  +  3IC1  +  3H2O. 

2)329.52        2)212.4 
10)164.76       10)106.2 

16.476  gms.  =  10.62  gms.=  1000  cc.  —  V.  S. 

10 

If  a  standard  solution  of  acid  potassium  iodate  is  used  under  the 
above  conditions,  that  is,  with  a  great  excess  of  acid,  the  reaction  is 
probably  as  follows: 


4KI  +  KH(IO3)2  +  i2HCl=6ICl+5KCl+HCl+6H2O. 

4)65.904      4)386.94 
10)164.76        10)96.735 

16.476  gms.      9.6735  gms.  or  1000  cc.  standard  solution. 

Titration  of  Free  Iodin  0.3  gm.  of  the  iodin  is  dissolved  in 
10  cc.  of  a  solution  of  potassium  iodid  which  is  checked  against  the 
standard  iodate  solution.  10  cc.  of  hydrochloric  acid  and  5  cc.  of 
chloroform  are  added  and  the  mixture  titrated  with  the  standard 
iodate,  as  above  described.  If  the  potassium  iodid  solution  used 
contains  16.476  gms.  per  liter,  then  10  cc.  will  take  up  10  cc.  of  the 
standard  iodate  solution.  Therefore  10  cc.  deducted  from  the  quan- 
tity of  the  latter  required,  gives  the  quantity  which  reacted  with  the 
free  iodin.  Each  cc.  represents  0.02518  gm.  of  free  iodin. 

Titration  of  Chlorates.  To  the  solution  of  the  chlorate  add  an 
exactly  known  amount  of  pure  potassium  iodid  in  a  glass-stoppered 


DIRECT  TITRATION   WITH   CHLORIN   WATER  265 

bottle  and  an  amount  of  fuming,  pure  hydrochloric  acid  at  least  one- 
third  greater  than  the  volume  of  the  solution.  Close  the  bottle  tightly 
and  allow  it  to  stand  fifteen  minutes  after  shaking,  then  add  5  cc.  of 
chloroform.  On  now  shaking,  the  chloroform  must  become  deep 
violet.  (If  the  color  is  pale  an  insufficiency  of  iodid  has  been  added, 
and  it  is  advisable  to  begin  again.)  Then  add  the  decinomal  iodate 
with  intermittent  shaking  until  the  chloroform  becomes  colorless, 
which  point  may  be  estimated  with  the  utmost  precision.  Each  cc.  of 

N 

—  iodate  solution  is  equivalent  to  0.00268  gm.  of  ClOa. 

10 

The  estimation  of  Mates*  bromates,^  and  hypochlorites  ,%  may 
be  made  by  the  use  of  hydrazin  (diamid)  which  is  a  very  powerful 
reducing  agent.  The  halogen  in  each  case  is  reduced  to  the  halid 
form,  while  the  nitrogen  of  the  hydrazin  is  set  free.  The  halid  may 
then  be  titrated  with  standard  silver  nitrate,  or  if  preferred  the  nitrogen 
may  be  measured. 

lodids  may  also  be  estimated  by  means  of  standard  potassium 
bi-iodate,  in  faintly  acid  solutions  with  very  satisfactory  results  (see 
standardization  of  sodium  thiosulphate  solution),  or  by  means  of  potas- 
sium dichromate  (also  see  standardization  of  sodium  thiosulphate 
solution),  or  by  means  of  potassium  permanganate  (see  iodometric 
standardization  of  permanganate). 

Estimation  of  Bromids  or  lodids  by  Direct  Titration  with 
Chlorin  Water.  If  for  any  reason  the  determination  of  these  salts 
cannot  conveniently  or  satisfactorily  be  made  by  titration  with  standard 
silver  nitrate,  the  much  less  accurate  method  of  titration  with  chlorin 
water  may  be  employed. 

This  method  depends  upon  determining  the  quantity  of  chlorin 
water  of  known  strength  required  to  convert  the  bromin  into  BrCl. 
If  to  an  aqueous  solution  of  a  bromid  a  small  quantity  of  chlorin 
water  is  added,  bromin  is  set  free. 

2KBr+Cl2=2KCl+Br2. 

If  a  larger  quantity  of  chlorin  water  is  added  the  bromin  is  con- 
verted into  bromin  monochlorid,  BrCl. 

KBr  +  Cl2=KCl+BrCl. 

The  addition  of  a  still  larger  quantity  of  chlorin  water  will  form 
BrCl5. 


*  Riegler,  Z.  anal.  Chem.,  42,  677. 

t  Schlotter,  Z.  anorg.  Chem.,  37,  164. 

%  Roberts  and  Rancoli,  Chem.  Centrlbl.,  1904,  i,  1294. 


266  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

which  is  finally  converted  into  bromic  acid. 

BrCl5  +3H20  =HBrO3  +5HC1. 

The  bromid  in  solution  is  treated  with  chlorin  water  in  the  pres- 
ence of  chloroform  (carbon  disulphid  or  carbon  tetrachlorid  may  also 
be  used).  The  chloroform  takes  up  the  liberated  bromin  and  becomes 
yellow  or  dark  brown  in  color  according  to  the  quantity  of  bromin  dis- 
solved, but  as  more  chlorin  water  is  added  the  color  of  the  chloroform 
solution  becomes  paler  through  conversion  of  the  bromin  into  bromin 
chlorid  (BrCl),  and  finally,  when  a  sufficient  excess  has  been  added, 
the  solvent  becomes  quite  colorless.  The  point  at  which  all  of  the 
bromin  is  converted  into  BrCl  is  easily  recognized  after  a  little  prac- 
tice by  the  solvent  becoming  yellowish  white.  This  indication  is 
fairly  sharp  if  the  vessel  in  which  the  titration  is  performed  is  placed 
upon  a  white  surface.  The  complete  decolorization  of  the  solvent 
cannot  be  utilized  as  the  end-reaction,  as  this  decolorization  does  not 
occur  until  the  chlorin  water  has  been  added  in  decided  excess. 

The  method  is  carried  out  according  to  Reimann,  as  follows: 

N 

The  titer  of  the  chlorin  water  as  compared  to  sodium  thio- 

100 

sulphate  is  first  found.  Then  knowing  how  many  cubic  centimeters 
of  the  sodium  thiosulphate  solution  correspond  to  a  given  volume  of 
the  chlorin  water  it  is  easy  to  calculate  how  much  bromin  the  same 
volume  of  chlorin  water  will  convert  into  BrCl. 

Into  a  flask  provided  with  a  well-fitting  ground-glass  stopper  a 
weighed  quantity  of  the  bromid  to  be  analyzed  is  introduced,  dissolved 
in  water,  a  small  quantity  of  chloroform  added,  and  the  mixture  titrated 
slowly  with  the  chlorin  water,  closing  the  flask  and  shaking  briskly 
after  each  addition,  and  observing  the  color  of  the  chloroform  layer;  as 
soon  as  this  is  pale  yellowish-white,  the  delivery  of  the  chlorin  water 
is  stopped,  and  the  quantity  used  noted,  and  the  calculation  made. 

By  reference  to  the  equation  it  is  seen  that  one  molecule  of  KBr 
requires  two  atoms  of  chlorin  to  transform  the  bromin  into  BrCl, 

N 
hence  one  cc.  of sodium  thiosulphate  represents  in  this  case,  not 

0.0007936  gm.  of  bromin,  but  rather  one  half  of  this  quantity,  which 
is  0.0003968  gm.  and  hence  likewise  0.0005911  gm.  of  KBr,  or  0.0005112 
gm.  of  NaBr. 

Example.     To  10  cc.  of  chlorin  water,  i  gm.  of  potassium  iodid 

N 

is  added,  and  the  solution  titrated  with  —  thiosulphate  in  the  usual 

10 

manner.     12  cc.  are  required.     Thus  the  10  cc.  of  chlorin  water  are 


DIRECT   TIT  RAT  ION  WITH   CHLORIN  WATER  267 

N 
equivalent  to  120  cc.  of  sodium  thiosulphate.     0.21  gm.  of  the 

potassium  bromid  to  be  analyzed  is  now  dissolved  in  10  cc.  of  water, 
2  cc.  of  chloroform  added,  and  the  solution  titrated  with  the  above 
chlorin  water.  28.6  cc.  are  required.  If  10  cc.  of  chlorin  water 

N 
are  equivalent  to  12  cc.  of  —  thiosulphate,  28.6  cc.  are  equivalent  to 

N 

34.3  cc.,  or  343  cc.  of sodium  thiosulphate,  and  since  each  cc.  of 

100 

the  latter  represent  0.0005911  gm.  of  pure  KBr  the  0.21  gm.  taken 
contain  0.0005911  gm.  X343  =0.202747  gm.  of  pure  KBr,  or  96.54  per 
cent. 

The  estimation  of  iodids  *  by  this  method  is  similar  to  that  of 
bromids,  the  reaction  with  chlorin  being  the  same. 

The  first  addition  of  chlorin  water  liberates  iodin,  then  it  reacts 
with  iodin  to  form  IC1,  then  IC15  and  finally,  HIO3. 

Free  iodin  and  iodin  monochlorid  (IC1)  dissolve  in  chloroform 
or  carbon  disulphid,  forming  a  violet  to  blue  solution,  according  to 
the  quantity  of  the  substance  present.  While  iodin  pentachlorid 
(IC15)  or  HIO3  on  the  other  hand  form  colorless  solutions.  Therefore 
in  titrating  with  chlorin  the  disappearance  of  the  violet  color  may  be 
taken  as  the  end-reaction  and  the  quantities  of  chlorin  used  as  the 
measure  of  the  iodin  which  was  converted  into  IC15.  The  reaction  is 

KI+6C1=IC15  +  KC1. 

Thus  it  is  seen  that  one  atom  of  iodin  or  one  molecule  of  potas- 
sium iodid  is  the  equivalent  of  six  atoms  of  chlorin. 

Therefore  if  the  quantity  of  chlorin  water  used  in  the  titration  is 

N 

calculated  into  cubic  centimeters  of sodium  thiosulphate,  as  directed 

100  'N 

under  Estimation  of  Bromids,  by  this  method,  each  cc.  of  —  sodium 

100 

thiosulphate  used  will  represent  in  this  case 

0.0002098    gm.  of  I; 
0.00027445    "     "  KI; 
0.00024797    "     "  Nal. 

The  presence  of  hydrochloric,  or  rather  of  a  large  excess  of  hydro- 
chloric acid,  will  materially  alter  the  reaction  in  this  case.  A  much 
smaller  quantity  of  chlorin  water  will  be  required.  Andrews,  J.  A.  C.  S., 
xxv,  757. 

*Dupre,  Ann.  Chem.  (Liebig),  94,  365,  1855. 


268  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  Determination  of  Chlorids  or  Bromids  in  the  Presence 
of  Sulphocyanate.  The  method  proposed  by  Rosanoff  and 
Hill,  J.  A.  C.  S.,  xxix,  1467,  depends  upon  oxidizing  the  sulpho- 
cyanate  to  hydrocyanic  acid  by  means  of  nitric  acid,  expelling  nearly 
all  of  the  hydrocyanic  acid,  and  determining  the  residual  chlorid  or 
bromid,  in  the  presence  of  the  traces  of  hydrocyanic  acid  by  the  Vol- 
hard  method.  The  sulphocyanate  originally  present  is  found  by 
difference.  The  sum  of  chlorid  and  sulphocyanate,  or  of  bromid  and 
sulphocyanate,  is  established  by  precipitation  with  excess  of  standard 
silver  nitrate  V.  S.  and  determination  of  the  excess  by  titration  with 
standard  ammonium  sulphocyanate  V.  S.  Precipitates  containing 
silver  chlorid  must  be  filtered  off  before  the  excess  of  silver  nitrate  is 
determined,  while  in  the  case  of  the  bromid  filtration  is  unnecessary. 

The  method  in  detail  is  as  follows: 

"  Determination  of  Chlorids.  The  following  solutions  are  required ; 
(a)  a  fifteenth -normal  solution  of  silver  nitrate,  standardized  gravi- 
metrically  or  by  weight  of  silver  nitrate  used;  (b)  a  fifteenth-normal 
solution  of  ammonium  sulphocyanate,  standardized  against  the  silver 
nitrate  by  Volhard's  method;  (c)  a  saturated  solution  of  iron-ammo- 
nium alum,  free  from  chlorids  and  treated  with  nitric  acid  to  lighten 
the  color. 

"A  measured  volume  of  the  chlorid-sulphocyanate  mixture  is 
diluted  so  that  the  normality  of  the  chlorid  is  about  one-fiftieth.  The 
solution  is  treated  with  i  cc.  of  the  iron-ammonium  alum  and  brought 
to  a  boil  in  a  large  Erlenmeyer  flask.  To  the  gently  boiling  solution 
concentrated  nitric  acid  is  added  at  the  rate  of  about  three  drops  a 
minute,  during  which  operation  the  liquid  should  be  frequently  stirred 
to  prevent  superebullition.  The  addition  of  the  acid  in  the  above 
described  manner  is  continued  until  the  color  of  the  solution  has  faded 
to  a  pale  orange,  and  then  without  further  addition  of  acid  the  solution 
is  allowed  tojx>il  for  a  few  minutes  longer,  by  which  treatment  the 
last  traces  of  sulphocyanate  are  oxidized.  The  complete  oxidation 
requires  about  twenty  minutes  time;  a  change  of  color  to  pure  yellow 
indicates  the  completion  of  the  operation.  The  odor  of  hydrocyanic 
acid  is  now  practically  gone. 

"  Cool  the  solution  thoroughly  under  the  tap  and  add  to  it  one  third 
its  volume  of  concentrated  nitric  acid.  Add  a  measured  amount  of 

N 
silver  nitrate  in  moderate  excess  above  the  quantity  required  to 

precipitate  the  chlorid  and  shake  the  mixture  until  the  precipitate  is 
well  coagulated,  which  requires  but  a  few  minutes'  agitation  when 
the  solution  is  cool,  but  a  much  longer  time  if  the  solution  is  heated. 
When  the  precipitate  is  well  coagulated  bring  the  solution  again  to  a 


DETERMINATION  OF   BROMIDS  269 

gentle  boil,  maintaining  it  at  this  temperature  for  about  five  minutes. 
The  liquid  above  the  precipitate  must  become  perfectly  clear. 

"Filter  the  solution  while  still  hot  through  a  rapid-running  double 
filter,  paying  no  attention  to  the  opalescence  which  forms  in  the  filtrate 
upon  cooling.  Wash  the  precipitate  with  hot  water  containing  a  few 
drops  of  nitric  acid.  Allow  the  liquid  to  cool  thoroughly,  dilute  until 
the  volume  is  about  three  times  that  of  the  solution  before  filtration, 
add  2  cc.  of  the  iron  indicator  for  every  100  cc.  of  solution  and  run 
in  the  standardized  sulphocyanate  to  a  strong  red  color.  Stir  the 
solution  for  five  minutes  and  then  add  the  standardized  silver  nitrate 
drop  by  drop,  with  stirring,  until  the  color  just  disappears.  A  single 
drop  of  the  sulphocyanate  solution  should  then  .give  a  permanent 
pink  color;  should  it  fail  to  do  so,  titrate  back  and  forth  with  the 
silver  nitrate  and  ammonium  sulphocyanate  until  the  color  changes 
on  addition  of  a  single  drop.  The  titration  is  best  conducted  in  a 
porcelain  dish,  against  the  white  ground  of  which  the  end-point  is 
most  accurately  determined;  in  glassware  transmitted  light  effects 
may  be  misleading.  The  total  silver  nitrate  added,  minus  the  total 
ammonium  sulphocyanate  added,  is  the  silver  nitrate  equivalent  of 
the  chlorid  present. 

"  Determination  of  Bromids.  The  analysis  of  bromid-sulphocyan- 
ate  mixtures  is  conducted  precisely  as  that  of  the  chlorid  mixtures,  with 
three  modifications:  (a)  the  solution  to  be  treated  with  nitric  acid 
for  the  destruction  of  the  sulphocyanates  should  be  diluted  to  roughly 
one-hundredth  normality  with  respect  to  bromids;  (b)  after  the  sulpho- 
cyanate has  been  decomposed  at  the  boiling  temperature  and  the 
liquid  thoroughly  cooled,  only  one  sixth  its  volume  of  concentrated 
nitric  acid  is  added;  (c)  the  precipitated  silver  salts  should  not  be 
filtered  off  before  the  excess  silver  nitrate  is  determined." 

This  method  depends  upon  the  following  assumptions:  (i)  that 
sulphocyanates  may  be  decomposed  at  a  temperature  of  about  100°  C. 
by  such  concentrations  of  nitric  acid  as  are  insufficient  either  to  oxidize 
chlorids  or  bromids  or  to  cause  the  volatilization  of  hydrochloric  or 
hydrobromic  acid;  (2)  that  all  but  traces  of  the  hydrocyanic  acid 
formed  by  this  decomposition  can  be  volatilized  during  the  time  required 
for  the  oxidation  itself;  (3)  that  the  silver  cyanid  formed  is  sufficiently 
soluble  in  hot  nitric  acid  to  be  separated  quantitatively  from  the  silver 
chlorid  by  filtration;  (4)  that  small  amounts  of  silver  cyanid  do  not 
interfere  with  the  determination  of  the  excess  of  silver  by  means  of  a 
sulphocyanate  titration  since  it  acts  as  a  soluble  silver  salt. 


CHAPTER  XVII 
CITRIC  ACID  AND  CITRATES 

FREE  citric  acid  may  be  estimated  by  titration  with  standard  sodium 
hydroxid  solution  in  the  presence  of  phenlophthalein  as  indicator. 
Citrates  of  potassium  sodium  and  lithium  may  be  estimated  as  directed 
under  Estimation  of  Organic  Salts  of  the  Alkalies,  page  83.  The 
citrate  being  converted  into  a  carbonate,  and  then  titrated  with  standard 
acid,  using  methyl  orange  as  indicator. 

Citrates  of  the  Alkalies  and  Earths  may  be  estimated  by  treating 
with  a  solution  of  lead  nitrate  or  acetate.  The  resulting  precipitate 
of  lead  citrate  is  then  washed  with  a  mixture  of  equal  parts  of  alcohol 
and  water,  and  then  suspended  in  water  and  treated  with  H2S  gas 
until  all  the  lead  is  precipitated  as  sulphid.  The  lead  sulphid  is  sepa- 
rated and  the  clear  solution  boiled  to  expel  H2S,  and  then  titrated  with 
normal  alkali  solution. 

Each  cc.  of  the  latter  used  =0.070  gm.  of  crystallized  citric  acid* 

This  method  may  be  employed  for  estimating  solutions  of  citrate 
of  magnesia. 

Lime-juice  or  Lemon-juice,  the  chief  constituent   of  which  is 

N 

citric  acid,  may  be  estimated  by  titrating  with  —  potassium  hydroxid 

10 

in  the  same  manner  as  other  acid  solutions. 

Lime-juice  contains  on  an  average  7.84  per  cent,  rarely  as  much 
as  10  per  cent,  and  very  seldom  as  little  as  7  per  cent,  of  citric  acid. 

Commercial  lime-juice  frequently  contains  sulphuric,  hydrochloric, 
or  tartaric  acid.  Therefore,  before  applying  this  test,  the  absence  of 
notable  quantities  of  these  acids  must  be  insured  by  qualitative  tests. 

W arringtorf s  Method  (Jour.  Chem,  Soc.,  1875,  934).  20  cc,  of 
ordinary  juice  or  4  cc.  of  concentrated  juice  are  neutralized  with  normal 
sodium  hydroxid  solution  and  diluted  to  about  50  cc.  The  mixture  is 
heated  to  boiling  and  a  small  excess  of  calcium  chlorid  solution  added. 
The  boiling  is  continued  for  about  half  an  hour,  the  precipitate  col- 
lected on  a  filter  and  washed  with  hot  water. 

The  filtrate  and  washings  are  mixed  and  concentrated  to  about 
15  cc.;  a  few  drops  of  ammonia-water  are  added  and  the  precipitate 
produced,  collected  separately  and  washed. 

270 


CITRIC  ACID  AND   CITRATES  271 

Both  filters  and  their  precipitates  are  then  dried  and  incinerated 

N 
at  a  low  red  heat  and  the  residue  titrated  with  —  hydrochloric  acid. 

The  process  depends  upon  the  formation  of  sodium  citrate,  which 
is  precipitated  as  calcium  citrate.  This  is  converted  by  ignition  into 
carbonate,  which  is  finally  titrated  with  the  normal  acid. 

Each  cc.  represents  0.0695  gm.  of 


CHAPTER  XVIII 

CYANOGEN  AND  ITS  COMPOUNDS 

THE  estimation  of  cyanogen  in  the  form  of  alkali  cyanids  or  hydro- 
cyanic acid  may  be  effected  in  a  number  of  ways.  The  most  satis- 
factory and  simplest  are  those  depending  upon  the  reaction  with  silver 
nitrate.  The  method  of  Liebig  (see  page  125),  in  which  standard 
silver  nitrate  solution  is  added,  until  the  first  appearance  of  a  perma- 
nent precipitate  of  silver  cyan  id,  is  one  of  the  oldest  and  best  known, 
and  the  results  obtained  with  it  are  quire  accurate  in  the  absence  of 
certain  impurities.  The  cyanogen  in  mercuric  cyanid  in  cyanogen 
bromid,  and  in  the  double  cyanids  of  silver,  gold,  nickel,  cobalt,  iron, 
copper,  zinc,  and  a  few  other  metals  cannot  be  estimated  by  this  method. 

The  method  of  Vielhaber  (see  page  127),  using  chromate  as  an 
indicator,  and  titrating  with  silver  nitrate,  is  very  popular,  has  a  well- 
defined  end-reaction,  and  is  quite  accurate,  but  like  the  foregoing  is 
useless  in  the  presence  of  certain  metallic  impurities. 

The  Denige-Sharwood  method  in  which  potassium  iodid  and 
ammonia  are  used  as  indicators  (see  page  128)  is  equally  if  not  more 
accurate  than  the  before-mentioned  methods,  and  has  the  advantage 
that  it  can  be  employed  in  the  presence  of  many  of  the  impurities 
which  interfere  with  the  working  of  the  others.  Sharwood  Q.  A.  C.  S., 
1897,  400),  in  an  extensive  series  of  well-conducted  experiments,  com- 
paring this  method  with  Liebig's,  shows  the  special  applicability  of 
the  method,  for  spent  cyanid  solutions,  etc.,  in  the  cyanid  method  of 
gold  extraction,  and  proves  the  superiority  of  the  method  over  Liebig's 
where  the  cyanid  solutions  tested  are  impure.  With  pure  solutions 
the  results  are  practically  identical.  The  standard  solution,  he  says, 
should  not  be  over  twentieth  normal.  See  also  Engineering  and  Mining 
Journal,  1898,  216. 

In  Volhard's  method,  an  excess  of  standard  silver  nitrate  solution 
is  used,  and  the  excess  determined  by  residual  titration  with  sulpho- 
cyanate  solution,  using  ferric  alum  as  an  indicator.  This  method 
can  be  made  to  give  reliable  results.  Its  disadvantage  over  the  others 
is  that  the  precipitated  silver  cyanid  and  the  sulphocyanate  react  upon 
one  another  during  the  titration,  and  hence  make  it  necessary  to  remove 
the  former  by  filtration  before  titrating  back  with  the  sulphocyanate 

272 


CYANOGEN  AND   ITS  COMPOUNDS  273 

solution.  This  is  a  decided  inconvenience,  and  hence  the  foregoing 
methods  are  usually  preferred.  The  irregularities  caused  by  the 
reaction  between  silver  cyanid  and  sulphocyanate  may  to  some  extent 
be  avoided  by  using  a  very  weak  sulphocyanate  solution,  which  less 
readily  dissolves  the  silver  cyanid. 

A  weighed  quantity  of  the  cyanid  is  treated  with  a  measured 
excess  of  decinormal  silver  nitrate  solution  in  order  to  unite  the  cyanogen 
entirely  with  the  silver  as  silver  cyanid;  the  mixture  is  then  made  up 
to  a  certain  volume  by  the  addition  of  distilled  water,  thoroughly 
shaken  and  a  portion  filtered  through  a  dry  filter.  An  aliquot  portion 
of  the  whole  solution  is  then  removed  by  means  of  a  pipette,  some 
ammonio-ferric  sulphate  solution  added,  then  strongly  acidulated 

N 

with  nitric  acid,  and  titrated  for  the  excess  of  silver  nitrate  with  — 

10 

potassium  sulphocyanate  solution.  The  quantity  of  silver  being  found 
in  the  aliquot  portion,  it  is  multiplied  by  the  proper  figure,  and  the 
quantity  of  silver  in  the  entire  solution  is  ascertained.  This  deducted 
from  the  quantity  originally  added  gives  the  quantity  which  went 
into  combination  with  the  cyanogen. 

N 

Each  cc.  of  —  AgNO3  V.  $.=0.002584  gm.  CN". 
10 

=0.002684  i;  HCN. 

=0.006470  "  KCN. 

=0.004872  lt  NaCN. 

=0.012509  H  Hg(CN)3. 

This  is  essentially  the  method  of  Volhard,  described  on  page  122. 
The  end-reaction  is  known  by  the  appearance  of  a  red  color.  In  esti- 
mating cyan  ids  by  this  method  the  above-described  procedure  must 
be  carefully  followed. 

The  following  precautions  must  be  taken,  which  are  not  necessary 
when  the  halogens  are  estimated  by  this  method,  namely: 

The  cyanogen  must  be  completely  combined  with  the  silver  and 
under  no  circumstances  must  the  solution  be  acidulated  before  the 
silver  solution  is  added,  otherwise  the  cyanid  will  be  converted  into 
hydrocyanic  acid,  as  the  equation  shows: 

KCN  +HNO3  =  KNO3  +  HCN. 

This  would  not  only  cause  a  loss  by  volatilization,  but  would  seri- 
ously endanger  the  health  or  even  the  life  of  the  analyst. 


274  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Furthermore,  the  sulphocyanate  solution  must  not  be  added  to 
the  solution  which  contains  the  silver  cyanid  in  suspension,  as  is  done 
in  the  case  of  the  halogens,  because  the  sulphocyanate  will  react  with 
silver  cyanid,  which  is  not  the  case  with  the  haloid  salts  of  silver. 
Hence  it  is  directed  to  filter  the  liquid  and  operate  upon  an  aliquot 
portion. 

In  the  estimation  of  solutions  containing  free  HCN,  such  as  bitter- 
almond  water,  a  weighed  quantity  of  the  latter  is  poured  directly  into 
the  silver  nitrate  solution  in  order  to  lose  as  little  as  possible  of  the 
hydrocyanic  acid. 

Cyan  ids  insoluble  in  water  are  treated  with  an  acid  in  order  to 
set  free  the  hydrocyanic  acid,  which  is  distilled  over  and  received  in  a 
solution  of  potassa. 

Titration  with  Standard  lodin  Solution  (Fordos  and  Gelis.*) 
This  method  is  based  upon  the  reaction  of  free  iodin  on  potassium 
cyanid,  which  is  as  follows: 

KCN+2l=KI+ICN. 

Two  equivalents  of  iodin  correspond  to  one  equivalent  of  cyanogen. 

The  standard  iodin  solution  is  added  to  the  cyanid  solution, 
until  it  ceases  to  lose  its  color.  The  end-reaction  is  known  by  the 
solution  becoming  yellow.  If  free  hydrocyanic  acid  is  to  be  deter- 
mined, the  latter  should  first  be  made  alkaline  by  the  cautious  addi- 
tion of  sodium  or  potassium  hydroxid,  any  excess  of  alkali  hydroxid 
is  then  converted  into  bicarbonate  by  the  addition  of  carbonic  acid 
water.  Hydroxid,  and  even  normal  carbonate,  will  decolorize  iodin. 
Pure  alkali  cyanids  do  not  need  this  preparation  for  analysis — they 
are  simply  dissolved  in  water  and  titrated  with  standard  iodin.  Com- 
mercial cyanids  are,  however,  frequently  contaminated  with  alkali 
hydroxid  or  carbonate,  and  must  therefore  be  treated  with  carbonic 
acid  water  as  described. 

Sulphids  must  be  absent,  as  this  will  vitiate  the  results. 

The  Process.  Five  grams  of  potassium  cyanid  accurately  weighed 
are  dissolved  in  water  to  make  500  cc.  Of  this  solution,  10  cc.  (rep- 
resenting o.i  gm.  of  the  cyanid)  are  removed  for  analysis,  about  250  cc. 
of  water  are  added  and  then  100  cc.  of  carbonic  acid  water,  the  solu- 

N 

tion  shaken  and  then  titrated  with  —  iodin  to  a  slight  but  perma- 
nent yellow  color.  Guerin  suggests  the  employment  of  borax,  instead 
of  alkali  hydroxid,  before  titration  of  hydrocyanic  acid  by  this  method 


*  Jour,  de  chim.  et  de  Pharm.,  XXIII,  48;   Jour.  f.  prakt.  Chem.,  LIX,  255. 


CYANOGEN   AND   ITS  COMPOUNDS  275 

or  by  that  of  Liebig.     10  cc.  of  the  dilute  acid  are  treated  with  10  cc. 
of  3  per  cent  borax  solution. 

2HCN  +Na2B4O7  =H2B4O7  +2NaCN. 

N 

Each  cc.  of  — •  iodin  V.  S.  =0.001342  gm.  of  HCN; 
10 

=0.003235    "    "  KCN. 

Titration  with  Standard  Mercuric  Chlorid  Solution  (Hannay).* 
In  this  method  the  cyanid  in  solution,  made  alkaline  by  the  addition  of 
ammonia  water,  is  titrated  with  a  standard  solution  of  mercuric  chlorid 
(13.443  gm.  per  1000  cc.),  stirring  constantly  until  a  bluish-white  opales- 
cence  is  produced  (mercurammonium  chlorid  (NH2Hg)Cl).  With 
pure  cyanid  the  reaction  is  very  delicate,  but  it  is  not  so  accurate  with 
impure  commercial  cyan  ids,  though  good  results  may  be  obtained  in 
the  presence  of  some  impurities,  such  as  alkaline  salts,  sulphocyanate, 
and  cyanates.  The  end-reaction  is  best  observed  if  the  beaker  is 
placed  on  a  black  surface. 

Titration  with  Standard  Copper  Solution  (Flajolot).  This 
method  depends  upon  the  reaction  between  copper  sulphate  and  an 
alkali  cyanid,  a  double  cyanid  of  copper  and  the  alkali  being  formed. 
The  reaction  is  illustrated  by  the  following  equation: 

4KCN+CuSO4  =  (KCN)2  .  Cu(CN)2+K2SO4. 

The  end-reaction  is  recognized  by  the  decolorization  of  the  alka- 
line copper  sulphate  solution. 

According  to  the  equation,  247.85  gms.  of  crystallized  copper 
sulphate,  CuSO4+5H2O,  are  decolorized  by  258.80  gms.  of  potassium 
cyanid. 

To  carry  out  this  method  a  standard  solution  is  made,  containing 
in  1000  cc.  24.785  gms.  of  pure  crystallized  copper  sulphate.  Twenty 
cubic  centimeters  of  this  solution  are  introduced  into  a  small  flask  and 
ammonia  water  added  until  the  precipitate  which  at  first  forms  is  just 
redissolved,  and  the  solution  assumes  a  deep-blue  color.  The  cyanid 
solution  to  be  tested  (and  which  should  contain  about  5  or  6  gms. 
per  liter),  is  delivered  from  a  burette,  drop  by  drop,  until  the  last  drop 
added  just  decolorizes  the  copper  solution.  In  order  to  see  this  change 
clearly  it  is  well  to  place  the  flask  on  a  white  surface. 

The  20  cc.  of  the  copper  solution  made  as  above  described  represent 
0.5177  gm.  of  potassium  cyanid.  Therefore  whatever  quantity  of  the 

*  J.  C.  S.,  1878,  245. 


276  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

cyanid  solution  is  taken  to  bring  about  the  discoloration,  that  quantity 
contains  0.5177  gm.  of  potassium  cyanid,  KCN,  or  its  equivalent  of 
HCN,  NaCN,  or  CN. 

Example.  Assuming  that  160  cc.  of  the  potassium  cyanid  solu- 
tion were  used  for  the  decolorization  of  the  20  cc.  of  copper  solution, 
then  the  160  cc.  contains  0.5177  gm.  of  pure  KCN.  We  may  now 
calculate  the  per  cent  of  pure  KCN  in  the  sample  of  commercial  salt 
taken  for  analysis  If  5  gms.  of  the  sample  were  dissolved  in  sufficient 
water  to  make  1000  cc.,  160  cc.  will  contain  0.8  gm.  of  the  sample. 
And  since  the  assay  showed  that  160  cc.  of  the  solution  contained 
0.5177  gm.  of  pure  KCN,  therefore  0.8  gm.  of  the  sample  contains 
0.5177  gm.  of  KCN. 

0.5177X100 

— — =64.71  per  cent. 

By  the  Modified  Kjeldahl  Process.  To  0.2  gm.  of  the  cyanid 
add  2  or  3  gms.  of  phenolsulphuric  acid,*  10  to  20  cc.  of  highly  con- 
centrated sulphuric,  and  o.i  to  0.2  gm.  of  yellow  mercuric  oxid.j 

These  substances  are  to  be  introduced  into  a  round-bottomed  flask 
of  200  to  250  cc.  capacity,  which  is  placed  in  an  oblique  position  on 
a  square  of  wire  gauze  or  of  asbestos  and  heated  to  just  below  the 
boiling  point  of  the  liquid.  This  digestion  should  be  continued  until 
the  solution  is  clear,"  and  all  of  the  nitrogen  is  converted  into  ammo- 
nium sulphate.  Several  hours  will  be  consumed  before  this  end  is 
attained. 

In  order  to  condense  any  acid  vapors  and  to  prevent  loss  by  spurting, 
a  glass  bulb  tube  may  be  inserted  into  the  neck  of  the  flask.  At  the 
end  of  the  operation  the  heat  is  withdrawn  and  the  solution,  which 
should  be  colorless,  is  transferred  (after  cooling)  to  the  distillation 
flask.  See  Fig.  57. 

An  excess  of  sodium  hydroxid  is  added  to  it  after  the  apparatus 
is  set  up,  and  the  .distillation  begun,  the  liberated  ammonia  gas  being 

N 

conducted  into  a  receiving  flask  containing  a  measured  quantity  of  — - 

10 

sulphuric  acid.  This  absorbs  the  ammonia,  forming  mmonium  sul- 
phate. When  about  half  of  the  liquid  has  distilled  over,  the  process 
is  completed,  and  the  contents  of  the  receiver  are  titrated  in  the  pres- 


*  Dissolve  50  gms.  of  phenol  in  a  small  quantity  of  concentrated  sulphuric 
acid,  and  then  make  up  the  solution  to  100  cc.  with  more  concentrated  sulphuric 
acid. 

t  The  mercuric  oxid  made  from  mercuric  nitrate  is  not  suitable — that  made 
in  the  wet  way  is  to  be  used. 


ASSAY   OF   INSOLUBLE   CYANIDS  277 

N 

ence  of  methyl  orange  with  —  sodium  hydroxid  solution.    The  volume 

10 

N 

of  the  latter  required,  deducted  from  the  quantity  of  —   sulphuric 

10 

acid  originally  taken,  gives  the  quantity  of  the  latter  which  was  neu- 
tralized by  ammonia,  and  hence  represents  the  nitrogen  of  the  cyanid 
tested. 

N 
Example.     50  cc.  of   —  sulphuric  acid  were  in  the  receiver.     After 

distillation  the  contents  of  the  receiver  required  for  complete  neutraliza- 

N 
tion,  just  21  cc.  of  —  sodium  hydroxid  solution.     Hence  50  cc.—  21  cc. 

N 
=29  cc.  is  the  quantity  of  —  sulphuric  acid  which  was  neutralized 

by  the  ammonia  which  distilled  over. 

16.93  gms.  of  ammonia  represents  64.70  gms.  of  potassium  cyanid. 


.2)33-86  2)97.35 
6.93  10)48.67 
1.693  4.8675  gms.=  1000  cc.  —  acid. 


10)16.93  10)48.675 


10 


N 
Thus  each  cc.  of  —  sulphuric  acid  represents  0.001693  gm- 


,  which  is  equivalent  to  0.006470  gm.  of  KCN. 

If  29  cc.  were  neutralized  by  the  evolved  ammonia,  then  29X0.006470 
gm.  =0.18763  gm.  Therefore  the  sample  tested  is  93.81  per  cent  pure. 

Assay  of  Insoluble  Cyanids.  Cyan  ids  insoluble  in  water  are 
treated  with  an  acid  in  order  to  set  free  the  hydrocyanic  acid  which 
is  distilled  over  and  received  in  a  solution  of  potassa. 

Care  must  be  taken  in  distilling  hydrocyanic  acid,  insamuch  as 
it  is  partially  decomposed  into  formic  acid  and  ammonia  in  the  pres- 
ence of  much  free  acid.  To  overcome  this,  the  distilling  apparatus 
shown  in  Fig.  68  is  used.  The  hydrochloric  acid  is  contained  in  the 
pipette.  The  cyanid  together  with  some  water  is  placed  in  the  flask. 
The  water  in  the  flask  is  heated  to  boiling,  and  the  hydrochloric  acid 
allowed  to  flow  in  slowly,  drop  by  drop.  In  this  way  the  hydrocyanic 
acid  formed  by  each  drop  of  hydrochloric  acid  distils  over  immediately 
with  the  vapor  of  water,  and  is  condensed  by  means  of  the  Liebig's 
condenser.  The  reaction  is  as  follows: 

Hg(CN)2  4-2HC1  =  2HCN  +HgCl2. 


278 


A   MANUAL  OF  VOLUMETRIC  ANALYSIS 


If  any  hydrochloric  acid  distills  over  with  the  hydrocyanic  acid, 
the  analysis  is  spoiled,  or  at  least  cannot  be  concluded  by  titration  with 
silver  nitrate,  because  hydrochloric  acid  reacts  with  silver.  This  may 
be  avoided  by  the  use  of  nitric,  sulphuric,  or  phosphoric  acid. 

If  potassium  ferrocyanid  is  to  be  estimated  in  this  way  it  is  treated 
with  sulphuric  acid;  but  it  must  be  remembered  that  only  half  of 
the  contained  cyanogen  distils  over,  the  rest  remains  in  combination 
with  potassium  and  iron. 

2K4Fe(CN)6+3H2S04=6HCN+K2Fe2(CN)6+3K2SO4. 


FIG.  68. 


Ferrocyanids.  Alkali  ferrocyanids  may  be  estimated  by  potas- 
sium permanganate  or  dichromate.  The  reaction  is  as  follows: 

5K4Fe(CN)6+KMnO4+4H2SO4 

=5K3Fe(CN)6+MnSO4+3K2SO4+4H2O. 

The  ferrocyanid  is  thus  oxidized  to  ferricyanid.  The  former  is 
yellow  in  color,  the  latter  red.  Therefore  the  end-reaction  is  the 
appearance  of  a  red  color,  but  much  practice  is  required  in  order  to 
recognize  the  first  appearance  of  a  red,  which  is  very  difficult  in  the 
greenish-yellow  solution.  The  end-reaction  may  also  be  found  by 
bringing  a  drop  of  the  solution  in  contact,  on  a  white  slab,  with  a 
drop  of  ferric  chlorid  solution;  when  a  blue  color  is  no  longer  produced 
by  this  contact  the  end-point  is  reached. 

The  process  is  conducted  as  follows:  2  gms.  of  the  ferrocyanid  are 
dissolved  in  sufficient  water  to  make  i  liter  of  solution.  100  cc.  of  this, 


FERRICYAN1DS  279 

representing  0.2  gm.  of  the  salt,  are  acidulated  with  sulphuric  acid, 
placed  in  a  white  porcelain  dish,  and  titrated  with  the  permanganate. 
i  cc.  of  the  permanganate  represents 

0.036598  gm.  of  K4Fe(CN)6, 
or  0.041962  gm.  of  K4Fe(CN)6+3H2O. 

Ferricyanids.  These  salts  may  be  estimated  after  reduction  to 
ferrocyanids  by  titrating  with  permanganate,  as  described  in  the  pre- 
ceding. 

The  ferricyanid  is  treated  with  an  excess  of  potassa  or  soda  and 
boiled,  while  small  quantities  of  strong  solution  of  ferrous  sulphate  are 
added  from  time  to  'time,  until  the  precipitate  produced  is  black  in 
color.  The  solution  is  then  diluted  to  a  convenient  quantity,  say  300  cc. 

100  cc.  of  this  solution  are  then  taken  out,  acidified  strongly  with 

N 
sulphuric  acid,  and  titrated  with  -  -   permanganate,  as  directed  for 

the  estimation  of  ferrocyanids. 

The  process  is  based  upon  the  fact  that  ferrous  sulphate  reduces 
the  ferricyanid  to  ferrocyanid  in  the  form  of  a  blue  precipitate,  Turn- 
bull's  blue  (Fe3Fe2(CN)i2),  as  the  equation  shows: 


K6Fe2(CN)i2  +3FeSO4  =Fe3Fe2(CN)12 

This  blue  precipitate  when  boiled  with  an  alkali  is  immediately 
reduced  to  magnetic  oxid  (Fe3O4),  and  the  alkali  ferrocyanid  goes 
in  solution,  as  shown  by  equation: 

Fe3Fe2(CN)12  +8KOH  =Fe3O4  +2K4Fe(CN)6  +4H2O. 

In  the  analysis,  whatever  quantity  of  permanganate  is  used  must 
be  multiplied  by  three,  because  only  one  third  of  the  entire  solution  is 
titrated. 

Each  cc.  of  permanganate  represents 

0.036598  gm.  of  K4Fe(CN)6, 
which  equals 

0.032712  gm.  of  K3Fe(CN)e. 

•  Most  of  the  insoluble  ferricyanids  are  converted  into  potassium 
ferricyanid  by  boiling  with  KOH. 

The  reduction  of  ferricyanid  may  also  be  effected  by  means  of 
nascent  hydrogen  developed  from  zinc  and  potassa,  or  by  means  of 
sodium  amalgam.  De  Haen  employs  lead  oxid  for  this  purpose  in 
alkaline  solution,  the  reaction  being  expressed  thus: 

2K3Fe(CN)6  +PbO  +  2KOH  =  2K4Fe(CN)6  +PbO2  +  H2O. 


280  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Sodium  peroxid  has  also  been  suggested  as  an  agent  for  completely 
and  rapidly  reducing  ferri  to  ferrocyanid.  About  0.06  gm.  of  the 
peroxid  is  required  for  0.5  gm.  of  ferricyanid  in  100  cc.  of  water. 
The  mixture  is  heated  until  all  effervescence  is  over,  acidified  with 
sulphuric  acid,  cooled,  and  titrated  with  permanganate,  as  above  de- 
scribed. 

Another  method  consists  in  boiling  with  an  excess  of  potassium 
hydroxid,  then  cooling,  and  adding  sufficient  H2O2  to  make  the  solu- 
tion yellow.  The  excess  of  H2O2  is  then  gotten  rid  of  by  boiling, 
and  the  solution  acidified  with  sulphuric  acid  and  titrated  with  per- 
manganate. Whatever  method  of  reduction  is  used  one  molecule 
of  ferricyanid  becomes  one  molecule  of  ferrocyanid. 

The  lodometric  Method  (Lenssen).  Hydroferricyanic  acid  reacts 
with  potassium  iodid  in  the  same  manner  as  ferric  salts  do,  hence 
the  estimation  of  ferricyanids  may  be  readily  effected  by  the  following 
process,  in  which  the  ferricyanid  in  strongly  acidulated  solution  is 
digested  with  potassium  iodid.  Sufficient  acid  must  be  added  to 
liberate  the  hydroferricyanic  acid.  The  reaction  may  be  expressed 
by  the  equation  below: 

2K3Fe(CN)6  +2KI +2HC1 =2K4Fe(CN)6  +I2  +2HC1. 

According  to  this  equation  potassium  ferricyanid  is  reduced  to 
potassium  ferrocyanid  and  an  equivalent  of  iodin  is  set  free.  In  this 
reaction  there  is  always  produced  more  or  less  hydroferrocyanic  acid, 
the  presence  of  which  has  a  disturbing  influence  upon  the  accuracy 
of  the  result.  This  disturbing  effect  can  be  entirely  avoided,  accord- 
ing to  C.  Mohr,  by  the  addition  of  a  quantity  of  iron-free  zinc  sulphate, 
which  forms  zinc  ferrocyanid. 

The  process  is  as  follows: 

An  accurately  weighed  quantity  of  the  ferricyanid  is  dissolved  in  a 
convenient  quantity  of  water,  some  dry  potassium  iodid  is  added, 
together  with  a  tolerable  quantity  of  pure  hydrochloric  and  an  excess 
of  iron-free  zinc  sulphate  solution.  After  a  few  minutes  standing 
the  excess  of  acid  is  partially  neutralized  with  sodium  bicarbonate, 
so  that  the  solution  is  only  very  faintly  acid.  At  this  stage  the  zinc 
ferricyanid  first  formed  is  converted  into  zinc  ferrocyanid  and  an 

N 
equivalent  of  iodin  is  set  free,  which  may  be  titrated  with  —  sodium 

N 
thiosulphate.     i  cc.  of  —  sodium  thiosulphate  represents 

0.01259  gm.  of  iodin; 

0.032712  gm.  of  potassium  ferricyanid. 


SULPHOCYANATES  281 

The  estimation  of  mixed  potassium  ferro-  and  ferricyanids  may 
be  done  by  first  estimating  the  former  by  titration  with  permanga- 
nate, as  described,  and  then  estimating  in  another  portion  the  quantity 
of  the  combined  salts,  after  reduction  to  ferrocyanid. 

Suiphocyanates.  These  salts,  when  not  containing  too  much 
impurity,  may  be  readily  estimated  by  means  of  standard  silver  nitrate 
solution,  with  which  they  form  silver  sulphocyanate.  Ammonio- 
ferric  alum  serves  as  indicator.  The  reaction  is  fully  explained  under 
Estimation  of  Silver,  Volhard's  method,  page  131. 

To  carry  out  the  process,  7  to  10  gms.  of  the  sample  are  dissolved 
in  sufficient  water  to  make  1000  cc.  This  solution,  which  should  be 
clear,  is  filled  into  a  burette  and  carefully  delivered  into  a  white  porce- 

N 
lain  dish  containing  20  cc.  of  —  silver  nitrate  (or  some  other  measured 

quantity),  and  5  cc.  of  ammonio-ferric  sulphate  T.  S.  together  with 
enough  nitric  acid  to  make  the  solution  colorless.  The  titration  is 
continued  until  the  solution  assumes  a  permanent  reddish  color. 

N 

Each  cc.  of  —  silver  nitrate  solution  represents 
10 

0.009653  gm.  of  KCNS, 
or  0.00756     "    "  NH4CNS. 

Example.     10  gms.  of  potassium  sulphocyanate  are  dissolved  in 

N 

sufficient  water  to  make  1000  cc.     20  cc.  of  —  silver  nitrate  solution 

10 

were  taken  and  24  cc.  of  the  sulphocyanate  solution  were  used  up. 

N 
The  20  cc.  of  —  silver  nitrate  represents   20X0.009653  gm.— 

0.19306  gm.  of  KCNS.  Then  the  24  cc.  of  the  sulphocyanate  solution 
analyzed  contain  0.19306  gm.  of  pure  KCNS. 

1000  cc.  of  the  solution  contain  10  gms.  of  the  sample,  therefore 
24  cc.  will  represent  0.24  gm.  of  the  sample.  Then  if  0.24  gm.  of  the 
sample  contain  0.19306  gm.  of  KCN,  100  gms.  will  contain  80.44  gm-J 
thus  the  salt  is  80.44  per  cent  pure. 

Estimation  by  Means  of  Cupric  Sulphate  in  Presence  of  a 
Reducing  Agent  (Barnes  and  Liddle).*  This  method,  which  is 
easily  carried  out  and  gives  good  technical  results,  depends  upon  the 
fact  that  when  a  solution  of  cupric  sulphate  is  added  to  a  solution  of  a 
sulphocyanate,  in  the  presence  of  sodium  bisulphite,  the  insoluble 
cuprous  sulphocyanate  is  precipitated.  The  end-reaction  is  found  by 
bringing  a  drop  of  the  solution  in  contact  with  a  drop  of  ferrocyanid 

*  J.  S.  C.  I.,  II,  122. 


282  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

solution,  when  a  brown  color  will  be  produced.     The  reaction  is  as 
follows  : 


The  standard  solution  of  cupric  sulphate  used  contains  6.196  gms. 
per  liter,  i  cc.  of  which  represents  0.0014417  gm.  of  CNS. 

The  Process.  3  gms.  of  the  sample  are  dissolved  in  sufficient  water 
to  make  1000  cc.  25  cc.  of  this  solution  are  introduced  into  a  flask, 
together  with  3  cc.  of  a  concentrated  solution  of  sodium  bisulphite, 
and  the  mixture  boiled. 

The  copper  sulphate  solution  is  filled  into  a  burette. 

When  the  liquid  in  the  flask  has  reached  the  boiling  point,  about 
20  cc.  of  the  copper  solution  are  run  in,  the  flask  shaken,  and  the 
precipitate  allowed  to  settle.  A  drop  of  the  solution  is  then  taken 
out  on  a  glass  rod  and  brought  in  contact  with  a  drop  of  five  per  cent 
potassium  ferrocyanid  solution  on  a  white  porcelain  tile.  If  no  brown 
color  appears,  more  copper  solution  must  be  added;  this  addition  (i  cc. 
at  a  time,  or  less)  should  be  continued,  with  frequent  testing,  as  just 
described,  until  the  brown  color  is  produced  immediately  upon  contact. 
A  brown  color  which  develops  in  the  mixed  drops  after  a  short  time 
(say  one  minute  or  a  little  less),  is  no  indication  of  the  completion 
of  the  reaction. 

Two  or  more  trials  should  be  made;  the  first  gives  approximately 
the  quantity  of  copper  solution  required.  In  the  subsequent  trials 
the  amount  of  copper  solution  first  added  should  be  as  near  as  possible 
the  quantity  which  is  required  to  complete  the  reaction. 


CHAPTER  XIX 
NITROGEN  AND  ITS  COMPOUNDS 

SEVERAL  of  the  better  methods  for  the  estimation  of  nitrogen  in 
organic  substances  depend  upon  converting  the  nitrogen  into  ammonia, 
which  is  then  estimated  by  the  usual  methods.  The  older  method  of 
Will  and  Varrentrapp  possesses  merely  an  historic  interest,  in  that 
it  is  now  almost  entirely  replaced  by  the  equally  exact,  but  more  rapid, 
and  readily  performed  method  of  Kjeldahl  and  its  modifications.  The 
former  method  depends  upon  the  fact  that  the  N  of  most  nitrogenous 
organic  bodies  may  be  converted  into  ammonia  by  heating  to  a  dull 
red  heat  with  soda-lime  (a  mixture  of  caustic  lime  and  caustic  soda). 
The  ammonia  thus  evolved  is  passed  into  a  measured  quantity  of 
standard  acid,  and  its  quantity  determined  by  retitration  with  standard 
alkali. 

The  substance  to  be  analyzed,  intimately  mixed  with  soda  lime, 


FIG.  69. 


is  introduced  into  a  combustion  tube  made  of  hard  glass,  which  is 
open  at  one  end  and  drawn  to  a  closed  point  at  the  other.  See 
Fig.  69.  The  tube  should  be  about  15  mm.  in  diameter  and  50  cm. 
in  length.  The  open  end  is  fitted  with  cork,  through  which  a  glass 
tube  passes,  and  which  conveys  the  evolved  ammonia  into  an  absorp- 
tion flask  containing  a  measured  quantity  of  standard  sulphuric  acid. 
In  filling  the  tube  an  asbestos  plug  is  first  introduced  into  the  far 
end  to  prevent  plugging  of  the  point  (a)  by  the  soda  lime,  the  soda 
lime  in  coarse  pieces,  is  then  introduced  to  (b)  and  from  (b)  to  (c), 
the  weighed  quantity  of  the  substance  to  be  analyzed  mixed  intimately 
with  powdered  soda  lime;  then  from  (c)  to  (d)  the  tube  is  filled  with 
more  coarsely-  powdered  soda  lime,  and  finally,  a  small  asbestos  plug, 
to  prevent  any  of  the  soda  lime  from  being  passed  over  into  the  flask. 
The  tube  is  now  placed  in  a  combustion  furnace,  the  flask  containing 
the  standard  acid  connected,  and  heat  applied,  first  to  that  portion 

283 


284  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

of  the  tube  from  (c)  to  (d)  until  the  soda  lime  in  this  portion  is  dull 
red,  then  the  other  end  of  the  tube  is  heated  in  the  same  way,  the 
middle  portion  being  kept  cool  until  the  soda  lime  at  both  ends  has 
been  heated  to  the  proper  point.  The  middle  portion  is  then  heated, 
and  the  heat  throughout  the  tube  continued  until  no  more  ammonia 
passes  over,  and  the  acid  solution  begins  to  rise  toward  the  tube;  an 
aspirator  is  now  applied  to  the  outer  end  of  the  absorption  flask,  the 
distal  end  of -the  combustion  tube  is  broken  off  and  a  current  of  air 
is  drawn  through  the  whole  apparatus.  Finally,  the  acid  in  the  absorp- 
tion flask  is  retitrated  with  standard  alkali  solution,  and  thus  the 
quantity  of  liberated  NH3  found,  and  from  this  the  N  is  readily  calcu- 
lated. 

This  method  is  not  applicable  in  the  presence  of  nitrates. 

N 

For  Example.     25  cc.  of   —  £[2804  were  originally  placed  in  the 

10 

absorption  flask,  and  after  the  completion  of  the  combustion  5  cc.  of 

N  N 

—  NaOH  were  required  for  neutralization.      Therefore,  20  cc.  of  — 

JO  10 

H2SO4  were  neutralized  by  the  evolved  NH3. 

Each  cc.  of  --  H2SO4  represents  0.001693  gm-  °f  NH3  which  is 

equivalent  to  0.001393  gm.  of  N. 

Precaution.  The  soda  lime  must  not  be  heated  above  a  dark 
red,  otherwise  the  liberated  NH3  will  be  decomposed  into  N 
andH. 

The  method  of  carrying  out  this  process  as  adopted  by  the  Asso- 
ciation of  Official  Agricultural  Chemists  *  is  as  follows: 
(i)  PREPARATION  OF  REAGENTS. 

(a)  Standard  solution  of  hydrochloric  acid,  standard  alkali  solution, 
and  cochineal  indicator,  the  preparation  of  which  are  described 
'under  the  Kjeldahl  method,  as  carried  out  by  the  A.  O.  A.  C. 

See  page  290. 

(b)  Soda  Lime.     Excellent  soda  lime  may  be  easily  and  quickly 
prepared  by  adding  2.5  parts  of  quicklime,  to  i  part  by  weight, 
of  commercial  caustic  soda  (such  soda  as  is  used  in  the  Kjeldahl 
method)  dissolved  in  a  sufficient  amount  of  water  to  slake  the 
lime.     The  mixture  is  then  dried  and  heated  in  an  iron  pot  to 
incipient  fusion,  and  when  cold  is  ground  and  sifted.    Two  sizes 
of  granules  are  required  in  this  method: 

(bi)  Fine  enough  to  pass  through  a  2.5  mm.  sieve. 
(62)  Fine  enough  to  pass  through  a  1.25  mm.  sieve. 

*  Bulletin  No.  46. 


NITROGEN  AND  ITS  COMPOUNDS  285 

(c)  Sodium  Carbonate  and  Lime  or  Slaked  Lime.  Instead  of  soda 
lime,  Johnson's  mixture  of  sodium  carbonate  and  lime  or  slaked 
lime  may  be  used. 

Slaked  lime  may  be  granulated  by  mixing  it  with  a  little  water  to 
form  a  thick  mass,  which  is  dried  in  the  water  oven  until  hard  and 
brittle.  It  is  then  ground  and  sifted  as  above.  Slaked  lime  is  much 
easier  to  work  with  than  soda  lime  and  gives  excellent  results,  though 
it  is  probable  that  more  of  it  should  be  used  in  proportion  to  the  sub- 
stance to  be  analyzed  than  is  the  case  with  soda  lime. 

(2)  APPARATUS. 

(a)  Asbestos.     The  asbestos  used  should  be  ignited  and  kept  in  a 
glass-stoppered  bottle. 

(b)  Combustion  Tubes..  These  are  about  40  cm.  long  and  of  12 
mm.  internal  diameter,  drawn  out   to  a  point  and  closed  at  one 

end. 

(c)  U-tubes.    Large-bulb  U-tubes  with  glass  stopcocks,  or  Will's 
tubes  with  four  bulbs. 

(3)  DETERMINATION. 

The  substance  to  be  analyzed  should  be  powdered  finely  enough 
to  pass  through  a  sieve  of  i  mm.  mesh.  0.7  or  1.4  gms.,  according  to 
the  amount  of  nitrogen  present,  are  used  for  the  determination.  Into 
the  closed  end  of  the  combustion  tube  put  a  small  loose  plug  of  asbestos, 
and  upon  it  about  4  cm.  of  fine  soda  lime.  In  a  porcelain  dish  or 
mortar  mix  the  substance  to  be  analyzed,  thoroughly  but  quickly, 
with  enough  fine  soda  lime  to  fill  about  16  cm.  of  the  tube,  or  about 
forty  times  as  much  soda  lime  as  substance,  and  put  the  mixture  into 
the  combustion  tube  as  quickly  as  possible  by  means  of  a  wide-neck 
funnel,  rinsing  out  the  dish  and  funnel  with  a  little  more  fine  soda 
lime,  which  is  to  be  put  in  on  top  of  the  mixture.  Fill  the  rest  of  the 
tube  to  about  5  cm.  from  the  end  with  granulated  soda  lime,  making 
it  as  compact  as  possible  by  tapping  the  tube  gently  while  held  in  a 
nearly  upright  position  during  the  filling.  The  layer  of  granulated 
soda  lime  should  not  be  less  than  12  cm.  long.  Lastly,  put  in  a  plug 
of  asbestos  about  2  cm.  long,  pressed  rather  tightly,  and  wipe  out 
the  end  of  the  tube  to  free  it  from  adhering  particles. 

Connect  the  tube  by  means  of  a  well-fitting  rubber  stopper  or  cork 
with  the  U-tube,  or  Will's  bulbs,  containing  10  cc.  of  standard  acid, 
and  adjust  it  in  the  combustion  furnace  so  that  the  end  projects  about 
4  cm.  from  the  furnace,  supporting  the  U-tube  or  Will's  bulbs  suitably. 
Heat  the  portion  of  the  tube  containing  the  granulated  soda  lime  to 
a  moderate  redness,  and  when  this  is  attained  extend  the  heat  gradu- 
ally through  the  portion  containing  the  substance,  so  as  to  keep  up 
a  moderate  and  regular  flow  of  gases  through  the  bulbs,  maintaining 
the  heat  of  the  first  part  until  the  whole  tube  is  heated  uniformly  to 


286  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

the  same  degree.  Keep  up  the  heat  until  gases  have  .ceased  bubbling 
through  the  acids  in  the  bulbs,  and  the  mixture  of  substance  and  soda 
lime  has  become  white,  or  nearly  so,  which  shows  that  the  combustion 
is  finished.  The  combustion,  should  occupy  about  three-quarters  of 
an  hour,  or  not  more  than  one  hour.  Remove  the  heat,  and  \\hen 
the  tube  has  cooled  below  redness  break  off  the  closed  tip  and  aspirate 
air  slowly  through  the  apparatus  for  two  or  three  minutes,  to  bring 
all  the  ammonia  into  the  acid.  Disconnect,  wash  the  acid  into  a 
beaker  or  flask,  and  titrate  with  the  standard  alkali. 

During  the  combustion  the  end  of  the  tube  projecting  from  the 
furnace  must  be  kept  heated  sufficiently  to  prevent  the  condensation 
of  moisture,  yet  not  enough  to  char  the  stopper.  The  heat  may  be 
regulated  by  a  shield  of  tin  slipped  over  the  projecting  end  of  the 
combustion  tube. 

It  is  found  very  advantageous  to  attach  a  Bunsen  valve  to  the 
exit  tube,  allowing  the  evolved  gases  to  pass  out  freely,  but  preventing 
a  violent  "sucking  back  "  in  case  of  a  sudden  condensation  of  steam 
in  the  bulbs. 

The  Ruffle  Method  (as  adopted  by  A.  O.  A.  C.).* 

(1)  PREPARATION  OF  REAGENTS. 

(a)  Standard  solutions   and   indicator,   the   same   as   in   foregoing 
method. 

(b)  A  mixture  of  equal  parts  by  weight  of  fine  slaked  lime  and  finely 
powdered  sodium  thiosulphate,  dried  at  100°. 

(c)  A  mixture  of  equal  parts  of  finely  powdered  granulated  sugar 
and  flowers  of  sulphur. 

(d)  Granulated  soda  lime. 

(2)  APPARATUS. 

(a)  Combustion  tubes  of  hard,  Bohemian  glass,  70  cm.  long  and 
1.3  cm.  in  diameter. 

(b)  Bulbed  U-tubes  or  Will's  bulbs. 

(3)  THE  DETERMINATION. 

Clean  the  U-tube  and  introduce  10  cc.  of  standard  acid. 
Fill  the  combustion  tube  as  follows: 

(a)  A  loosely  fitting  plug  of  asbestos,  previously  ignited,  and  then 
from  2.5  to  3.5  cm.  of  the  thiosulphate  mixture. 

(b)  The  weighed  portion  of  the  substance   to  be  analyzed  is  inti- 
mately mixed  with  from  5  to  10  gms.  of  the  sugar  and  sulphur 
mixture. 

(c}  Pour  on  a  piece  of  glazed  paper  or  in  a  porcelain  mortar  a  suffi- 
cient quantity  of  the  thiosulphate  mixture  to  fill  25  cm.  of  tube, 
then  add  the  substance  to  be  analyzed,  as  previously  prepared, 

*  Bulletin,  No.  46. 


THE    KJELDAHL    METHOD  287 

mix  carefully,  and  pour  into  the  tube;  shake  down  the  contents 
of  the  tube;  rinse  off  the  paper  or  mortar  with  a  small  quantity 
of  the  thiosulphate  mixture,  then  fill  up  with  soda  lime  to  within 
5  cm.  of  the  end. 

(d)  Place  another  plug  of  ignited  asbestos  at  the  end  of  the  tube 
and  close  with  a  cork. 

(e)  Hold  the  tube  in  a  horizontal  position  and  tap  on  the  tube  until 
there  is  a  gas  channel  all  along  the  top.     Make  connections  with 
the  U-tube  containing  the  acid;   aspirate  and  see  that  the  appa- 
ratus is  tight. 

THE  COMBUSTION. 

Place  the  prepared  combustion  tube  in  the  furnace,  letting  the 
open  end  project  a  little,  so  as  not  to  burn  the  cork.  Commence  by 
heating  the  soda  lime  portion  until  it  is  brought  to  a  full  red  heat. 
Then  turn  on  slowly  jet  after  jet  toward  the  farther  end  of  the  tube, 
so  that  the  bubbles  come  off  two  or  three  a  second.  When  the  whole 
tube  is  red  hot  the  evolution  of  the  gas  has  ceased  and  the  liquid  in 
the  U-tube  begins  to  recede  toward  the  furnace.  Attach  the  aspirator 
to  the  other  limb  of  the  U-tube,  break  off  the  end  of  the  combustion 
tube,  and  draw  a  current  of  air  through  for  a  few  minutes.  Detach 
the  U-tube  and  wash  its  contents  into  a  beaker  or  porcelain  dish,  add 
a  few  drops  of  cochineal  solution,  and  titrate. 

The  Kjeldahl  Method  (modified).  In  this  method  the  nitrogenous 
organic  substance  is  digested  with  concentrated  sulphuric  acid  in  the 
presence  of  an  oxidizing  agent.  The  nitrogen  is  thus  converted  into 
ammonia,  which  in  the  presence  of  the  large  excess  of  sulphuric  acid, 
forms  ammonium  sulphate.  The  solution  is  then  cooled,  diluted 
with  water,  an  excess  of  caustic  soda  added,  and  the  evolved  ammonia 
(NH3)  distilled  over  into  a  measured  volume  of  standard  acid,  and 
the  amount  found  by  titration  in  the  usual  way.  As  oxidizing  agent, 
either  of  the  following  substances  may  be  employed :  Metallic  mercury, 
yellow  mercuric  oxid,  copper  oxid,  platinic  chlorid,  potassium  per- 
manganate, or  acid  potassium  sulphate. 

The  flask  in  which  the  digestion  is  done  should  be  well  annealed, 
should  have  a  rather  long  neck,  and  a  rounded  bottom,  and  should  hold 
150  to  250  cc. 

The  distillation  flask  should  be  of  the  Erlenmeyer  pattern,  made  of 
hard  Bohemian  glass,  and  of  about  500  cc.  capacity.  It  should  be 
fitted  with  a  rubber  stopper,  and  a  bulbed  delivery  tube,  to  prevent 
the  spray  of  the  boiling  alkaline  liquid  from  being  carried  over  into 
the  acid  solution  in  the  receiver,  with  which  it  is  connected.  A  con- 
densing apparatus  need  not  be  connected  unless  the  temperature  of 
the  laboratory  in  which  the  distillation  is  conducted  be  high,  though 
usually  a  condenser  is  used.  Fig.  57  shows  the  apparatus  without 


288 


A    MANUAL    OF    VOLUMETRIC'  ANALYSIS 


condenser,  while  Fig.  70  shows  same  with  condenser  attached,  and 
arranged  for  making  a  number  of  distillations  simultaneously. 


FIG.  70. 

The  Process.  0.2  to  0.5  gm.  or  a  larger  quantity  of  the  nitrogenous 
substance  (the  amount  taken  depending  upon  the  nature  of  the  sub- 
stance and  the  amount  of  nitrogen  present  in  it),  is  introduced  into 
the  digestion  flask,  10  to  20  cc.  of  pure  concentrated  sulphuric  acid 
are  added,  followed  by  o.i  to  0.2  gm.  of  yellow  mercuric  oxid,  and  the 
flask  placed  in  an  inclined  position  on  wire  gauze  or  asbestos,  and 

cautiously  heated  for  ten  to  fifteen 
minutes  or  until  frothing  has  ceased. 
The  arrangement  shown  in  Fig.  71  is 
suitable  for  this  purpose;  as  many 
as  six  flasks  may  be  heated  at  once  on 
this  apparatus. 

The  heat  is  now  raised  so  that 
the  acid  liquid  boils  briskly,  and 
is  continued  at  this  point  for  about 
fifteen  minutes.  5  to  10  gms.  of 
potassium  sulphate  are  then  added 
and  the  heating  continued  until 
the  black  color  of  the  liquid  is 
destroyed,  and  it  is  clear  and  color- 
less. 

FlG    7I>  The    flask    is   now  removed  and 

cooled   and   its    contents    transferred 
with  the  aid  of  several  portions  of  water  to  the  distilling  flask. 


THE    KJELDAHL    METHOD 


289 


About  10  cc.  of  4  per  cent  potassium  sulphid  solution  are  added, 
followed  by  a  decided  excess  of  saturated  sodium  hydroxid  solution. 
A  few  small  pieces  of  granulated  zinc  are  now  added,  and  the  flask  at 
once  connected  with  the  rest  of  the  apparatus.  The  distillation  is 
now  begun  by  applying  heat,  and  continued  until  all  of  the  ammonia 
has  passed  over  into  the  standard  acid,  and  the  concentrated  solution 
can  no  longer  be  safely  boiled.  This  usually  requires  half  an  hour. 
The  contents  of  the  receiving  flask  are  then  titrated  with  standard 
alkali  solution  in  the  usual  manner. 

The  use  of  mercuric  oxid  hastens  the  oxidation.  The  use  of  potas- 
sium sulphid  is  to  remove  the  mercury,  as  mercurous  sulphid,  and 
thus  prevent  the  formation  of  mercur-ammonium  compounds  which 


FIG.  72.* 

the  soda  solution  cannot  completely  decompose.  The  use  of  zinc 
is  to  prevent  violent  bumping  by  evolving  hydrogen  gas. 

The  sulphuric  acid  used  must  be  free  nitrates  and  ammonium 
sulphate. 

The  solution  of  sodium  hydroxid  used  must  be  free  from  nitrates 
and  nitrites.  The  indicator  may  be  methyl  orange  or  litmus,  but 
not  phenolphthalein. 

Example.  0.5  gm.  of  the  nitrogenous  substance  are  digested  with 
20  cc.  of  H2SO4  and  0.2  gm.  of  HgO.  The  evolved  NH3  is  received 

N 
in  100  cc.  of  —  H2SO4.     Methyl  orange  is  added  as  indicator,  and 


10 


N 


the  solution  titrated  with  — •  NaOH,  60  cc.  of  which  is  required  for 


10 


N 


neutralization.     Hence  40  cc.  of  —  sulphuric  acid  were  neutralized  by 
the  evolved  NH3. 

*  Fig.  72  shows  a  shelf  upon  which  the  flasks  can  be  placed  in  a  reclining 
position  for  digestion,  and  which  may  likewise  be  employed  for  heating  the  distil- 
lation flasks. 


290  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

N 

Each  cc.  of  —  H2SO4  is  neutralized  by  0.001693  gm-  °f  NH3,  and 
10 

hence  represents  0.001393  gm.  of  N.  Therefore  the  0.5  gm.  of  the 
substance  analyzed  contained  0.001393X40=0.05572  gm.  of  N  or 
11.14  per  cent. 

The  above  method  must  be  somewhat  modified,  if  the  substance 
examined  contains  any  nitrates. 

The  Kjeldahl  Method  (modified  by  Jodlbauer)  gives  in  such 
cases  very  accurate  results.  This  is  conducted  as  follows: 

The  substance  to  be  analyzed  (in  requisite  amount)  is  put  into 
the  digestion  flask,  together  with  i  or  2  gms.  of  zinc  dust.  20  to  30  cc. 
of  concentrated  sulphuric  acid  containing  2  gms.  of  salicylic  acid  are 
then  poured  over  the  mixture  so  as  to  cover  it  at  once.  Heat  is  then 
applied  gently  until  frothing  is  over,  and  after  the  addition  of  potas- 
sium sulphate,  the  process  is  completed  as  described  in  the  foregoing. 
Instead  of  salicylic  acid,  phenol  may  be  taken  (see  Estimation  of  Cyanids 
by  Modified  Kjeldahl  Process).  A  globule  of  mercury  or  a  small 
quantity  of  the  yellow  mercuric  oxid  may  be  added  as  well.  This 
method  may  be  used  for  nitrates  alone. 

Small  quantities  of  nitrates  or  nitrites  may  be  estimated  by  mere 
distillation  with  zinc,  or  still  better,  with  aluminum  and  caustic  soda. 
Large  quantities  of  nitrates  or  nitrites  cannot,  however,  be  accurately 
estimated  by  this  means,  because  too  much  time  is  consumed. 

The  estimation  of  potassium  nitrate  by  the  Jodlbauer  modification 
of  Kjeldahl 's  process,  is  illustrated  by  the  following  example: 

0.2  gm.  of  KNO3  are  put  into  the  digestion  flask  together  with 
2  cc.  of  phenol-sulphuric  acid,  10  to  20  cc.  of  concentrated  H2SO4 
and  0.2  gm.  of  HgO,  and  heated  until  colorless.  The  evolved  ammonia 

N 
is  received  in  50  cc.  —  H2SO4,  and  for  the  neutralization  30  cc.  of 

N  .I0 

—  NaOH  were  required. 

10  N 

Thus  20  cc.  of  —  H2SC>4  were  neutralized  by  the  evolved  NH3. 
10 

N 
Each  cc.  of  —  H2SO4  =0.001393  gm.  of  nitrogen  or  0.010043  gm-  °f 

KN02. 

The  Kjeldahl  Method  (as  adopted  by  the  Association  of  Official 
Agricultural  Chemists).*     (Not  applicable  in  the  presence  of  nitrates}. 
(i)  PREPARATION  OF  REAGENTS. 

(a)  Standard  Hydrochloric  Acid,  the  absolute  strength  of  which 
has  been  determined  by  precipitating  with  silver  nitrate  and 
weighing  the  silver  chlorid,  as  follows : 

*  Bulletin  No.  107,  U.  S.  Dept.  of  Agriculture. 


THE    KJELDAHL    METHOD  291 

By  means  of  a  preliminary  test,  with  silver  nitrate  solution,  to  be 
measured  from  a  burette,  with  excess  of  calcium  carbonate  to 
neutralize  free  acid,  and  potassium  chromate  as  indicator,  deter- 
mine exactly  the  amount  of  silver  nitrate  required  to  precipi- 
tate all  the  hydrochloric  acid. 

To  a  measured  and  also  weighed  portion  of  the  standard  acid, 
add  from  a  burette  one  drop  more  of  the  silver  nitrate  solution 
than  is  required  to  precipitate  the  hydrochloric  acid.  Heat 
to  boiling,  cover  from  light,  and  allow  to  stand  until  the  pre- 
cipitate is  granular.  Then  wash  with  hot  water  through  a 
Gooch  crucible,  testing  the  filtrate  to  prove  excess  of  silver 
nitrate.  Dry  the  silver  chlorid  at  140°  to  150°  C. 

(b)  Standard  Sulphuric  Acid.     For  ordinary  work,  half  normal  acid 
is  recommended.     For  works  determining  very  small  quantities 
of  nitrogen,  one  tenth  normal  is  recommended. 

(c)  Standard  Alkali  Solution.     The  strength  of  this  solution,  relative 
to  the  acid,  must  be  accurately  determined;    one  tenth   normal 
ammonia  solution  is  recommended. 

(d)  Sulphuric  Acid,  specific  gravity    1.84,  free  from  nitrates    and 
also  from  ammonium  sulphate. 

(e)  Metallic  Mercury  or  Mercuric  Oxid.     The  latter  should  be  pre- 
pared in  the  wet  way,  but  not  from  mercuric  nitrate. 

(f)  Potassium  permanganate  in  fine  powder. 

(g)  Granulated  Zinc,  Pumice  Stone,  or  Zinc  Dust.     Used  to  prevent 
bumping.     When  zinc  dust  is  used,  0.5  will  be  sufficient. 

(h)  Potassium  Sulphid  Solution,  40  gms.  to  one  liter. 

(i)  Sodium   Hydroxid  Solution.    A   saturated   solution    free    from 
nitrates. 

(/)  Indicator.  A  solution  of  cochineal  is  prepared  by  digesting 
and  frequently  agitating  3  gms.  of  pulverized  cochineal  in  a  mix- 
ture of  50  cc.  of  strong  alcohol  and  200  cc.  of  distilled  water  for 
a  day  or  two  at  ordinary  temperatures.  The  filtered  solution  is 
employed  as  indicator. 
(2)  APPARATUS. 

(a)  Kjeldahl  Digestion  Flasks.    These  are  pear-shaped,  round-bot- 
tom flasks,  made  of  hard,  moderately  thick,  well-annealed  glass, 
having  a  total  capacity  of  about  250  cc.     They  are  22  cm.  long 
and  have  a  maximum  diameter  of  6  cm.,  tapering  gradually  to  a 
long  neck,  which  is  2  cm.  in  diameter  at  the  narrowest  part  and 
flared  a  little  at  the  edge. 

(b)  Distillation  Flasks.     A  flask  of  ordinary  shape,  of  about  550  cc. 
capacity.     It  is   fitted  with  a  rubber  stopper,  and  with  a  bulb 
tube  above  to  prevent  the  possibility  of  sodium  hydroxid  being 
carried   over  mechanically  during  distillation.     The  bulbs   may 


292  A    MANUAL   OF   VOLUMETRIC    ANALYSIS 

be  about  3  cm.  in  diameter,  the  tubes  being  of  the  same  diame- 
ter as   the    condenser  and  cut   off  obliquely  at   the  lower  end, 
which  is  fastened  to  the  condenser  by  a  rubber  tube. 
(c)  Kjeldahl  Flasks  for  both  Digestion  and  Distillation.     These  are 
pear-shaped,    round-bottom    flasks,    having   a   total   capacity   of 
about  550  cc.,  made  cf  hard,  moderately  thick,  and  well-annealed 
glass.     When  used  for  distillation,  the  flasks  are  fitted  with  rubber 
stoppers  and  bulb  tubes,  as  given  under  Distillation  Flasks. 
(3)  DETERMINATION. 

(a)  The  Digestion.     From  0.7  to  3.5  gms.  of  the  substance  to  be 
analyzed,  according  to   its  proportion  of  nitrogen,  are  brought 
into  a  digestion  flask  with  approximately  0.7  gm.  of  mercuric 
oxid,  or  its  equivalent  in  metallic  mercury,  and  20  cc.  of  sulphuric 
acid.     The  flask  is  placed  in  an  inclined  position,  and  heated 
below  the  boiling  point  of  the  acid  for  from  five  to  fifteen  minutes, 
or  until  frothing  has  ceased.     If  the  mixture  froth  badly,  a  small 
piece  of  paraffin  may  be  added  to  prevent  it.     The  heat  is  then 
raised  until  the  acid  boils  briskly.     No  further  attention  is  required 
till  the  contents  of  the  flask  have  become  a  clear  liquid,  which  is 
colorless,  or  at  least  has  only  a  very  pale  straw  color.     The  flask 
is  then  removed  from  the  flame,  held  upright,  and    while  still 
hot,  potassium  permanganate  is  dropped  in  carefully  and  in  small 
quantities  at  a  time,  till,  after  shaking,  the  liquid  remains  of  a 
green  or  purple  color. 

(b)  The  Distillation.     After  cooling,  the  contents  of  the  flask  are 
transferred  to  the  distilling  flask  with  about  200  cc.  of  water,  a 
few  pieces  of  granulated  zinc,  pumice  stone,  or  0.5  gms.  of  zinc 
dust  when  found  necessary  to  keep  the  contents  of  the  flask  from 
bumping,  and  25  cc.  of  potassium  sulphid  solution  are  added, 
with  shaking.     Next  add  50  cc.  of  the  soda  solution,  or  sufficient 
to  make  the  reaction  strongly  alkaline,  pouring  it  down  the  side 
of  the  flask  so  that  it  does  not  mix  at  once  with  the  acid  solution. 
Connect  the  flask  with  the  condenser,  mix  the  contents  by  shaking, 
and  distil  until  all  ammonia  has  passed  over  into  the  standard 
acid.     The  first   150  cc.  of  the  distillate  will  generally  contain 
all  the  ammonia.     This   operation   usually  requires   from   forty 
minutes  to  one  hour  and  a  half.     The  distillate  is  then  titrated 
with  standard  alkali. 

The  use  of  mercuric  oxid  in  this  operation  greatly  shortens  the 
time  necessary  for  digestion,  which  is  rarely  over  an  hour  and  a  half 
in  case  of  substances  most  difficult  ot  oxidize,  and  is  more  commonly 
less  than  an  hour.  In  most  instances  the  use  of  potassium  perman- 
ganate is  quite  unnecessary,  but  it  is  believed  that  in  exceptional  cases 
it  is  required  for  complete  oxidation,  and  in  view  of  the  uncertainty  it  is 


GUNNING    METHOD  293 

always  used.  The  potassium  sulphid  removes  all  the  mercury  from 
the  solution,  and  so  prevents  the  formation  of  mercur-ammonium 
compounds  which  are  not  completely  decomposed  by  the  sodium 
hydroxid.  The  addition  of  zinc  gives  rise  to  an  evolution  of  hydrogen 
and  prevents  violent  bumping.  Previous  to  use  the  reagents  should 
be  tested  by  a  blank  experiment  with  sugar,  which  will  partially 
reduce  any  nitrates  that  are  present,  which  might  otherwise  escape 
notice. 

Gunning  Method.*     (Not  applicable  in  the  presence  of  nitrates.) 

(1)  PREPARATION  OF  REAGENTS. 

(a)  Potassium  Sulphate.     This  reagent  should  be  pulverized  before 

using. 

The  other  standard  solutions  and  reagents  used  are  the  same  as 
those  described  under  Kjeldhal  method. 

(2)  APPARATUS. 

The  apparatus  used  is  the  same  as  that  described  in  the  Kjeldahl 
method. 

(3)  DETERMINATION.  In  a  digestion  flask  holding  from  250  to  500 
cc.  place  from  0.7  to  3.5  gms.  of  the  substance  to  be  analyzed, 
according  to  its  proportion  of  nitrogen.  Then  add  10  gms.  of 
powdered  potassium  sulphate  and  from  15  to  25  cc.  (ordinarily 
about  20  cc.)  of  concentrated  sulphuric  acid.  Conduct  the 
digestion  as  in  the  Kjeldahl  process,  starting  with  a  temperature 
below  boiling  point  and  increasing  .the  heat  gradually  until 
frothing  ceases.  Digest  until  the  mixture  is  colorless  or  nearly 
so.  Do  not  add  either  potassium  permanganate  or  potassium 
sulphid.  Dilute,  neutralize,  and  distil  as  in  the  Kjeldahl  method. 
In  neutralizing  it  is  convenient  to  add  a  few  dops  of  phen- 
olphthalein  indicator,  by  which  one  can  tell  when  the  acid  is 
completely  neutralized,  remembering  that  the  pink  color,  which 
indicates  an  alkaline  reaction,  is  destroyed  by  a  considerable 
excess  of  strong  fixed  alkali.  The  distillation  and  titration  are 
conducted  as  in  the  Kjeldahl  method. 

Kjeldahl  Method  (modified  to  include  the  nitrogen  of  nitrates),  f 
(i)  PREPARATION  OF  REAGENTS. 

Besides  the  reagents  given  under  the  Kjeldahl  method,  there  will 
be  needed — 

(a)  Zinc  Dust.     This  should  be  an  impalpable  powder;   granulated 
zinc  or  zinc  filings  will  not  answer. 

(b)  Sodium  Thiosulphate. 

(c)  Commercial  Salicylic  Acid. 

*  A.  O.  A.  C.,  Bulletin  No.  46,  U.  S.  Dept.  Agric.  t  Ibid.,  No.  107. 


294  A    MANUAL  OF    VOLUMETRIC   ANALYSIS 

(2)  APPARATUS. 

The  apparatus  used  is  the  same  as  in  the  Kjeldahl  method. 

(3)  DETERMINATION.    Place  from  0.7  to  3.5  gms.  of  the  substance 
to  be  analyzed  into  a  Kjeldahl  digestion  flask,  add  30  cc.  of 
sulphuric  acid  containing  i  gm.  of  salicylic  acid  and  shake  until 
thoroughly  mixed,  then  add  5  gms.  of  crystallized  sodium  thio- 
sulphate;  or  add  to  the  substance  30  cc.  of  sulphuric  acid  con- 
taining 2  gms.  of  salicylic  acid,  then  add  gradually  2  gms.  of  zinc 
dust,  shaking  the  contents  of  the  flask  at  the  same  time.    Finally, 
place  the  flask  on  the  stand  for  holding  the  digestion  flasks,  where 
it  is  heated  over  a  low  flame  until  all  danger  from  frothing  has 
passed.     The  heat  is  then  raised  until  the  acid  boils  briskly 
and  the  boiling  continued  until  white  fumes  no  longer  escape 
from  the  flask.     This  requires  about  five  or  ten  minutes.     Add 
approximately  0.7  gm.   of  mercuric  oxid  or  its  equivalent  in 
metallic  mercury,  and  continue  the  boiling  until  the  liquid  in 
the  flask  is  colorless  or  nearly  so.     In  case  the  contents  of  the 
flask  are  likely  to  become  solid  before  this  point  is  reached,  add 
10  cc.  more  of  sulphuric  acid.     Complete  the  oxidation  with  a 
little  potassium  permanganate  in  the  usual  way,  and  proceed 
with   the   distillation  as  described  in   the    Kjeldahl    method. 
The  reagents  should  be  tested  by  blank  experiments. 

Gunning  Method  (modified  to  include  the  nitrogen  of  nitrates).* 

(1)  PREPARATION  OF  REAGENTS. 

Besides  the  reagents  given  under  the  Gunning  method,  there 
will  be  needed — 

(a)  Sodium  Thiosulphate. 

(b)  Commercial  Salicylic  Acid. 

(2)  APPARATUS. 

The   apparatus  used   is   the   same   as   that   given  under  the 
Kjeldahl  method. 

(3)  DETERMINATION.     In  a  digestion  flask,  holding  from  250  to  500 
cc.,  place  from  0.7  to  3.5  gms.  of  the  substance  to  be  analyzed, 
according  to  the  amount  of  nitrogen  present.     Add  from  30  to 
35  cc.  of  salicylic  acid  mixture,  namely,  30  cc.  sulphuric  acid 
to  i  gm.  of  salicylic  acid,  shake  until  thoroughly  mixed,  and 
allow  to  stand  from  five  to  ten  minutes,  with  frequent  shaking; 
then  add  5  gms.  of  sodium  thiosulphate  and  10  gms.  of  potas- 
sium sulphate.     Heat  very  gently  until  frothing  ceases,  then 
heat  strongly  until  nearly  colorless.      Dilute,    neutralize,   and 
distil,  as  in  the  Gunning  method. 

Nitric    Acid   Nitrates.     The  assaying  of  nitric  acid,  in  a  solution 
containing  no  other  acid  may  be  accomplished  most  simply  by  neu- 

*  A.  O.  A.  C.  Bulletin  No.  107. 


NITRIC    ACID    AND    NITRATES  295 

tralization,  as  described  on  page  103.  The  strength  of  a  solution  of 
pure  nitric  acid  may  also  be  readily  found  by  taking  its  specific  gravity 
and  referring  to  a  table  such  as  that  in  the  U.  S.  P. 

The  determination  of  combined  nitric  acid  is,  on  the  other  hand,  a 
much  more  difficult  problem,  though  one  of  greatest  importance.  A 
great  many  methods  have  been  prepared,  all  of  them  more  or  less 
complicated.  Only  the  simplest  and  best  will  be  described  here. 
Whichever  method  is  selected  for  use  it  is  advised  that  it  be  tried  several 
times  upon  weighed  quantities  of  pure  potassium  nitrate,  in  order  to 
become  familiar  with  the  method  and  to  acquire  sufficient  skill  in  its 
use  to  attain  accurate  results. 

1.  Conversion  with  Chlorids.    Nitrates  may  be  evaporated  with 
concentrated   hydrochloric   acid,  and   the   resulting 

chlorid   dissolved   in  water  and  titrated  with  deci- 
normal  silver  nitrate. 

2.  Distillation  with   Sulphuric   Acid.      Nitrates 
are   decomposed    by   distillation    with    moderately 
dilute    sulphuric    acid.       The    nitric    acid    which 
passes  over  is  received  in  a  measured  quantity  of 
standard  alkali. 

The  Process,  i  gm.  of  nitrate  is  introduced  into 
a  tubulated  retort,  together  with  a  cooled  mixture 
of  5  cc.  of  sulphuric  acid  and  10  cc.  of  water.  The 
distillation  is  performed  at  a  temperature  of  160°  pIG  7 

to  170°  C.  on  a  paraffin  or  sand-bath,  and  occupies 
about  three  or  four  hours.     The  retort  is  connected  with  a  bulbed  U-tube 
(see  Fig.  73)  containing  a  measured  quantity  of  standard  soda  solution. 

After  the  distillation  is  complete,  the  U-tube  is  washed  out  and 
the  excess  of  alkali  found  by  titration  with  standard  acid. 

The  distillation  may  be  performed  in  a  partial  vacuum,*  by  connect- 
ing the  retort  air  tight  with  a  receiver  having  two  tubulures.  Place  the 
normal  alkali,  diluted  with  water,  in  the  receiver,  and  the  dilute  sul- 
phuric acid  in  the  retort.  Heat  the  contents  of  both  vessels  to  boiling, 
with  the  tubulures  open.  When  the  air  is  expelled  from  both  vessels, 
drop  the  nitrate  contained  in  a  small  glass  tube  through  the  tubulure 
of  the  retort,  quickly  insert  the  stopper,  and  then  insert  the  stopper 
of  the  receiver  and  take  away  both  lamps.  The  retort  is  placed  upon 
a  water-bath,  and,  as  the  nitric  acid  will  all  go  over  at  this  tempera- 
ture in  the  vacuum,  the  distillation  may  be  left  to  take  care  of  itself. 
The  greatest  advantage  obtained  by  this  process  is  that  there  is  far 
less  danger  of  sulphuric  acid  going  over  with  the  nitric  acid  than  when 
the  temperature  is  raised  to  170°  C. 

*  Finkener,  Zeit.  Anal.  Chem.,  i,  309. 


296 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


3.  Decomposition  of  Nitrates  by  Alkali  Hydroxids  or  Carbonates. 
Nitric  acid,  combined  with  bases  which  are  precipitable  by  alkali 
hydroxid  or  carbonate,  may  be  estimated  by  simply  boiling  with  an 
excess  of  standard  alkali  or  carbonate.    After  cooling,  the  solution  is 
diluted  to  about  200  cc.,  the  precipitate  allowed  to  settle,  and  a  portion 
of  the  clear  liquid  drawn  off,  and  the  free  alkali  remaining  in  it  deter- 
mined by  titration  with  standard  acid. 

In  many  nitrates,  the  bases  of  which  are  precipitable  by  hydrogen 
sulphid,  the  nitric  acid  may  be  determined  by  adding  to  the  salt  in 
solution  about  its  own  weight  of  some  neutral  organic  salt,  e.g.,  Rochelle 
salt,  and  precipitating  the  metal  by  hydrogen  sulphid.  The  filtrate 
and  washings  are  brought  to  a  definite  bulk,  and  free  acid  is  deter- 
mined in  aliquot  portions. 

4.  Conversion  of  Nitric  Acid  into  Ammonia.     If  a  nitrate  is  heated 
in  an  alkaline  solution  in  which  nascent  hydrogen  is  being  evolved  in 


FIG.  74. 

sufficient  quantity,  the  nitric  acid  is  converted  into  ammonia.  From 
the  quantity  of  ammonia  so  produced,  the  quantity  of  the  nitric  acid 
present  is  readily  determined. 

Fr.  Schultze  was  the  first  to  utilize  this  reaction  for  the  quantitative 
estimation  of  nitric  acid.  He  was  soon  followed  by  W.  Wolf,  Har- 
court,  and  Siewert.  Schultze  employed  platinized  zinc,  Harcourt  and 
Siewert  used  zinc  and  iron  filings.  The  combination  of  zinc  and  iron 
affords  most  satisfactory  results. 

Harcourt  employs  the  apparatus  shown  in  Fig.  74. 

The  distilling  flask  (a)  holds  about  200  cc.,  and  is  connected  with 
a  smaller  flask  in  such  a  way  that  both  may  be  placed  obliquely  upon 
a  sand-bath,  as  shown  in  the  figure;  this  arrangement  is  to  prevent  the 
spurting  of  the  boiling  liquids  into  the  exit  tubes — the  latter  as  an  extra 


THE  ZINC-IRON    METHOD  297 

precaution,  are  bent  into  the  form  of  hooks.  From  the  small  flask  a 
long  tube  passes  through  a  Liebig's  condenser  into  a  tubulated 
receiver  containing  normal  sulphuric  acid  colored  with  litmus  and 
provided  with  a  bulb-tube  (b)  of  peculiar  form.  This  also  contains 
some  of  the  colored  sulphuric  acid.  The  tube  which  passes  through 
the  condenser  has  a  small  tubulure  (c)  closed  with  a  rubber  stopper; 
through  this  tubulure  water  may  be  passed  when  the  distillation  is 
over,  in  order  to  wash  any  traces  of  ammonia  into  the  receiver.  All 
the  corks  of  the  apparatus  should  be  soaked  in  paraffin. 

The  bulb-tube  (b)  on  the  receiver  is  turned  half  way  round  so  as 
to  bring  it  into  a  vertical  position,  and  a  measured  quantity  (more 
than  sufficient  to  fix  the  ammonia)  of  normal  sulphuric  acid  is  passed 
into  it,  to  the  receiver,  a  small  quantity  of  litmus  tincture  having  been 
added.  The  tube  is  then  turned  back  to  the  position  shown  in  the 
figure,  and  a  little  more  normal  sulphuric  acid  measured  into  it.  The 
flasks  are  now  removed,  and  into  the  smaller  tube,  some  water  is  intro- 
duced, and  the  flask  placed  back  into  position.  Into  the  larger  flask  (a) 
is  put  50  gms.  of  finely-granulated  zinc  and  25  gms.  of  pure  iron  filings 
(purified  by  sifting,  and  then  heating  in  a  current  of  hydrogen,  or 
igniting  in  a  covered  crucible).  The  weighed  quantity  of  the  nitrate 
is  then  added  (say  0.5  gm.),  followed  by  25  cc.  of  water  and  25  cc. 
of  potassium  hydroxid  solution,  of  sp.gr.  1.3.  The  flask  is  then 
quickly  connected,  and  after  standing  for  about  one  hour,  heat  is 
applied  directly  under  the  distillation  flask  (a),  until  its  contents  boil. 
When  distillation  begins,  the  heat  is  so  applied  that  the  water  in  the 
smaller  flask  boils  gently.  In  this  manner  the  fluid  is  twice  distilled 
and  any  potassium  hydroxid  which  may  have  been  carried  over  from  (a) 
is  completely  retained  in  the  smaller  flask. 

From  one  to  two  hours  are  required  for  the  distillation.  It  may 
be  stopped  when  the  hydrogen,  which  is  evolved  more  fully  as  the 
potash  solution  becomes  more  concentrated,  has  passed  through  the 
bulb  tube  (b)  for  five  or  ten  minutes.  When  the  apparatus  has  cooled, 
and  the  fluid  in  the  bulb  tube  has  receded,  the  rubber  stopper  is  re- 
moved from  the  tubulure  (c)  and  the  tube  rinsed  into  the  receiver. 
The  bulb  tube  is  also  rinsed  with  water.  The  receiver  is  then  dis- 
connected, the  lower  end  of  the  condenser  tube  which  has  projected 
to  the  middle  of  the  receiver  is  also  rinsed  off  into  the  acid  solution, 
and  the  latter  then  titrated  with  normal  alkali. 

The  Zinc-Iron  Method.  Under  this  name  the  Association  of  Offi- 
cial Agricultural  Chemists  adopted  the  following  simplied  procedure.* 

"Ten  grams  of  the  sample  are  dissolved  in  500  cc.  of  water.  Of 
this  solution  25  cc.,  corresponding  to  one -half  gram,  are  placed  in  a 

*  Bulletin  No.  107,  U.  S.  Dept.  of  Agriculture. 


298  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

distillation  flask  of  about  400  cc.  capacity,  120  cc.  of  water  added,  also 
about  5  gms.  of  well  washed  and  dried  zinc  dust  and  an  equal  weight 
of  reduced  iron.  To  the  solution  are  added  80  cc.  of  sodium  hydroxid 
of  32°  B.  The  flask  is  then  connected  with  the  condensing  apparatus 
and  the  distillation  carried  on  synchronously  with  the  reduction,  the 
ammonia  being  collected  in  carefully  standardized  acid.  The  distilla- 
tion is  continued  for  one  or  two  hours,  or  until  100  cc.  have  been  dis- 
tilled. The  resulting  distillate  is  titrated  as  in  the  Kjeldahl  method." 

This  method  is  applicable  to  the  determination  of  nitric  and  ammo- 
niacal  nitrogen. 

The  Ulsch  Method.  This  method  may  be  used  for  the  estimation 
of  alkali  nitrates  when  no  ammonium  salt  or  other  form  of  nitrogen 
is  present.  It  depends  upon  the  fact  that  when  a  solution  of  an  alkali 
nitrate  is  heated  with  dilute  sulphuric  acid,  together  with  some  reduced 
iron,  the  nitrogen  is  converted  into  ammonium  sulphate,  and  the  ammo- 
nia is  then  distilled  off  by  boiling  with  sodium  hydroxid,  as  in  Kjeldahl 's 
method. 

The  Process.  0.5  gm.  of  the  alkali  nitrate  is  dissolved  in  25  cc. 
of  water  (or  an  equivalent  quantity  of  manure  solution,  though  not 
more  than  30  cc.),  and  put  into  a  150  cc.  flask,  together  with  5  gms. 
of  reduced  iron  and  20  cc.  of  dilute  sulphuric  acid  (1.3).  The  reaction 
is  allowed  to  go  on  in  the  cold  until  effervesence  ceases,  the  flask  being 
placed  in  an  oblique  position.  Then  the  contents  of  the  flask  are 
boiled  for  six  or  seven  minutes  and  allowed  to  cool.  The  liquid  is  then 
transferred  to  a  Kjeldahl  distilling  flask,  an  excess  of  soduim  hydroxid 
and  a  few  pieces  of  zinc  are  added.  The  distillation  of  the  ammonia 
is  then  done  as  in  the  Kjeldahl  process. 

If  the  iron  is  suspected  of  containing  some  form  of  nitrogen  or 
ammonia,  a  blank  experiment  should  be  made  with  the  iron. 

The  following  modification  is  that  adopted  by  the  Association  of 
Official  Agricultural  Chemists,  Bulletin  No.  107,  U.  S.  Dept.  Agriculture. 

Ulsch  Method  (Modified  by  Street).  (Applicable  to  all  nitric  and 
ammoniacal  nitrogen  determinations.) 

Place  i  gm.  of  the  sample  in  a  half  liter  flat-bottom  flask.  Add 
about  30  cc.  of  water,  2  to  3  gms.  of  reduced  iron,  and  10  cc.  of  a  mix- 
ture of  strong  sulphuric  acid  with  an  equal  volume  of  water;  shake 
well  and  allow  to  stand  for  a  short  time.  This  will  remove  the  danger 
of  an  explosion  caused  by  the  otherwise  violent  action  which  takes 
place.  Close  the  neck  of  the  flask  with  a  rubber  stopper  through 
which  passes  a  dropping  bulb  filled  with  water.  The  flask  having 
thus  been  stoppered,  place  it  on  a  slab  to  which  a  moderate  heat  is 
applied.  Heat  the  solution  slowly,  boil  it  for  five  minutes,  and  cool. 
Add  about  100  cc.  of  water,  a  little  paraffin,  and  from  7  to  10  gms. 
of  magnesium  oxid.  The  flask  is  then  connected  with  a  condenser, 


SCHLOSING'S   METHOD  299 

such  as  is  used  in  the  Kjeldahl  method,  and  the  mixture  boiled  for 
forty  minutes,  the  ammonia  being  collected  in  a  known  amount  of 
standard  acid.  When  magnesia  is  used,  assurance  must  be  had  that 
it  is  strongly  in  excess,  and  forty  minutes  are  necessary  for  the  com- 
plete distillation  of  the  ammonia.  The  contents  of  the  receiver  are 
titrated,  as  in  the  Kjeldahl  method. 

In  the  analysis  of  nitrate  salts,  proceed  as  above,  except  that  25  cc. 
of  the  nitrate  solution  equivalent  to  0.25  gm.  of  the  sample  are  employed 
with  5  gms.  of  reduced  iron.  After  boiling  add  75  cc.  of  water,  and 
an  excess  of  sodium  hydroxid,  and  complete  the  determination  as 
above. 

The  Kjeldahl  process,  as  modified  by  Jodlbauer,  may  be  used 
in  the  presence  of  non-nitrogenous  organic  matter,  with  very  satisfactory 
results.  This  method  is  described  on  page  290. 

The  Kjeldahl  and  Gunning  methods,  modified  to  include  nitrogen 
of  nitrates,  as  adopted  by  the  Association  of  Official  Agricultural 
Chemists,  may  also  be  employed  with  good  results. 

The  Pelouze  method  depends  upon  the  oxidation  of  ferrous  salts 
by  the  nitrate  in  the  presence  of  hydrochloric  acid.  This  method, 
which  is  described  on  page  172,  is  theoretically  perfect,  but  in  practice 
serious  errors  are  liable  to  creep  in.  It  is  not  available  in  the  presence 
of  organic  matter.  This  method  has  been  variously  modified,  and 
with  care  good  results  may  be  obtained.  Among  the  modifications 
may  be  mentioned,  (a)  the  use  of  potassium  dichromate  instead  of 
permanganate  in  the  titration  of  the  residual  ferrous  salt;  (b)  the 
direct  titration  of  the  resulting  ferric  salt  by  means  of  standard  stannous 
chlorid,  after  the  manner  described  on  page  231 ;  (c)  the  iodometric 
estimation  of  the  resulting  ferric  salt,  after  the  manner  described  on 
page  224.  An  excess  of  potassium  iodid  is  added,  and  the  liberated 
iodin  estimated  with  sodium  thiosulphate. 

Schlosing's  Method.  This  method  was  devised  for  the  estima- 
tion of  nitric  acid  in  tobacco.  It  affords  the  very  important  advantage 
that  it  may  be  used  in  the  presence  of  organic  matter,  and  has  suc- 
cessfully passed  through  the  ordeal  of  numerous  and  searching  experi- 
ments. It  is  conducted  in  the  apparatus  shown  in  Fig.  75. 

In  this  method  the  solution  of  nitrate  is  boiled  in  a  flask  until  all 
air  is  expelled,  then  an  acid  solution  of  ferrous  chlorid  is  drawn  in, 
the  mixture  boiled,  and  the  resulting  nitric  oxid  collected  over  mercury 
in  a  bell  jar  filled  with  mercury  and  milk  of  lime;  the  gas  is  finally 
brought  without  loss  in  contact  with  oxygen  and  water,  so  as  to  con- 
vert it  into  nitric  acid,  which  latter  is  then  titrated  with  standard  alkali 
in  the  usual  manner.  The  reaction  is: 

6FeCl2 +2KN03  +8HC1  =3Fe2Cl6 +2KC1 +4H2O  +N2O2. 


300  A    MANUAL  OF   VOLUMETRIC   ANALYSIS 

The  nitrate  is  dissolved  and  introduced  into  the  flask  A ,  the  drawn- 
out  neck  of  which  is  connected  by  means  of  a  rubber  tube  a  with 
the  glass  tube  b,  at  the  lower  end  of  which  is  another  rubber  tube  c 
six  inches  in  length.  The  solution  of  the  salt  which  must  be  neutral 
or  alkaline  is  boiled  down  to  a  small  volume,  so  that  the  aqueous  vapor 
will  completely  expel  all  of  the  air  from  the  flask  and  tubes.  The 
end  of  the  tube  c  is  then  placed  into  a  solution  of  ferrous  chlorid  in 
hydrochloric  acid  contained  in  a  small  glass  vessel.  The  flame  is 
now  removed,  and  the  condensation  of  the  aqueous  vapor  in  the  flask 
will  cause  the  ferrous  chlorid  solution  to  rise  and  enter  the  flask  A. 
This  may  be  regulated  by  compressing  the  tube  c  with  the  fingers. 
When  sufficient  of  the  iron  solution  is  absorbed,  a  little  hydrochloric 


.'•"'I:'":;!!!!:  I",;'!.:'      1  ,',:;•"!';;:•:'!  : .-^i;.  l:..;.       1      v  11      


Fw.  75- 

acid  is  allowed  to  pass  in,  in  the  same  manner,  in  three  or  four  portions  in 
order  to  completely  wash  the  tubes  of  any  ferrous  chlorids.  In  these 
operations,  as  well  as  in  all  subsequent  transfers,  it  is  important  to  ex- 
clude all  traces  of  air.  This  is  easy,  provided  the  drop  of  liquid  which 
hangs  to  c  when  it  is  carried  from  one  liquid  to  another  is  not  allowed  to 
fall  off.  Finally,  the  tube  is  rinsed  by  allowing  a  little  boiled  water 
to  recede,  and  then  the  tube  c  closed  by  means  of  an  iron  compression- 
cock  while  still  filled  with  water,  and  its  end  immersed  into  the  mer- 
cury in  the  trough  and  brought  up  under  the  bell  jar  B.  Heat  is  again 
applied  under  the  flask  A,  and  its  contents  will  boil  violently  and 
become  black.  As  soon  as  pressure  is  evident  within  the  tube  c  the 
compression-cock  is  removed  and  the  nitric  oxid  gas  allowed  to  pass 
into  the  bell  jar  receiver  B.  The  reaction  is  usually  complete  in  about 
eight  minutes,  when  the  tube  may  be  removed  from  under  B.  The 
receiver  B  is  a  small  bell  jar  made  from  an  adapter,  and  should  have 
a  capacity  of  three  or  four  times  the  volume  of  the  gas  to  be  received. 


NITRIC    AND     CHLORIC    ACIDS  301 

In  cases  where  the  evolution  of  gas  is  too  rapid  it  may  be  necessary 
to  immerse  the  entire  receiver,  hi  order  to  facilitate  the  condensation 
of  the  vapors.  The  upper  part  of  the  receiver  is  drawn  out  so  as  to 
admit  of  its  insertion  into  the  rubber  tube  from  C  and  the  breaking 
off  of  its  point.  The  receiver  is  first  filled  with  water  in  order  to  expel 
all  the  air,  and  then  with  mercury. 

Then  by  means  of  a  curved  pipette  a  sufficient  quantity  of  thick 
and  well-boiled  milk-of-lime  is  introduced.  This  is  -to  absorb  the 
hydrochloric  acid  which  boils  over  from  the  flask,  and  thus  the  nitric 
oxid  is  obtained  free  from  traces  of  acid  vapor.  Should  the  nitric 
oxid  be  evolved  in  quantities  greater  than  the  receiver  will  hold  at 
once,  the  cock  is  closed,  and  the  flame  removed  from  under  the  flask. 
The  receiver  is  then  emptied,  as  below  described,  charged  anew  with 
water,  mercury,  and  milk-of-lime,  reconnected  and  the  boiling  resumed. 
When  the  nitric  oxid  has  been  completely  collected  in  the  receiver  it 
is  transferred  to  the  flask  C,  there  to  be  converted  into  nitric  acid  by 
means  of  oxygen.  The  flask  C  contains  a  small  quantity  of  water 
(about  100  cc.);  it  is  provided  with  a  rubber  tube  d  connected  with 
a  glass  tube  e,  the  other  end  of  which  carries  a  rubber  tube /five  inches 
long.  The  water  in  C  is  boiled  to  expel  all  atmospheric  air,  and 
while  still  vigorously  boiling,  /  is  connected  with  the  tip  of  the  bell- 
jar  B  which  has  previously  been  slightly  scratched  with  a  diamond 
and  the  tip  then  broken  off.  At  first  the  aqueous  vapor  condenses 
in  the  bell  jar  and  expels  at  the  same  time  the  small  quantity  of  milk- 
of-lime  remaining  in  the  tip.  On  now  removing  the  flame  a  return 
current  is  soon  established,  which  draws  the  nitric  oxid  into  C.  If 
this  proceeds  too  rapidly  /  is  compressed  by  the  fingers ;  as  soon  as 
the  milk-of-lime  in  the  bell  jar  has  reached  the  rim  of  /,  the  latter  is 
closed  by  a  compression-cock;  20  to  30  cc.  of  pure  hydrogen  (washed 
by  passing  through  sulphuric  acid  and  through  milk-of-lime)  are  now 
introduced  into  the  bell  jar  in  order  to  complete  the  transferral  of  the 
nitric  oxid  into  C.  The  tube  /  is  now  removed  from  the  tip  of  the 
bell  jar  and  connected  with  the  glass  tube  h  of  the  jar  D,  which  con- 
tains oxygen  under  pressure;  the  cock  r  is  now  opened  and  the 
oxygen  allowed  to  enter  C.  It  is  absorbed  with  the  appearance  of 
red  fumes  and  the  formation  of  nitric  acid.  The  cock  r  is  closed,  the 
jar  D  separated,  and  after  half  an  hour,  the  flask  being  occasionally 
shaken,  the  nitric  acid  is  dissolved  in  the  water  and  may  be  estimated 
by  a  standard  alkali  solution. 

The  lodometric  Estimation  of  Nitric  and  Chloric  Acids. 
McGowan,  J.  C.  S.,  LXIX,  530  and  LXI,  87,  describes  methods  for  the 
iodometric  estimation  of  these  acids. 

The  method  is  based  upon  the  principle  that  when  a  concentrated 
solution  of  a  nitrate  or  chlorate  is  warmed  with  an  excess  of  pure, 


302  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

strong  hydrochloric  acid,  the  salts  are  completely  decomposed,  the 
nitrate  being  resolved  into  nitrosyl  chlorid  and  chlorin.  The  reaction 
is  as  follows: 

HN03  +  3HC1= NOC1  +  C12  +  2H20. 

With  chlorates  only  chlorin  is  evolved.  The  operation  is  to  be 
conducted  in  an  atmosphere  of  carbon  dioxid,  and  the  escaping  gases 
passed  through  a  solution  of  potassium  iodid.  The  amount  of  iodin 
liberated  is  exactly  equivalent  to  the  whole  of  the  chlorin  present 
(free  and  combined),  nitric  oxid  escaping  in  the  operation  upon  nitric 
acid.  One  molecule  of  nitric  acid  thus  liberates  three  atoms  of  iodin 
which  can  be  titrated  in  the  usual  manner  with  standard  thiosulphate 

The  details  of  the  methods  are  given  in  Button's  "Volumetric 
Analysis,"  and  in  Z.  f.  angew.  Chem.,  Aug  15,  1890,  Details  of  a 
process  depending  upon  the  same  principle  are  given  by  De  Koninck 
and  Nihoul. 

Nitrous  Acid.  Estimation  by  Means  of  Standard  Permanganate. 
The  direct  titration  of  nitrites  by  means  of  potassium  permanganate 
is  not  usually  employed,  because  nitrites  in  acidulated  solution  evolve 
nitrous  acid,  which  being  very  volatile  will  occasion  a  considerable 
loss.  This  especially,  since  the  complete  oxidation  of  the  nitrous 
acid  requires  a  slightly  raised  temperature.  This  loss  may  be  to 
some  extent  obviated  by  taking  very  dilute  solutions  of  nitrite  for 
analysis,  say  not  stronger  than  one  part  of  nitrite  in  one  liter  of 
water.  The  most  satisfactory  ways  of  estimating  nitrites  by  means 
of  potassium  permanganate  are  described  on  page  165.  The  per- 
manganate may  be  added  in  excess  to  the  nitrite  solution  and  the 
excess  then  found  by  retitrating  with  oxalic  acid,  or  the  nitrite  in  solu- 
tion may  be  delivered  from  a  burette  into  a  measured  quantity  of  stand- 
ard permanganate  until  the  latter  is  just  decolorized. 

The  standardization  of  the  permanganate  solution  used  in  this 
estimation  is  best  accomplished  by  titration  against  a  weighed 
amount  of  pure  silver  nitrite,  which  is  prepared  as  follows:  Pure 
potassium  nitrate  is  fused  at  a  strong  red  heat  for  some  time,  allowed 
to  cool,  dissolved  in  hot  water,  and  the  greater  part  of  the  undecom- 
posed  nitrate  crystallized  out ;  the  potassium  nitrite,  being  very  soluble, 
remains  in  the  mother  liquor.  A  small  portion  of  the  mother  liquor 
is  then  treated  with  a  few  drops  of  silver  nitrate.  If  the  precipitate  is 
brown  or  yellowish,  the  heat  has  been  too  intense,  and  a  portion  of 
the  nitrite  been  decomposed.  Add  nitric  acid  to  the  rest  of  the  mother 
liquor  until  a  portion  tested  with  silver  nitrate  gives  a  white  precipitate, 
and  then  add  silver  nitrate  until  a  precipitate  ceases  to  form.  The 
precipitate  is  washed  with  a  little  cold  water,  dissolved  in  boiling  water, 


THE    IODOMETRIC    METHOD  303 

and  recrystallized  once  or  twice.  The  crystals  obtained  have  the 
composition  AgNC>2. 

In  the  absence  of  nitrates,  nitrous  acid  and  nitrites  may  be  esti- 
mated by  converting  the  nitrogen  into  ammonia,  and  estimating  the 
latter,  as  described  under  nitric  acid,  or  by  determining  the  oxidizing 
action  on  ferrous  salt.  In  the  presence  of  nitric  acid  the  permanganate 
method  may  be  used. 

To  estimate  the  quantity  of  nitrogen  tetroxid  in  red  fuming  nitric 
acid,  place  a  few  cubic  centimeters  in  about  500  cc.  of  cold,  distilled 
water,  and  determine  the  quantity  of  nitrous  acid  produced— one 
molecule  of  nitrous  anhydrid  found  corresponds  to  two  molecules  of 
nitrogen  tetroxid.  The  latter,  when  mixed  with  a  large  quantity  of 
water  as  above,  is  decomposed  into  nitric  and  nitrous  acid. 

N2O4+H2O=HNO3+HNO2. 

The  Estimation  of  Nitrous  Acid  in  the  Chamber  Acid  of 
Sulphuric  Acid  Works.  The  specific  gravity  is  first  determined,  then 
the  acid  is  delivered  out  of  a  small  burette  into  a  measured  quantity 
of  standard  permanganate.  The  latter  must  be  largely  diluted,  pref- 
erably with  hot  water.  The  addition  of  sulphuric  acid  is  not  abso- 
lutely necessary,  but  it  is  a  good  plan  to  add  a  small  quantity  before 
titrating,  in  order  to  avoid  any  possibility  of  a  precipitation  of  manga- 
nese dioxid.  The  calculation  is  then  made  as  follows: 

The  weight  of  the  acid  taken  is  determined  by  multiplying  the 
volume  consumed  in  the  titration  by  the  specific  gravity.  Then,  know- 
ing the  titer  of  the  permanganate  solution,  we  can  readily  calculate  the 
quantity  of  nitrous  acid. 

N 
Each  cc.  of  —  potassium  permanganate =0.0023345  gm.  of  HNC>2. 

The  lodometric  Method.*  This  method  is  based  upon  the 
reaction  between  nitrous  acid  and  an  iodid  in  which  an  equivalent  of 
iodin  is  liberated,  as  shown  in  the  following  equation: 

2HN02+2HI=2H20-f2NO+I2. 

The  liberated  iodin  is  then  estimated  by  means  of  standard  thio- 
sulphate  in  the  usual  manner. 

The  process  must  be  conducted  with  complete  exclusion  of  air, 
in  order  to  avoid  oxidation  of  the  NO,  and  a  consequent  excessive 
liberation  of  iodin.  If  the  operation  is  conducted  in  an  atmosphere 
of  carbon  dioxid  or  hydrogen,  very  good  results  may  be  obtained. 

*  Legler,  Ph.  Centralh.  March  9,  1905. 


304  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  simple  apparatus  described  below  will  answer  very  well  for 
this  purpose. 

A  small,  wide-mouthed  flask  is  fitted  with  a  rubber  stopper  having 
four  perforations;  through  one  of  these  an  inlet  tube  is  passed  which 
reaches  below  the  surface  of  the  liquid  in  the  flask;  into  another, 
an  outlet  tube  is  fitted,  the  inner  end  of  which  is  flush  with  the  lower 
surface  of  the  stopper,  and  the  other  end  dips  into  water  to  prevent 
entrance  of  atmospheric  air.  Into  the  other  holes  the  tips  of  two 
burettes  are  fitted.  One  of  these  burettes  contains  dilute  sulphuric 

N 

acid  and  the  other  —  thiosulphate. 
10 

The  Process.  Into  the  flask  is  introduced  a  quantity  of  the  nitrite 
solution  to  be  examined,  containing  about  o.i  gm.  of  nitrous  acid, 
and  the  solution  boiled  to  drive  out  air.  5  cc.  of  a  10  per  cent  solu- 
tion of  potassium  iodid,  or  a  few  small  crystals  of  the  dry  salt,  together 
with  a  few  drops  of  starch  solution  are  added,  and  the  flask  imme- 
diately closed  with  the  stopper  and  attached  apparatus.  A  good 
stream  of  CC>2  or  Hk  is  then  passed  through  the  apparatus  in  order 
to  completely  drive  out  all  the  air.  5  or  10  cc.  of  diluted  sulphuric 
acid  are  then  introduced,  and  after  standing  a  short  time  the  liberated 

N 
iodin  is  estimated  in  the  flask  by  means  of  the  —  thiosulphate.    The 

tips  of  the  two  burettes  must  of  course  be  completely  filled  with  the 
reagents,  so  that  no  air  will  be  introduced  when  their  contents  are 

N 
delivered  into  the  flask,     i  cc.  of  —  thiosulphate =0.01259  gm-  of 

iodin,  which  in  turn  represents  0.004669  gm.  of  HNC>2. 


CHAPTER  XX 
OXALIC  ACID  AND  OXALATES 

FREE  oxalic  acid  may  be  estimated  by  direct  titration  with  standard 
alkali.  See  page  105. 

It  may  likewise  be  estimated  by  adding  an  excess  of  calcium  acetate 
to  the  solution,  which  must  be  neutral  or  very  faintly  acidified  with 
acetic  acid.  Alumina,  chromium  sesqui  oxid,  copper,  and  ferric  salts 
must  be  absent.  The  precipitation  is  made  in  a  hot  solution.  The 
precipitate  of  calcium  oxalate  is  washed  with  hot  water,  heated  to 
redness  in  a  crucible,  whereby  it  is  converted  into  a  mixture  of  calcium 
carbonate  and  oxid,  and  is  then  dissolved  in  an  excess  of  normal  acid, 
and  the  excess  found  by  retitration  with  normal  alkali. 

The  best  method,  however,  for  oxalic  acid  and  oxalates  as  well, 
is  the  permanganate  method,  described  on  page  153. 

If  the  titration  is  conducted  with  an  empirical  permanganate  solu- 
tion, standardized  by  means  of  iron,  we  must  take  into  consideration 
that  2KMnO4  is  equivalent  to  five  molecules  of  H2C2C>4  (=446.70) 
on  the  one  hand  and  ten  atoms  of  iron  (=555)  on  the  other. 

Hence  if  in  the  estimation  of  a  weighed  quantity  of  oxalic  acid, 
40  cc.  of  a  permanganate  solution  (i  cc.  of  which= 0.0057  SP*-  Fe) 
are  consumed.  The  quantity  of  iron  represented  by  40X0.0057  is 

calculated  into  oxalic  acid  by  multiplying  it  by  -  —  for  anhydrous 
oxalic  acid. 
Thus 

40X0.0057X^-^=0.183+  gm. 

For  crystallized  oxalic  acid  multiply  by 

5X125.1 

555 

For  oxalic  acid  anhydrid  (C2O3)  multiply  by 

357-3 


v 

5  X  —  or 

5      555  555 


3«>S 


306  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

In  the  titration  of  salts  of  oxalic  acid  the  acidulation  with  sul- 
phuric acid  serves  not  only  to  prevent  the  precipitation  of  manganese 
dioxid,  but  also  to  liberate  the  oxalic  acid,  a  sulphate  of  the  metal 
being  formed  as  a  by-product.  The  titration  of  oxalates  by  this  method 
can  be  carried  out  not  only  with  such  salts  as  form  clear  solutions 
with  the  excess  of  sulphuric  acid,  but  also  with  those  which  form 
insoluble  or  scantily  soluble  sulphates,  as  calcium,  strontium,  barium, 
and  lead.  With  the  raised  temperature  and  frequent  shaking,  the 
reaction  is  a  quantitative  one,  and  the  presence  of  the  white  precipitate 
of  sulphate  in  no  way  interferes  with  the  recognition  of  the  end-reaction. 


CHAPTER  XXI 
OXYGEN  AND  PEROXIDS 

Oxygen  Dissolved  in  Water.  Mohr's  method,  which  depends 
upon  the  oxidation  of  ferrous  salts  and  subsequent  titration  with  potas- 
sium permanganate,  is  briefly  as  follows: 

When  a  solution  of  ferrous  sulphate  made  strongly  alkaline  is 
poured  into  water  containing  dissolved  oxygen,  the  latter  is  com- 
pletely absorbed,  and  upon  then  acidifying  the  solution,  an  amount 
of  ferric  sulphate  is  produced  equivalent  to  the  dissolved  oxygen  of 
the  water. 

The  water  to  be  tested  is  placed  in  a  bottle  provided  with  a  care- 
fully ground  stopper,  and  a  weighed  amount  of  iron  wire  dissolved 
in  sulphuric  acid,  or  of  ferrous  -ammonium  sulphate,  dissolved  in 
water,  is  added.  The  bottle  is  then  filled  with  pure  carbon  dioxid,  an 
excess  of  sodium  hydroxid  solution  added,  the  bottle  quickly  closed, 
shaken,  and  allowed  to  stand  for  ten  or  fifteen  minutes.  The  stopper 
is  then  removed,  an  excess  of  sulphuric  acid  introduced,  and  finally,  the 

N 
mixture  titrated  with  —  potassium  permanganate  for  excess  of  ferrous 

salt.  In  filling  the  bottle  with  carbon  dioxid,  the  gas  must  not  be 
allowed  to  run  into  the  bottle  longer  than  is  necessary  to  fill  it,  as  other- 
wise a  part  of  the  oxygen  dissolved  in  the  water  will  diffuse  into  it, 
and  the  results  obtained  will  be  too  low. 

N 
Each  cc.  of  —  permanganate  represents  0.038934  gm.  of 

FeSO4+(NH4)2SO4+6H2O  and  0.000794  gm.  of  oxygen. 

Therefore  by  multiplying  the  quantity  of  ferrous  ammonium  sul- 
phate which  was  oxidized  by  0.0204  gm.  we  arrive  at  the  quantity  of 
oxygen  which  was  contained,  dissolved  in  the  water.  Of  course  the 
quantity  of  the  iron  salt  found  by  titration  must  be  deducted  from 
the  quantity  originally  added  to  the  water. 

The  reaction  may  be  expressed  by  this  equation: 


307 


308  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

Example.     0.5  gm.  of  the  ferrous  ammonium  sulphate  were  em- 

N 
ployed,  and  in  the  titration  10.4  cc.  of  —  permanganate  were  required. 

0.5  gm.  of  ferrous  ammonium  sulphate  are  represented  by  12.84  cc. 

N 
of  —  permanganate,  therefore  12.84  —  10.40=2.44,    the   quantity   of 

N 
—  permanganate  which  represents  the  dissolved  oxygen  in  the  water. 

2.44X0.000794=0.00193736  gm.  oxygen. 

The  calculation  may  also  be  made  as  follows: 
15.88  parts  by  weight  of  oxygen,  will  oxidize  2X55.5  parts  of  iron, 
as  the  above  equation  shows. 

N 
One  cubic  centimeter  of  —  permanganate =0.00555  gm-  °f  iron- 

0.5  gm.  of  ferrous  ammonium  sulphate= 0.0719  gm.  of  iron.     If  10.4  cc. 
of  the  permanganate  solution  are  required,  then  10.4X0.00555  deducted 
from  0.0719=0.01418  gm.  iron  oxidized. 
Then 

0.01418X15.88 

—  =  0.00202+  gm.  oxygen. 

2X55-5 

Schutzenberger's  Method.  This  method  depends  upon  the  use 
of  sodium  hyposulphite  (Na2SC>2)  as  a  reducing  agent.  20  to  30  cc. 
of  water  are  introduced  into  a  large  Woulff's  bottle  of  about  2000  cc. 
capacity,  filled  with  pure  hydrogen.  Indigo,  carmine  solution  is  added 
in  sufficient  quantity  to  color  the  solution  slightly  blue.  Standard 
hyposulphite  solution  is  then  cautiously  added  so  as  to  just  discharge 
the  color.  To  this  decolorized  liquid,  250  cc.  of  the  water  to  be  ex- 
amined are  added.  The  dissolved  oxygen  present  restores  the  blue 
color  by  oxidation,  and  the  amount  of  hyposulphite  solution  now 
required  to  again  decolorize  the  liquid  is  the  measure  of  the  dissolved 
oxygen  present.  The  hyposulphite  solution  is  prepared  by  the  action 
of  zinc  dust  on  a  saturated  solution  of  sodium  bisulphite,  containing 
an  excess  of  sulphurous  acid.  For  more  detailed  descriptions  of  this 
method  and  its  modifications,  see  Sutton's  "Volumetric  Analysis," 
9th  edition. 

See  also  International  Scientific  Series,  "Fermentation,"  by  P. 
Schiitzenberger;  Schutzenberger  and  Risler,  J.  Chem.  Soc.,  1873, 
p.  936;  Roscoe  and  Lunt,  J.  Chem.  Soc.,  1889,  p.  552;  Ramsay  and 
Williams,  J.  Chem.  Soc.,  1886,  p.  751;  Dupre,  Analyst.,  x,  156;  M.  A. 
Adams,  J.  Chem.  Soc.,  LXI,  310. 


HYDROGEN  DIOXID  309 

Other  methods  are. 

The  lodometric  Method  of  Thresh,  J.  Chem.  Soc.,  LVII,  185. 

The  Manganous  Hydrate  Method  of  Winkler,  Berichte,  1888-2851. 

Seyler's  Modification  of  Foregoing,  C.  N.,  LXX,  151. 

Hydrogen  Dioxid  (Peroxid  of  Hydrogen).  The  assay  of  hy- 
en  dioxid  solution  may  be  conducted  titrimetrically  by  either  the 
permanganate  method  or  the  iodometric  method  of  Kingzett;  the  former 
is  described  on  page  156,  the  latter  on  page  213.  Gasometric  methods 
are  described  in  Part  IV.  Of  all  of  these  methods  the  Kingzett  method 
is  decidedly  the  best,  in  that  it  is  very  easily  carried  out,  and  that  its 
accuracy  is  practically  unaffected  by  the  presence  of  any  of  the  sub- 
stances usually  added  as  preservatives.  It  is  the  best  method  for 
ethereal  peroxid  solutions. 

The  accuracy  of  the  permanganate  methods  is  slightly  affected  by 
the  presence  of  moderate  quantities  of  glycerin  or  boroglycerin,  but 
in  the  presence  of  the  quantities  that  would  be  added  to  the  solution 
by  manufacturers  (which  would  hardly  exceed  i  per  cent)  the  differ- 
ence in  results  would  scarcely  be  noticeable.  In  the  presence  of  sali- 
cylic acid,  however,  this  method  is  worthless.  Ether  also  has  a  marked 
disturbing  influence  upon  the  accuracy  of  the  results.  In  fact,  gener- 
ally speaking,  in  the  presence  of  organic  matter  the  permanganate 
methods  are  not  to  be  depended  upon.  As  regards  the  gasometric 
methods  in  general,  they  require  more  time,  attention,  and  apparatus 
than  titration  methods,  and  the  results  obtained  by  them  do  not  ap- 
proach the  latter  in  accuracy,  unless  suitable  corrections,  requiring 
tedious  calculation,  are  made  for  variation  in  temperature  and  atmo- 
spheric pressure. 

Among  the  other  methods  for  the  assay  of  hydrogen  dioxid  may  be 
mentioned 

(a)  Decomposition  by  Means  of  Silver  Oxid  and  Measurement  of 
the  Oxygen  Liberated.  The  reaction,  according  to  E.  Rieger,  is  rep- 
resented by  the  following  equation: 


The  Ag4O  being  further  decomposed  into  Ag2O  and  Ag2. 

(b)  Decomposition  by  Means  of  Potassium  Ferricyanid  in  Alkaline 
Solution.  Quincke,  Zeitschr.  f.  anorg.  Chem.,  13,  i.  The  volume  of 
oxygen  liberated  is  noted,  and  the  strength  of  the  hydrogen  dioxid 
solution  calculated  therefrom. 

The  equation  is 


(c)  By  Oxidation  of  Arsenous  Acid.     According  to  B.  Griitzner, 
Arch.  d.    Ph.,   237-507,  the   assay  of   hydrogen  dioxid  can  be  car- 


310  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

ried  out  with  exactness  by  oxidation  of  arsenous  acid  (As2O3)  in 
an  alkaline  solution,  and  then,  by  means  of  standard  iodin  V.  S.,  the 
excess  of  AsO  determined. 


Barium  Dioxid  may  be  assayed  by  the  method  described  on  page  161  . 

Sodium  Peroxid  may  be  assayed  by  titration  with  standard  per- 
manganate. The  peroxid  being  added  to  cold  water  acidulated  with 
sulphuric  acid  and  titrated  with  the  permanganate,  or  a  known  weight 
of  the  peroxid  may  be  added  to  an  excess  of  the  standard  permanga- 
nate, acidulated  with  sulphuric  acid,  and  the  excess  determined  by 
residual  titration  with  standard  oxalic  acid.  Archbutt  (Analyst,  xx,  5), 
recommends  a  gasometric  method  in  which  precipitated  cobalt  sesqui- 
oxid  is  employed  for  the  decomposition  of  the  peroxid. 

The  Estimation  of  the  Active  Oxygen  in  Inorganic  Per  sulphates 
may  be  carried  out  by  heating  a  weighed  amount  of  the  persulphate 
with  a  solution  of  ferrous  ammonium  sulphate  (Mohr's  salt)  and 
titrating  the  unoxidized  portion  with  standard  permanganate.  This 
method  cannot  be  used  for  organic  persulphates. 


CHAPTER  XXII 
PHOSPHORIC  ACID  AND  PHOSPHATES 

FREE  phosphoric  acid  is  usually  estimated  by  neutralization  in  the 
manner  described  on  page  101. 

Phosphoric  Acid  may  also  be  estimated  by  Stolba's*  method,  as 
Ammonio-magnesium  Phosphate. 

0.2  gm.  of  phosphoric  acid  is  supersaturated  with  ammonia  water, 
so  as  to  convert  all  of  the  acid  into  ammonium  phosphate  and  leave  an 
excess  of  the  alkali. 

H3P04+  2NH4OH=  (NH4)2HP04+  2H2O. 

97.29  131.15 

An  excess  of  magnesia  mixture  f  is  now  added  in  order  to  precipi- 
tate all  of  the  phosphoric  acid  in  the  form  of  ammonio-magnesium 
phosphate. 

(NH4)2HPO4+MgSO4=Mg(NH4)PO4+NH4HSO4 

131.15  136.40 

The  precipitate  is  washed,  first  with  ammonia  water,  and  then 
the  ammonia  is  entirely  removed  by  washing  with  alcohol  of  50  per 
cent  or  60  per  cent  strength. 

N 

The  precipitate  is  now  dissolved  in  a  measured  excess  of  —  hydro- 

10 

chloric  acid,  a  few  drops  of  methyl-orange  T.  S.  added,   and  the 

N 
excess  of  acid  found  by  titrating  back  with  —  potassium  hydroxid. 

N 

The  difference  between  the  number  of  cc.  of  —  HC1  added  and  the 

10 

*  Fres.  Zeitschrift.,  16,  100. 

f  Magnesia  Mixture.  Dissolve  10  gms.  of  magnesium  sulphate  or  chlorid 
and  20  gms.  of  ammonium  chlorid  in  80  cc.  of  water,  add  42  cc.  of  ammonia 
water,  set  aside  for  a  few  days  in  a  well-stoppered  bottle,  and  filter.  It  should 
never  be  used  freshly  made. 

3" 


312  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

N 
quantity  of  —  KOH  used  gives  the    quantity  of   the  former  which 

went  into  combination  with  the  ammonio-magnesium  phosphate. 

Mg(NH4)P04+2HCl=NH4H2P04+MgCl2. 
136.40  72.36 

By  consulting  the  equations  given,  it  will  be  seen  that  72.36  gms. 
of  HC1  are  equivalent  to  136.4  gms.  of  Mg(NH4)PO4,  or  131.15  gms. 
of  (NH4)2HPO4,  or  97.29  gms.  of  H3PO4. 


This  means  that  1000  cc.  of  a  decinormal  I  —  )  solution  of  HC1, 

\io/ 

containing  3.618  gms.  of  the  acid,  represents  -^V  of  each  of  these  quan- 

N 
tities;  and  one  cc.  of  —  HC1  thus  represents  0.004864  gm.  of  phosphoric 

acid. 

In  this  estimation  care  must  be  taken  that  all  free  ammonia  is 
removed  from  the  precipitate,  and  that  the  whole  of  the  ammonio- 
magnesium  phosphate  is  decomposed  by  the  acid  before  titration  with 

N 
the  —  alkali.     This  may  be  insured  by  using  a  rather  large  excess  of 

the  acid  and  warming. 

Example.     To  the  precipitate  of  ammonio-magnesium  phosphate 

N 
obtained  from  0.2  gm.  of  phosphoric  acid,  50  cc.  of  —  HC1  are  added. 

N 
In  titrating  back  15.3  cc.  of  —  KOH  are  required.     Hence  34.7  cc. 

of  the  acid  went  into  combination  with  the  double  salt. 
Then 

34.7X0.004864=0.16878  +  gm., 
and 

0.16878X100 


.2 


=  84.39  Per  cent  °f  absolute  phosphoric  acid. 


This  method  is  said  to  give  good  results. 

By  Precipitation  as  Uranium  Phosphate.  One  of  the  best 
methods  for  the  estimation  of  phosphates  is  by  means  of  a  standard 
solution  of  uranium.  The  process  is  conducted  in  hot  solutions  con- 
taining acetic  acid.  The  uranium  solution  is  run  into  the  solution 
under  examination,  until  a  drop  of  the  latter  gives  a  reddish  brown 
color  when  brought  in  contact  on  a  white  porcelain  slab  with  a  drop 
of  potassium  ferrocyanid  solution. 

The  Standard  Uranium  Solution  is  prepared  by  dissolving  38.5  gms. 
of  uranium  acetate  or  an  equivalent  quantity  of  yellow  uranium  oxid 


BY  PRECIPITATION    AS    URANIUM    PHOSPHATE        313 

and  about  50  cc.  of  glacial  acetic  acid  in  about  900  cc.  of  water  and 
then  so  adjusting  the  solution  that  it  and  the  standard  phosphate  solu- 
tion will  correspond,  volume  for  volume. 

The  Standard  Phosphate  Solution  is  prepared  by  dissolving  35.56 
gms.  of  crytallized  sodium  phosphate  (Na2HPO4+i2H2O)  in  sufficient 
water  to  make  1000  cc.  Each  cc.  of  this  solution  represents  0.007047  gm. 
of  P2O5. 

Instead  of  using  the  sodium  phosphate  we  may  prepare  the  standard 
phosphate  by  using  microcosmic  salt  (NafNHjHPC^  +  4H2O), 
20.762  gms.  of  which  are  dissolved  in  sufficient  water  to  make  1000  cc. 

Standardization  of  the  Uranium  Solution.  20  cc.  of  the  standard 
phosphate  solution  are  mixed  with  a  quantity  of  very  dilute  acetic 
acid,  the  liquid  is  heated  to  boiling,  then  after  removing  the  flame,  the 
uranium  solution  is  run  in  from  a  burette,  about  five  drops  at  a  time, 
until  a  drop  of  the  solution  gives  the  required  reaction  with  potassium 
ferrocyanid;  2  cc.  of  the  standard  phosphate  solution  are  then  added, 
the  liquid  again  boiled,  and  again  titrated  with  the  uranium  solution. 
The  total  quantity  of  uranium  solution  employed  corresponds  to  22  cc. 
of  the  standard  phosphate  solution,  and  whatever  that  quantity  is  it 
must  be  diluted  to  32  cc.  so  that  it  will  correspond,  volume  for  volume, 
with  the  standard  phosphate  solution.  One  cubic  centimeter  of  it  will 
then  represent  0.007047  gm.  of  P2O5. 

The  Process  for  the  Estimation  of  Phosphoric  Acid  is  carried  out 
exactly  in  the  manner  just  described  for  the  standardization  of  the 
uranium  solution.  It  is  a  good  plan  to  add  2  cc.  of  standard  phos- 
phate solution  to  facilitate  the  determination  of  the  exact  close  of  the 
titration;  the  phosphoric  acid  in  these  2  cc.  must  of  course  be  deducted 
from  the  total  phosphoric  acid  found.  In  determining  phosphoric 
acid  by  this  process,  the  absence  of  all  bases  except  the  alkalies  and 
alkaline  earths  and  manganese  must  be  assured,  likewise  all  non- 
volatile or  reducing  organic  acids,  such  as  citric,  tartaric,  oxalic,  and 
formic  acids,  must  be  absent.  The  presence  of  hydrogen  sulphid, 
sulphurous  oxid,  hydriodic  acid,  or  the  acids  of  arsenic  likewise  interfere. 

Instead  of  titrating  direct  with  the  uranium  solution  as  above 
described,  it  is  usually  a  better  plan  to  add  an  excess  of  the  standard 
uranium  solution,  and  then  retitrate  with  standard  phosphate. 

If  tricalcic  phosphate  is  to  be  estimated,  the  uranium  solution  should 
be  standardized  with  a  solution  of  tricalcic  phosphate,  and  in  the 
process  of  titration  it  is  necessary  to  add  nearly  the  full  amount  of 
uranium  solution  before  boiling  the  mixture,  so  as  to  prevent  the  pre- 
cipitation of  calcium  phosphate  *  which  is  apt  to  occur  in  acetic  acid 
solution  when  heated,  or  the  inverted  process  following,  may  be  used. 

*  Button's  "Volumetric  Analysis,"  Qth  edition. 


314  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  Inverted  Uranium  Method,  which  is  described  as  follows  by 
Muir,  in  his  translation  of  Fleischer's  "Volumetric  Analysis,"*  affords 
exceedingly  accurate  results.  The  alkali  or  alkali  earth  phosphate 
is  dissolved  in  acetic  acid,  the  liquid  made  up  to  a  fixed  volume 
(say  200  cc.),  and  a  portion  of  it  placed  in  a  burette.  20  cc.  of  standard 
uranium  solution  are  heated  almost  to  boiling,  with  the  addition  of 
a  few  drops  of  acetic  acid.  The  liquid  should  remain  perfectly  clear; 
if  a  turbidity  occurs,  more  acetic  acid  must  be  added.  The  phosphate 
solution  is  then  run  in  until  a  drop  of  the  hot  liquid  ceases  to  give  a 
reddish-brown  color  with  potassium  ferrocyanid,  and  the  number  of 
cc.  used  is  noted.  The  liquid  is  again  heated  nearly  to  boiling,  and 
the  standard  uranium  solution  is  cautiously  added  from  another  burette 
until  the  brown  color  is  again  obtained  with  ferrocyanid.  The  number 
of  cc.  of  uranium  solution  used  is  added  to  the  20  cc.  originally  taken; 
the  sum  represents  the  amount  corresponding  to  the  quantity  of  phos- 
phate solution  run  in  from  the  burette.  Each  cc.  of  the  standard 
uranium  solution  represents  0.007047  gm.  of  P2&5- 

Estimation  by  Uranic  Nitrate.f    The  solutions  required  are: 

1.  A  standard  uranium  solution. 

2.  A  standard  phosphate  solution. 

3.  A  solution  of  sodium  acetate  in  dilute  acetic  acid. 

4.  A  freshly  prepared  solution  of  potassium  ferrocyanid. 
Standard   Uranium  Solution.     Either  the  acetate  ^  or  nitrate  of 

uranium  may  be  employed. 

Thirty-five  grams  of  the  salt  are  dissolved  in  about  1000  cc.  of 
water.  The  solution  keeps  better  if  about  50  cc.  of  glacial  acetic  acid 
are  included. 

Standard  Phosphate  Solution.  5.886  gms.  of  crystallized  non- 
effloresced  microcosmic  salt  (ammonio-sodic  phosphate)  are  dissolved 
in  water  and  diluted  to  one  liter. 

Fifty  cubic  centimeters  of  this  solution  will  represent  o.i  gm.  of 
P205. 

The  Sodium  Acetate  Solution  is  made  by  dissolving  100  gms.  of 
sodium  acetate  in  water,  adding  50  cc.  of  glacial  acetic  acid,  and  diluting 
to  one  liter. 

The  standard  uranium  solution  is  titrated  against  the  above  standard 
phosphate  solution  and  diluted  so  that  20  cc.  of  the  uranium  solution 
will  be  equivalent  to  50  cc.  of  the  phosphate  solution.  50  cc.  of  the 
phosphate  solution  are  placed  in  a  beaker,  5  cc.  of  sodium  acetate 

*  Second  edition,  1877,  p.  116. 

f  For  fuller  details,  see  Button's  "Volumetric  Analysis,"  gth  ed. 
|  If  the  acetate  is  used,  the  addition  of  sodium  acetate  may  be  omitted,  since 
the  sodium  acetate  is  added  to  prevent  the  possible  occurrence  of  free  nitric  acid. 


ESTIMATION    BY    U  RAN  1C  NITRATE  315 

solution  are  added,  and  the  mixture  heated  to  nearly  boiling.  The 
uranium  solution  is  then  delivered  in  from  a  burette,  until  a  drop  of 
the  hot  solution  brought  in  contact  on  a  white  porcelain  plate  with  a 
drop  of  the  freshly  prepared  ferrocyanid  solution  produces  a  brown 
color  (uranic  ferrocyanid).  A  second  and  third  titration  should  always 
be  made  so  as  to  ascertain  the  exact  strength  of  the  uranium  solution, 
which  is  then  diluted  so  that  20  cc.  correspond  to  50  of  the  phosphate 
solution.  If  18.7  cc.  were  required,  then  each  18.7  cc.  must  be  diluted 
to  20,  or  935  to  1000. 

In  estimating  phosphoric  acid  by  standard  uranium  solution  it  is 
absolutely  essential  that  all  the  above  conditions  should  be  present. 
That  is,  the  bulk  of  fluid  should  be  the  same,  the  quantity  of  phosphate 
acted  upon  should  be  nearly  the  same  (o.i  gm.  in  50  cc.),  the  same 
relative  amount  of  sodium  acetate,  and  the  same  depth  of  color  in 
testing. 

In  the  analysis,  the  phosphate  is  dissolved  in  water,  if  no  ammonia 
is  present,  i  cc.  of  10  per  cent  solution  is  added  and  neutralized  with 
the  least  possible  quantity  of  acetic  acid;  then  5  cc.  of  sodium  acetate, 
and  water  to  make  50  cc.  The  solution  is  then  heated  to  near  boiling, 
and  the  uranium  solution  run  in  as  described. 

Several  titrations  should  be  made;  the  first  will  give  roughly  the 
amount  required,  and  that  may  be  taken  as  a  guide. 

Each  cc.  of  uranium  solution  =  0.005  gm-  °f  1*205=0069  H3PO4. 
This  method  depends  upon  the  fact  that  when  uranic  nitrate  is  added 
to  a  solution  of  an  orthophosphate  the  whole  of  the  P2O5  is  precipi- 
tated as  yellow  uranyl  phosphate.  If  ammonia  is  present,  the  P2Os 
is  precipitated  as  uranyl-ammoniurn  phosphate. 

If  a  mineral  acid  is  present,  as  when  phosphate  is  dissolved  by 
the  use  of  hydrochloric  or  nitric  acid,  a  corresponding  amount  of  ammo- 
nium acetate  and  ammonia-water  in  excess  must  be  added,  followed  by 
acetic  acid  to  neutralize.  The  reactions  may  be  expressed  as  follows: 

Na2HP04+UO2(NO3)2=UO2HP04+2NaNO3, 
or 


Gluckmann's  Method  (R.  Segalle's  Modification)  (Z.  f.  anal. 
Chem.,  1895).  To  the  solution  of  phosphoric  acid  add  a  measured 
excess  of  normal  ammonia  solution  ;  then  sufficient  of  a  neural  solu- 
tion of  magnesium  sulphate  to  cause  the  precipitation  of  all  of  the 
phosphoric  acid  as  ammonio-magnesium  phosphate.  The  mixture  is 
then  made  up  to  a  definite  volume,  shaken  vigorously,  and  immediately 
filtered. 


316  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

The  excess  of  ammonia  is  then  determined  by  titration  with  normal 
acid  solution.  The  number  of  cc.  of  acid  solution  used  is  deducted 
from  the  quantity  of  normal  ammonia  added,  and  the  difference  rep- 
resents the  phosphoric.  The  reactions  which  take  place  are  represented 
by  the  following  equations  : 


Thus  it  is  seen  that  one  molecule  of  phosphoric  acid  is  neutralized 
by  three  molecules  of  ammonia. 

H3P04  +  3NH3. 

3)97-29        3)5o-79  N 

32.43  gms.=  16.93  gms.  or  1000  cc.  —  V.  S. 

10 

N 

0.03243  gm.=  i  cc.  —  V.  S. 

10 

The  presence  of  the  magnesium  sulphate  does  not  interfere  in  the 
least  with  the  titration  of  the  ammonia. 

Pember  ton's  Molybdic  Method  (Ch.  News,  XLVI,  4).  This 
process  is  based  upon  the  fact  that  if  an  aqueous  solution  of  ammonium 
molybdate  be  added  to  a  hot  solution  of  a  phosphate  in  the  presence 
of  a  large  quantity  of  ammonium  nitrate  and  a  small  excess  of  nitric 
acid,  the  phosphoric  acid  will  be  completely  precipitated  in  the  form 
of  ammonium  phospho-molybdate. 

The  standard  molybdate  solution  is  made  by  dissolving  89.543  gms. 
of  the  crystallized  salt  in  about  900  cc.  of  water,  and  if  the  solution  is 
not  quite  clear  a  few  drops  of  ammonia-water  are  added,  and  it  is 
then  diluted  with  water  to  1000  cc. 

The  solution  should  be  standardized  with  a  solution  of  phosphate 
of  known  strength. 

The  Analysis.  Take  a  quantity  of  the  phosphate  not  containing 
more  than  o.i  gm.  of  ?2Os,  add  a  small  quantity  of  water,  then  2  cc. 
of  nitric  acid  (sp.gr.  1.4)  and  10  gms.  of  granular  ammonium  nitrate, 
and  heat  the  solution  to  140°  F.  or  over.  Then  run  in  some  of  the 
standard  molybdate  solution,  stirring  constantly;  set  aside  in  a  warm 
place  for  a  few  minutes  in  order  to  allow  the  yellow  precipitate  to 
settle  and  leave  the  supernatant  liquid,  not  clear,  but  containing  widely 
disseminated  particles,  in  which  the  yellow  cloud  produced  by  the 
further  addition  of  molybdate  solution  may  be  readily  seen. 

When  the  precipitation  is  thought  to  be  nearly  complete,  the  titra- 
tion is  continued  carefully,  with  the  aid  of  a  Beale's  filter  (Fig.  87). 
By  means  of  the  Beale's  filter  a  small  portion  is  taken  out  of  the  solution 


PEMBERTON'S   NEW  MOLYBDIC  METHOD   MODIFIED    317 

at  intervals,  and  tested  with  a  drop  or  two  of  the  molybdate  solution. 
If  a  precipitate  is  produced  the  solution  is  washed  back  into  the  beaker 
with  a  little  hot  water,  and  the  titration  continued  until  a  portion  of  the 
filtered  solution  tested  as  above  no  longer  yields  a  precipitate. 

If  the  end-point  has  been  overstepped,  a  measured  quantity  of 
phosphate  solution  of  known  strength  is  added,  and  the  titration  with 
molybdate  resumed,  the  quantity  of  phosphate  thus  added  being 
deducted  from  the  amount  found. 

Each  cc.  of  the  molybdate  solution  represents  0.003  g111-  °f  ^2^5 
or  0.004  gin.  H3PC>4. 

About  three  titrations  should  be  made:  the  first  shows  about  how 
much  of  the  molybdate  solution  is  required,  the  second  gives  approxi- 
mate results,  the  third  will  give  exact  results. 

The  process  is  not  reliable  in  the  presence  of  silicates,  organic  matter, 
or  organic  acids. 

Pemberton's  New  Molybdic  Method  Modified  (Bulletin  No.  107, 
U.  S.  Dept.  Agriculture).  This  method  depends  upon  the  precipita- 
tion of  ammonium  phospho-molybdate,  and  then  titrating  the  preci- 
pitate alkalimetrically.  The  process  requires  great  delicacy  of  manipu- 
lation, but  gives  excellent  results.  It  is  especially  suitable  for  fertilizers. 

The  solutions  required  are: 

Molybdate  Solution.  Dissolve  100  gms.  of  molybdic  acid  in  144  cc. 
of  ammonium  hydroxid,  specific  gravity  0.90,  and  271  cc.  of  water; 
slowly,  and  with  constant  stirring,  pour  the  solution  thus  obtained  into 
489  cc.  of  nitric  acid  (sp.gr.  1.42),  and  1148  cc.  of  water.  Keep  the 
mixture  in  a  warm  place  for  several  days,  or  until  a  portion  heated  to 
40°  C.  deposits  no  yellow  precipitate  of  ammonium  phosphomolyb- 
date.  Decant  the  solution  from  any  sediment  and  preserve  in  glass- 
stoppered  vessels. 

For  use  add  to  100  cc.  of  this  solution  5  cc.  of  nitric  acid,  sp.gr.  1.42. 
Filter  each  time  before  using. 

Standard  Potassium  Hydroxid  Solution.  This  solution  contains 
18.17106  gms.  of  potassium  hydroxid  to  the  liter.  It  is  prepared  by 
diluting  323.81  cc.  of  normal  potassium  hydroxid  (which  has  been 
freed  from  carbonates  by  barium  hydroxid)  to  one  liter.  100  cc.  of 
the  solution  should  neutralize  32.38  cc.  of  normal  acid.  One  cc.  of 
this  is  equal  to  o.ooi  of  P2Os,  or  i  per  cent  if  o.i  gm.  of  the  substance 
is  taken  for  analysis. 

Normal  sodium  hydroxid  may  be  used  instead  of  potassium.  The 
dilution  is  made  in  exactly  the  same  way. 

Standard  Nitric  Acid  Solution.  This  solution  should  correspond 
in  strength  to  the  standard  alkali  solution,  or  may  be  one  half  that 
strength.  It  is  standardized  by  titrating  against  that  solution,  using 
phenolphthalein  as  indicator.  Any  mineral  acid  may  be  used.  The 


318  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

indicator  is  made  by  dissolving  i  gm,  of  phenolphthalein  in  100  cc.  of 
alcohol. 

If  a  soluble  phosphate  is  to  be  analyzed,  dissolve  i  gm.  in  sufficient 
water  to  make  250  cc.  25  cc.  of  this  solution,  representing  o.i  gm. 
of  the  substance,  is  taken  for  analysis.  If  the  phosphate  is  in  an 
insoluble  compound  or  organic  substance,  2  gms.  are  treated  by  one 
of  the  methods  given  below.  After  solution,  cool,  dilute  to  200  or 
250  cc.,  mix  and  pour  on  a  dry  filter. 

Total  Phosphoric  Acid,  (a)  Dissolve  in  30  cc.  of  concentrated 
nitric  acid  and  a  small  quantity  of  hydrochloric  acid  and  boil  until 
organic  matter  is  destroyed. 

(b)  Evaporate  with  5  cc.  of  magnesium  nitrate,  ignite,  and  dis- 
solve in  hydrochloric  acid. 

(c)  Add  30  cc.  of  concentrated  hydrochloric  acid,  heat,  and  add 
cautiously,  in  small  quantities  at  a  time,  about  0.5  gm.  of  finely  pul- 
verized potassium  chlorate  to  destroy  organic  matter. 

(d)  Dissolve  in  from  15  to  30  cc.  of  strong  hydrochloric  acid  and 
from  3  to  10  cc.  of  nitric  acid.     This  method  is  recommended  for 
fertilizers  containing  much  iron  or  aluminum  phosphate. 

Determination,  (i)  For  percentages  of  5  or  below  use  an  aliquot 
corresponding  to  0.4  gm.  of  substance;  for  percentages  between  5  and 
20  use  an  aliquot  corresponding  to  0.2  gm.  of  substance,  and  for  per- 
centages above  20  use  an  aliquot  corresponding  to  o.i  gm.  of  substance. 
Add  from  5  to  10  cc.  of  nitric  acid,  depending  on  the  method  of  solu- 
tion (or  the  equivalent  in  ammonium  nitrate),  nearly  neutralize  with 
ammonium  hydroxid,  dilute  to  from  75  to  100  cc.,  heat  in  water  bath 
to  from  60°  to  65°  C.,  and  for  percentages  below  5  add  from  20  to 
25  cc.  of  freshly  filtered  molybdate  solution.  For  percentages  between 
5  and  20  add  from  30  to  35  cc.  of  molybdate  solution;  stir,  let  stand 
about  fifteen  minutes,  filter  at  once,  wash  once  or  twice  with  water  by 
decantation,  using  from  25  to  30  cc.  each  time,  agitating  the  precipitate 
thoroughly  and  allowing  to  settle;  transfer  to  filter  and  wash  with 
cold  water  until  two  fillings  of  the  filter  do  not  greatly  diminish  the 
color  produced  with  phenolphthalein  by  one  drop  of  the  standard 
alkali.  Transfer  precipitate  and  filter  to  beaker  or  precipitating  vessel, 
dissolve  in  small  excess  of  standard  alkali,  add  a  few  drops  of 
phenolphthalein  solution,  and  titrate  with  standard  acid. 

(2)  Proceed   as   directed  in   (i),  with  this  exception:    Heat  in  a 
water  bath  at  45°  to  50°  C.,  add  the  molybdate  solution,  and  allow 
to  remain  in  the  bath  with  occasional  stirring  for  thirty  minutes. 

(3)  Proceed  as  in  (i)  to  the  point  where  the  solution  is  ready  to 
place  in  the  water  bath.     Then  cool  solution  to   room  temperature, 
add  molybdate  solution  at   the  rate  of  75  cc.   for  each  decigram  of 
phosphoric  acid  present,  place  the  stoppered  flask  containing  the  solu- 


CITRATE-INSOLUBLE    PHOSPHORIC    ACID  319 

tion  in  a  shaking  apparatus  and  shake  for  thirty  minutes  at  room 
temperature,  filter  at  once,  wash,  and  titrate  as  in  preceding 
method. 

Water-soluble  Phosphoric  Acid.  Place  2  gms.  of  the  sample  on 
a  9-cm.  filter,  wash  with  successive  small  portions  of  water,  allowing 
each  portion  to  pass  through  before  adding  more,  until  the  filtrate 
measures  about  250  cc.  If  the  filtrate  be  turbid,  add  a  little  nitric  acid. 
Make  up  to  any  convenient  definite  volume,  mix  well,  use  an  aliquot 
portion  of  the  solution  corresponding  to  0.2  or  0.4  gms.,  add  10  cc. 
of  concentrated  nitric  acid  and  ammonium  hydroxid  until  a  slight 
permanent  precipitate  is  formed,  dilute  to  60  cc.,  and  proceed  as  under 
the  preceding  method  (i). 

Citrate-insoluble  Phosphoric  Acid.  Make  the  solution  accord- 
ing to  the  directions  given  before  and  determine  the  phosphoric  acid 
in  an  aliquot  corresponding  to  0.4  gms.,  as  directed  for  total  phosphates. 

Determination  in  Acidulated  Samples.  Heat  100  cc.  of  strictly 
neutral  ammonium  citrate  solution  of  1.09  sp.gr.  to  65°  C.  in  a  flask 
placed  in  a  warm-water  bath,  keeping  the  flask  loosely  stoppered  to 
prevent  evaporation.  When  the  citrate  solution  in  the  flask  has  reached 
65°  C.  drop  into  it  the  filter  containing  the  washed  residue  from  the 
water-soluble  phosphate  determination,  close  tightly  with  a  smooth 
rubber  stopper,  and  shake  violently  until  the  filter  paper  is  reduced 
to  a  pulp.  Place  the  flask  in  the  bath  and  maintain  it  at  such  a  tem- 
perature that  the  contents  of  the  flask  will  stand  at  exactly  65°  C. 
Shake  the  flask  every  five  minutes. 

At  the  expiration  of  exactly  thirty  minutes  from  the  time  the  filter 
and  residue  are  introduced,  remove  the  flask  from  the  bath  and  imme- 
diately filter  the  contents  as  rapidly  as  possible;  wash  thoroughly  with 
water  at  65°  C.  Then  proceed  as  under  (a)  or  (b). 

(a)  Transfer  the  filter  and  its  contents  to  a  crucible,  ignite  until 
all  organic  matter  is  destroyed,  add  from  10  to  15  cc.  of  strong  hydro- 
chloric acid,  and  digest  until  all  phosphate  is  dissolved,  or  (b)  return 
the  filter  with  contents  to  digestion  flask,  add  from  30  to  35  cc.  of 
strong  nitric  acid,  and  from  5  to  10  cc.  of  strong  hydrochloric  acid,  and 
boil  until  all  phosphate  is  dissolved.  Dilute  to  200  cc.,  mix  well,  and 
pass  through  a  dry  filter.  Take  a  definite  portion  of  the  filtrate  and 
proceed  as  under  total  phosphoric  acid. 

Determination  of  Non-acidulated  Samples.  Treat  2  gms.  of  the 
phosphatic  material  without  previous  washing  with  water,  precisely 
in  the  way  above  described,  except  that  in  case  the  substance  con- 
tains much  animal  matter  (bone,  fish,  etc.),  the  residue,  insoluble  in 
ammonium  citrate,  is  to  be  dissolved  by  the  treatment  described  under 
(&),  or  by  digestion  with  concentrated  sulphuric  acid  in  the  presence 
of  a  small  quantity  of  sodium  or  potassium  nitrate. 


320  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Citrate-soluble  Phosphoric  Acid.  The  sum  of  the  water-solu- 
ble and  citrate-insoluble  subtracted  from  the  total  phosphoric  acid, 
gives  the  citrate-soluble  phosphoric  acid. 

When  phosphoric  acid  is  determined  in  acid  phosphate  by  the 
Pemberton  volumetric  method  or  its  usual  modifications,  the  results 
do  not  agree  with  those  obtained  gravimetrically  by  the  A.  O.  A.  C. 
method,  asserts  W.  D.  Richardson,*  and  the  error  frequently  amounts 
to  +  1  per  cent  P2Os. 

The  disturbing  substance  is  probably  sulphuric  acid,  and  if  this  be 
removed  by  barium  chlorid,  the  volumetric  method  may  be  applied 
and  accurate  results  obtained. 

Richardson  recommends  the  following  procedure  : 

Weigh  2  gms.  into  a  250  cc.  flask,  digest  by  boiling  with  30  cc.  of 
concentrated  nitric  acid  and  5  cc.  concentrated  hydrochloric  acid, 
then  add  10  cc.  water  and  boil  for  five  minutes.  Add  25  to  30  cc.  of 
10  per  cent  barium  chlorid,  cool,  and  make  up  to  volume.  Filter 
through  a  dry  filter,  rejecting  the  first  portion  of  the  filtrate,  and  take 
25  cc.  for  the  determination.  From  this  point  on  follow  the  A.  O.  A.  C. 
modification  of  the  Pemberton  Method. 

Estimation  of  Mixed  Disodium  and  Trisodium  Phosphates 
(Ahlum).j*  Both  of  these  phosphates  may  be  titrated  with  standard 
acid,  both  are  alkaline  to  methyl  orange,  while  sodium-dihydrogen 
phosphate  is  neutral.  The  end  reaction  therefore  occurs  when  either 
or  both  are  converted  into  the  acid  phosphate.  The  equations  are  : 

HCl+Na2HP04=NaH2PO4+NaCl; 
2HCl+Na3PO4=NaH2PO4+2NaCl. 

Thus  trisodium  phosphate  requires  two  molecules  of  acid,  while 
the  disodium  hydrogen  phosphate  requires  but  one,  therefore  i  cc. 

N 
of  —  hydrochloric   acid   will   be   equivalent    to    0.0081465    gm.    of 

Na3PO4  or  0.014105  gm.  of  Na2HPO4. 

If  carbon  dioxid  is  passed  into  a  solution  of  trisodium  phosphate, 
the  latter  will  be  converted  into  disodium  hydrogen  phosphate,  with 
the  formation  of  a  definite  quantity  of  sodium  carbonate. 


As  the  sodium  carbonate  is  directly  proportional  to  the  trisodium 
phosphate,  a  determination  of  the  latter  salt,  when  in  mixture  with 

*  J.  A.  C.  S.,  XXIX,  1314. 

t  C.  Chester  Ahlum,  J.  A.  C.  S.,  XXVIII,  533. 


MIXED    DlSODIUM    AND    TRISODIUM   PHOSPHATES     321 

disodium  hydrogen  phosphate  can  be  easily  accomplished,  i.e.,  by 
passing  carbon  dioxid  into  a  solution  of  the  mixed  salts,  and  deter- 
mining the  sodium  carbonate  formed. 

The  Process.  Two  grams  of  the  mixed  salt  are  dissolved  in  water 
and  carbon  dioxid  passed  through  the  solution  until  the  reaction  is 
complete  (about  ten  or  fifteen  minutes).  The  solution  is  then  evaporated 
to  dryness  and  the  sodium  carbonate  estimated  by  the  Schrotter  appa- 
ratus. The  amount  of  carbon  dioxid  eliminated,  multiplied  by  7.46, 
gives  the  amount  of  trisodium  phosphate  present  in  the  mixture. 

One  gram  of  the  mixed  salts  is  dissolved  in  water  and  titrated 

N 

with  —  hvdrochloric  acid.     The  number  of  cubic  centimeters  required 
10 

by  the  trisodium  phosphate  Js  obtained  by  dividing  the  amount  (in 
grams)  of  trisodium  phosphate  found  in  the  above  determination  by 
0.016292  (2X0.00846).  The  number  of  cubic  centimeters  required 
by  the  disodium  hydrogen  phosphate  is  the  difference  between  the 
number  obtained  in  the  titration  and  the  number  required  by  the  tri- 
sodium phosphate.  The  number  of  cubic  centimeters  required  by  the 
disodium  hydrogen  phosphate,  multiplied  by  0.01405,  gives  the  amount 
of  disodium  hydrogen  phosphate  in  the  mixture. 

If  the  original  mixture  contains  sodium  carbonate,  a  determination 
of  this  ingredient  will  be  necessary  in  order  to  estimate  the  amount 
of  sodium  carbonate  formed  from  the  trisodium  phosphate  and  also  to 
make  a  correction  in  the  titration. 

Example. 

(a)  2  gms.  of  mixed  salts  gave  0.16  gms.  CC>2, 

0.16  gm.X  7.46=  1-1936  gms.  Na3PO4. 

N 

(b)  i  gm.  of  mixed  salts  required  101.83  cc.  —  HC1. 

N 

(c)  1.1936  gms.  Na3PO4  required  73.25  cc.  —  HC1. 

1.1936 


(d)  The  Na2HPO4  required   101.83  —  73.25  cc.  =  28.58  cc.  of 


10 

Therefore  i  gm.  of  the  sample  contained 

0.014105  X  28.  58=0.4037  +gm.  of 
an(j  H23?  =  0.5968  -f  gm.  of  Na3PO4. 


322  A    MANUAL   OF    VOLUMETRIC  ANALYSIS 

Ahlum's  Alternative  Method  (J.  A.  C.  S.,  xxvm,  535).  Ahlum 
also  proposes  the  following: 

Alternative  Method.  If  a  solution  containing  disodium  hydrogen 
phosphate  and  trisodium  phosphate  is  acidified  and  then  neutralized 
with  sodium  carbonate,  adding  an  excess  of  the  latter,  we  have  as  a 
result  a  solution  containing  disodium  hydrogen  phosphate  and  sodium 
carbonate. 

If  a  titration  were  to  be  made  of  the  mixed  salts  before  treatment 
and  a  titration  made,  after  making  allowance  for  the  amount  of  sodium 
carbonate  found  in  excess,  we  find  the  amount  of  acid  required  by 
the  phosphates  after  treatment  is  less  than  that  required  before,  the 
difference  being  directly  proportional  to  the  amount  of  trisodium 
phosphate  present. 

This  is  made  clear  by  referring  to  the  reactions  above,  noting  that 
disodium  hydrogen  phosphate  requires  but  one  molecule  of  acid  while 
the  trisodium  phosphate  requires  two. 

This  method  in  detail  is  as  follows:  0.5  gm.  is  dissolved  in  50  cc. 

N 

of  water  and  titrated  with  —  hydrochloric  acid,     i.o  gm.  is  dissolved 

10 

in  50  cc.  of  water  containing  a  drop  of  methyl  orange.  Hydrochloric 
acid  is  added  in  slight  excess  and  the  solution  boiled  for  ten  minutes. 
Sodium  carbonate  is  added  in  excess  and  the  solution  concentrated 
by  boiling  as  far  as  possible.  It  is  then  transferred  to  a  weighed 
platinum  dish,  evaporated  to  dryness  on  a  steam-bath,  dried  in  an  oven 
and  weighed. 

The  mass  in  the  dish  is  broken  up  and  pulverized  with  a  procelain 
pestle,  guarding  against  loss.  One-half  of  the  amount  of  solid  matter 
found  is  weighed  off  and  the  carbon  dioxid  determined  by  the  Schrotter 
apparatus.  The  remaining  half  of  the  solids  is  dissolved  in  50  cc.  of 

N 

water  and  titrated  with  —  hvdrochloric  acid. 
10 

The  following  example  will  be  explanatory:   0.5  gm.  was  titrated 

N 

with  —  hydrochloric  acid,  requiring  23.1  cc.     One  gram  was  treated 
10 

in  the  manner  described  above,  titrating  a  solution  of  one-half  of  the 
solids  and  estimating  the  carbon  dioxid  in  the  other  half.  The  amount 
of  carbon  dioxid  found  was  0.0149  gm-  The  amount  of  sodium  car- 
bonate equivalent  to  the  carbon  dioxid  eliminated= 0.0149X2. 41 15 
=  0.0359  gm.  N 

The  number  of  cubic  centimeters  of  —  hydrochloric  acid  equiva- 

10 

lent  to  the  sodium  carbonate  present =0.03 59  -4-0.0053  =  6. 7. 

N 
The  number  of  cubic  centimeters  of  —  hydrochloric  acid  required 

after  treatment  was  found  to  be  20.1. 


AHLUM'S   ALTERNATIVE  METHOD  323 

cc. 

N 
Total  —  acid  required 20.  i 

N      .I0 

—  acid  required  by  Na2COs 6.7 

10  

N 

—  acid  required  by  Na2HPC>4 13 .4 

10 

—  HC1  required  by  the  Na3PO4+Na2HPO4.  .      .  23.1 

10 

N 

—  HC1  required   by  the   Na2HPO4  (the  original 
10 

with  that  formed  from  the  trisodium  phosphate) . .   13.4 


Difference 9.7 

The  difference  in  the  amount  of  acid  required  is  due  to  the  loss 
in  alkalinity  of  the  trisodium  phosphate,  caused  by  the  conversion 
of  this  salt  into  disodium  hydrogen  phosphate,  thereby  requiring  just 
one  half  of  the  acid  as  when  in  the  tribasic  state. 

N 

In  the  above  example  the  amount  of  —  hydrochloric  acid  neces- 

10 

sary  to  completely  act  upon  the  trisodium  phosphate  would  be  9.7X2 
=  19.4  cc.  As  each  cubic  centimeter  is  equivalent  to  0.0082  gm.  of 
trisodium  phosphate,  the  amount  of  this  salt  in  the  mixture  will  be 
19. 4X0.0082X200= 31.9  per  cent. 

The  number  of  cubic  centimeters  required  by  the  disodium  hydrogen 
phosphate  is  obtained  by  subtracting  the  number  of  cubic  centimeters 
required  by  the  trisodium  phosphate  from  the  number  of  cubic  cen- 
timeters obtained  in  the  original  titration. 

In  the  above  example  this  would  be  23.1  cc.  — 19.4  cc.=3.7  cc- 

N 
As  each  cubic  centimeter  of  —  hydrochloric  add  is  equivalent  to 

0.0142  gm.  of  disodium  hydrogen  phosphate,  the  amount  of  this  salt 
present  in  the  mixture  would  be  3.7X0.0142X200=10.5  per  cent. 

The  accuracy  of  the  determinations  depends  upon  the  determina- 
tion of  the  sodium  carbonate.  As  all  sources  of  error  may  be  traced 
to  this  determination,  this  should  be  performed  with  care  and  results 
checked. 


CHAPTER  XXIII 
SALICYLIC  ACID  AND  SALICYLATES 

FREE  salicylic  acid  may  be  readily  estimated  by  titrating  its  aqueous 
solution  with  decinormal  sodium  hydroxid,  using  phenolphthalein  as 
indicator. 

HC7H503+NaOH=NaC7H503+H20. 

10)137.01       10)39-76 

j^ 

13.701  gms.     3.976  gms.=  1000  cc.  —  V.  S. 

10 

N 

Thus  each  cc.  of  —  NaOH  represents  0.013701  gm.  of  salicylic  acid. 
10 

Salicylates  of  the  alkalies  and  alkali  earths  may  be  estimated  by 
incineration  of  the  salt  (thereby  converting  it  into  carbonate),  and 
titrating  the  resulting  carbonate  with  standard  acid,  using  methyl 
orange  as  indicator  See  page  88. 

W.  Fresenius  and  L.  Grunhut  (Zeitschrift  fur  analytische  Chemie, 
1899,  292),  submitted  the  several  methods  which  have  been  proposed 
for  the  estimation  of  salicylic  acid  to  a  critical  study. 

Separation  by  Immiscible  Solvents.  From  a  solution  of  sodium 
salicylate  acidulated  with  sulphuric  acid  all  the  salicylic  acid  could 
readily  be  abstracted  with  chloroform.  This  chloroformic  solution 
could,  however,  not  be  brought  to  dryness  without  loss  of  acid,  and 
this  method  can  therefore  only  be  depended  upon  for  approximate 
results. 

The  lodometric  Method.  This  method  depends  upon  the  forma- 
tion of  iodin  compounds  of  salicylic  acid  when  an  alkaline  solution  of 
the  latter  is  treated  with  an  excess  of  iodin  solution  and  then  rendered 
acid.  From  the  amount  of  iodin  rendered  insoluble,  the  salicylic  acid 
is  calculated.  While  Vortmann  (Anleitung  zur  chemischen  Analyse 
organischer  Stoffe,  Wien,  1891,  pp.  320  and  401)  calculates  one  molecule 
of  acid  for  every  six  atoms  of  iodin  combined,  the  authors  demonstrate 
that  the  amount  of  iodin  combined  depends  largely  upon  the  condi- 
tions of  the  experiment;  thus  in  their  work  they  find  in  sodium  sali- 
cylate from  64.71  per  cent  to  92.70  per  cent  of  acid,  while  theory  requires 
86.23  per  cent. 

324 


SALICYLIC  ACID   AND   SALICYLATES  325 

Titration  with  Bromin  Solution.  This  method  of  Fr.  Freyer, 
1896  (Chemiker-Zeitung,  20,  p.  820)  is  based  upon  the  same  principle 
as  the  estimation  of  phenol  according  to  Koppeschaar.  Excellent  results 
were  obtained  when  of  the  volumetric  bromin  solution  (sodium  bromate 
3  gms.,  sodium  bromid  20  gms.,  water,  1000  cc.)*  100  cc.  were  diluted 
with  300  cc.  of  water,  to  this  20  cc.  of  hydrochloric  acid  added,  and 
to  this  mixture  20  cc.  of  an  approximately  i  per  cent  solution  of  sub- 
stance under  examination  was  gradually  added.  After  standing  for 
five  minutes,  occasionally  stirring,  30  to  40  cc.  of  a  10  per  cent  potas- 
sium iodid  solution  is  added  and  the  residual  iodin  titrated  in  the 
usual  way. 

The  reaction  in  the  process  is  as  follows : 

/OH 

C6H4<  +  8Br  =  C6HBr3 .  OBr3  +  4HBr+ CO2. 

XCOOH 

On  adding  the  solution  of  potassium  iodid,  not  only  does  the  excess 
of  bromin  liberate  an  equivalent  of  iodin,  but  the  tribromphenol  bromid 
also  reacts  thus, 

C6HBr3.OBr+2KI=C6HBr3.OK+KBr+l2. 

Hence  6  atoms  of  bromin  correspond  to  one  molecule  of  salicylic 
acid. 

*  See  Estimation  of  Phenol. 


CHAPTER  XXIV 

SULPHUR  AND  ITS  COMPOUNDS 

SULPHUR  may  be  estimated  by  converting  it  into  sulphuric  acid. 
This  is  accomplished  by  heating  the  finely-powdered  substance  with 
strong  nitric  acid  to  which  some  crystals  of  potassium  chlorate  have 
been  added.  The  solution  is  covered  with  a  watch-glass  until  all 
spurting  has  ceased,  and  then  evaporated  to  dryness  on  the  sand-bath 
with  an  excess  of  pure  hydrochloric  acid.  The  residue  is  dissolved 
in  hydrochloric  acid,  and  again  evaporated  to  dryness,  redissolved  in 
water,  a  few  drops  of  hydrochloric  acid  added,  and  the  sulphuric  acid 

N 

estimated  by  —  barium  chlorid.     (See  Sulphuric  Acid.)     The  sulphur 
10 

of  most  insoluble  sulphids  may  be  oxidized  to  sulphuric  acid  and 
determined  in  the  same  way.  The  object  of  the  evaporations  is  to 
drive  off  the  nitric  acid,  which  interferes  with  the  estimation  of  the 
sulphuric  acid. 

Another  way  for  estimating  sulphur  in  insoluble  sulphids,  as  in  iron 
or  copper  pyrites,  consists  in  igniting  the  sulphid  with  potassium 
chlorate  and  sodium  carbonate.  The  sulphur  is  converted  entirely 
into  sulphuric  acid  which  reacts  with  an  equivalent  of  sodium  carbonate 
forming  sodium  sulphate.  An  accurately  weighed  quantity  of  the 
substance  is  fused  with  a  known  weight  of  pure  sodium  carbonate  in 
excess,  and  in  the  presence  of  potassium  chlorate.  And  the  resulting 
mass  titrated  with  normal  acid  to  find  the  quantity  of  unaltered  sodium 
carbonate.  The  quantity  of  normal  acid  required,  subtracted  from 
the  quantity  required  to  saturate  the  sodium  carbonate  originally  added, 
is  the  quantity  of  normal  acid  representing  the  carbonate  which  reacted 
with  the  sulphuric  acid  produced,  and  from  which  the  proportion  of 
sulphur  is  easily  calculated. 

N 
Each  cc.  of  —  sulphuric  acid= 0.015915  gm.  of  sulphur. 

It  is  advisable  to  take  i  gm.  of  the  substance  and  5.2655  gms.  of 
pure  anhydrous  sodium  carbonate  for  the  assay.  5.2655  gms.  of 

N 
Na2CO3  represent  100  cc.  of  —  H2SO4,  therefore,  it  is  only  necessary 

10  N 

to  subtract  the  number  of  cc.  of  —  H2SO4  used  after  ignition  from 

10 

326 


ESTIMATION   OF  SULPHUR  AND  ITS  COMPOUNDS        327 

100,  and  multiply  the  remainder  by  the  factor  for  sulphur  0.015915  gm. 
In  order  to  arrive  at  the  weight  of  sulphur  in  i  gm.  of  the  substance. 
This  weight  multiplied  by  100  gives  the  percentage. 

Example,  i  gm.  of  finely  powdered  FeS2  is  mixed  thoroughly  with 
5.2655  gms.  of  anhydrous  sodium  carbonate,  and  8  gms.  each  of  potas- 
sium chlorate  and  sodium  chlorid,  and  gradually  exposed  in  a  platinum 
crucible  to  a  dull  red  heat  for  ten  minutes.  The  crucible  is  allowed 
to  cool,  its  contents  treated  with  warm  water,  and  the  solution  so 
obtained  filtered.  The  insoluble  residue  is  then  boiled  with  water, 
the  water  passed  through  a  filter,  and  the  residue  washed  on  the  filter 
until  all  soluble  matter  is  dissolved. 

The  mixed  filtrate  is  then  titrated  with  normal  sulphuric  acid, 
using  methyl  orange  as  the  indicator. 

If  66.8  cc.  of  normal  sulphuric  acid  are  required,  this  deducted 
from  100=33.2  cc.  This  multiplied  by  0.015915  =  0.  5283  +  gm.  or 
52.83  per  cent  of  sulphur. 

The  results  by  this  method  are  by  no  means  exact,  and  vary  at 
times  as  much  as  1.5  per  cent.  It  is,  however,  suitable  for  rough 
technical  purposes.  The  inaccuracy  of  this  process  is  caused  by  the 
volatilization  of  some  of  the  sulphur  in  the  form  of  chlorid,  and  also 
the  formation  of  ferric  sulphate  from  which  the  acid  is  expelled  at  a 
red  heat. 

The  reaction  involved  is  as  follows: 


The  sodium  chlorid  is  used  to  moderate  the  violence  of  the  reaction. 

Caution.  Great  caution  must  be  exercised  in  using  potassium 
chlorate  in  this  method,  because  many  sulphids,  especially  antimony 
sulphid,  afford  violent  explosions. 

Estimation  of  Sulphur  in  Alkali  Sulphid.  The  estimation  is 
the  exact  reverse  of  the  process  for  estimating  zinc  by  means  of  sodium 
sulphid. 

The  reaction  is  as  follows: 

Na2S  +  ZnSO4=ZnS+Na2SO4. 
77-59 

N 
The  —  zinc  solution  may  be  made  by  dissolving  3.245  gms.  of  pure 

metallic  zinc  in  hydrochloric  acid,  supersaturating  with  ammonia,  and 
diluting  to  i  liter.  Or  by  dissolving  14.275  gms.  of  pure  crystallized 
zinc  sulphate  (ZnSO4+  71120  =  285.  41)  in  water,  making  strongly 
alkaline  with  ammonia  water  and  diluting  to  i  liter.  The  indicator  is 
nickel  protochlorid  or  alkaline  lead  solution. 


328  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

Each  cc.  of  this  zinc  solution  will  indicate  respectively, 

0.0015915  gm.  sulphur; 
0.0038795    "    sodium  sulph  id; 
0.0054777    "    potassium  sulphid  ; 
0.0033845    '  '    ammonium  sulphid. 

The  sulphid  is  dissolved  in  water  and  the  zinc  solution  added 
from  a  burette  until  no  dark  color  is  shown,  when  a  drop  of  the  solution 
tested  is  brought  in  contact  with  a  drop  of  nickel  sulphate  on  a  white 
porcelain  tile.  Instead  of  the  nickel  sulphate,  an  alkaline  lead  solu- 
tion may  be  used  as  indicator. 

Sulphids  may  also  be  estimated  by  adding  an  excess  of  zinc  solu- 
tion, washing  the  precipitated  sulphid  rapidly,  out  of  contact  of  air, 
digesting  with  an  excess  of  ferric  sulphate,  and  estimating  the  ferrous 

N 
salt  with  permanganate  (see  Zinc).     —  permanganate  X  0.001591  5  =  8, 

etc.     * 


Estimation  of  Hydrosulphuric  Acid.  By  Permanganate  (Mohr). 
When  ferric  sulphate  is  added  to  a  solution  of  H2S  in  water  the 
following  reaction  takes  place: 

Fe2(SO4)3+H2S=2FeSO4+H2SO4+S. 

The  sulphur  separates  as  a  white  powder,  making  the  fluid  milky. 
The  FeSC>4  formed  is  a  measure  of  the  H2S,  and  may  be  estimated  by 
permanganate.  The  precipitated  sulphur  does  not  interfere. 

An  acid  solution  of  ferric  sulphate  which  must  be  free  from  ferrous 
sulphate  is  poured  into  a  flask  and  the  solution  of  hydrogen  sulphid 
measured  in.  with  a  pipette  whose  point  just  touches  the  surface  of 
the  solution.  The  flask  is  closed  and  allowed  to  stand  for  an  hour, 
shaking  frequently.  At  the  end  of  this  time  the  liquid  must  still 
possess  a  yellow  color,  from  excess  of  ferric  sulphate,  and  upon  opening 
the  flask  no  odor  of  H2S  should  be  present.  The  solution  is  consider- 
ably diluted  and  titrated  with  permanganate  until  the  rose  color  appears. 
If  the  solution  of  hydrogen  sulphid  is  very  dilute,  it  should  stand  three 
or  four  hours  before  being  titrated. 

When  the  H2S  solution  also  contains  thiosulphates,  or  other  sub- 
stances which  reduce  permanganate,  the  H2S  must  be  precipitated  by 
alkaline  zinc  solution  added  in  excess.  The  precipitate  is  rapidly 
washed,  transferred  to  a  flask  containing  an  acid  solution  of  ferric 
sulphate,  and  after  standing  half  an  hour  in  a  warm  place,  the  solution 
is  diluted  and  titrated  with  permanganate. 


ESTIMATION    OF    HYDROSULPHURIC    ACID  329 

N 
Each  cc.  of  —  permanganate =0.00 1 69 1 5  gm.  of  H2S. 

In  like  manner  metallic  sulphids  which  are  soluble  in  dilute  sul- 
phuric acid  may  be  estimated  by  adding  the  dry  sulphids  in  known 
quantity  to  an  acid  solution  of  ferric  sulphate,  and  after  standing 
in  a  warm  place  as  above  stated  may  be  titrated  with  per- 
manganate. 

Alkali  sulphids  may  be  estimated  in  the  same  manner  as  hydrogen 
sulphid. 

By  Iodin.  When  iodin  and  hydrogen  sulphid  are  brought  together 
Jn  solution  the  following  reaction  occurs : 

H2S+2l=2HI+S. 

The  reaction  is  not  regular,  however,  when  performed  in  an  acid 
solution,  but  in  the  presence  of  alkali  bicarbonates  the  results  are 
constant.  The  method  may  be  employed  for  the  estimation  of  alkali 
sulphates. 

The  process  may  be  conducted  as  follows,  according  to  Sutton : 

N 

10  cc.  or  any  other  necessary  volume  of   —    iodin  solution  are 

100 

measured  into  a  500  cc.  flask  and  the  H2S  solution  to  be  examined 
added  until  the  color  disappears.  5  cc.  of  starch  solution  are  then 

N 

added,  and  —  iodin  until  the  blue  color  appears.     The  flask  is  then 
100 

filled  to  the  500  cc.  mark  with  distilled  water.  The  respective 
volumes  of  iodin  and  starch  solution,  together  with  the  added  water, 
deducted  from  500  cc.  will  show  the  volume  of  water  actually  titrated 
by  the  iodin.  A  correction  should  be  made  for  the  excess  of  iodin 
necessary  to  produce  the  blue  color. 

Mohr's  Procedure.  The  H2S  solution  is  made  alkaline  with 
ammonium  carbonate  or  sodium  bicarbonate,  starch  solution  is  added, 
and  then  the  standard  iodin  solution  until  the  blue  color  appears. 

N 
Each  cc.  of  —  iodin =0.0001691 5  gm.  of  H2S. 

The  estimation  may  likewise  be  made  by  adding  the  H2S  solution  to 
a  solution  of  copper  sulphate,  boiling  for  a  few  minutes,  filtering  off 
the  precipitated  sulphid,  dissolving  it  in  nitric  acid,  evaporating  the 
solution  to  dryness  with  excess  of  sulphuric  acid,  and  estimating  the 
copper  by  means  of  potassium  iodid  and  sodium  thiosulphate.  See 
Copper. 

N 

Each  cc.  of  —  sodium  thiosulphate =0.00169 15  gm.  of  H2S. 
10 


330 


A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


The  estimation  of  H2S  in  sulphids  which  are  insoluble  in  water, 
may  be  carried  out  as  follows : 

A  weighed  quantity  of  the  sulphid  is  introduced  into  a  flask,  pro- 
vided with  a  double  perforated  stopper;  through  one  of  the  perforations 
the  stem  of  a  separatory  funnel  is  passed,  through  the  other  a  glass 
delivery  tube  (see  Fig.  76).  The  funnel  tube  extends  to  near  the 

bottom  of  the  flask  and  is  bent  to 
form  a  hook,  the  opening  of  which 
is  under  water.  The  delivery  tube 
begins  at  the  lower  end  of  the 
stopper  and  ends  in  another  flask 
containing  sodium  bicarbonate 
solution.  The  funnel  contains 
diluted  sulphuric  acid,  which,  upon 
opening  the  glass  stopcock,  is  allowed 
to  flow  into  the  flask,  upon  the 
contained  sulphid,  the  H2S  libera- 
ted is  conducted  into  the  solution 
of  sodium  bicarbonate  which  absorbs 
it  completely.  A  current  of  air 
aspirated  through  the  apparatus 
FIG.  76.  insures  absorption  of  the  entire  H2S 

developed.     The  sodium  bicarbonate 

solution  of  H2S  is  then  titrated  with  the  standard  iodin,  in  the 
presence  of  starch. 

By  Arsenous  Acid  (Mohr).  When  H2S  is  added  to  a  solution  of 
arsenous  acid,  the  following  reaction  occurs : 

3H2S + As2O3 = As2S3  +  3H2O. 

The  arsenous  acid  is  added  in  excess,  and  the  excess  then  found 

N 
by  titration  with  —  iodin  and  starch. 

10  N 

A  measured  quantity,  say  10  cc.  of  —  arsenous  acid  solution  is  put 

10 

into  a  flask  together  with  a  measured  quantity,  say  20  cc.  of  the  H2S 
solution,  sufficient  hydrochloric  acid  is  then  added  to  make  the  solution 
distinctly  acid  and  diluted  to  300  cc.  The  precipitated  arsenic  sulphid 
is  separated  by  filtration,  and  100  cc.  of  the  clear  colorless  filtrate 
taken  out,  neutralized  with  sodium  bicarbonate,  and  then  titrated 

N 
with  —  iodin,  using  starch  as  indicator.     The  quantity  of  arsenous 

acid  V.  S.  so  found  multiplied  by  3  is  deducted  from  the  original  TO  cc. 
and  the  remainder  multiplied  by  the  factor  for  H2S,  which  is  0.002537 
gm. 


SULPHUROUS    ACID    AND   SULPHITES  331 

By  Silver  Nitrate.  Hydrogen  sulphid  water  may  also  be  esti- 
mated by  adding  an  accurately  measured  quantity  of  the  water  to  be 
analyzed,  to  an  accurately  measured  quantity  of  standard  silver  nitrate 
solution,  and  after  thoroughly  shaking,  diluting  to  a  definite  volume. 
After  the  precipitate  of  silver  sulphid  has  subsided,  an  aliquot  portion 
of  the  clear  supernatant  liquid  is  removed  by  means  of  a  pipette,  and 
in  this  the  unchanged  silver  nitrate  determined  by  means  of  standard 
sulphocyanate  solution.  The  difference  between  the  quantity  of 
unchanged  silver  nitrate  (in  the  whole)  found  by  titration  and  the 
quantity  originally  added,  is  calculated  into  H2S. 

The  reaction  is  as  follows: 

H2S  +  2  AgN03 = Ag2S + 2HNO3. 
33-83 

N 
Each  cc.  of  —  AgN 03  =  0.001691 5  gm.  H2S. 

Sulphurous  Acid  and  Sulphites.  Sulphurous  acid  and  sulphites 
may  be  accurately  estimated  by  titration  with  standard  iodin,  as 
described  on  page  197. 

Sulphurous  acid  may  also  be  estimated  by  neutralization  with 
standard  alkali.  It  acts  in  about  the  same  way  as  does  phosphoric  acid, 
i.e.,  with  methyl  orange  as  indicator,  the  yellow  color  appears  upon 
the  formation  of  KHSC>3. 

H2S03  +  KOH=KHSO3+H2O, 

while  if  phenolphthalein  is  used  as  indicator,  the  end-point  does  not 
appear  until  normal  potassium  sulphite  KoSOa  is  produced. 

H2SO3  +  2KOH=K2SO3-f-2H2O. 

In  the  first  instance  one  molecule  of  sulphurous  acid  is  neutralized 
by  one  equivalent  of  KOH;  in  the  second  instance  two  equivalents 
of  KOH  are  required. 

Sulphuric  Acid  and  Sulphates.  The  free  acid  is  estimated  by 
neutralization  with  normal  alkali. 

Sulphuric  acid  in  sulphates  may  be  estimated  by  various  volu- 
metric methods,  though  the  gravimetric  method  is  undoubtedly  the 
most  satisfactory.  The  principal  volumetric  methods  are  as  follows: 

With  Barium  Chlorid.  The  sulphate  is  dissolved  in  water, 
acidified  with  hydrochloric  acid,  heated  to  boiling,  and  decinormal 
barium  chlorid  *  carefully  added  until  no  further  precipitation  occurs. 

*The  decinormal  barium  chlorid  solution  is  made  by  dissolving  12.126  gms. 
of  pure  crystallized  barium  chlorid  (BaCl2+2H2O)  in  water  to  make  one  liter. 


332  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  end  of  the  reaction  may  be  determined  by  the  use  of  Beale's 
filter,  Fig.  87,  or  by  placing  a  drop  of  the  clear  solution  on  a  plate 
of  black  glass  or  a  mirror,  and  bringing  in  contact  with  it  a  drop  of 
barium  chlorid  solution. 

Wildenstein   first   proposed   this  method   of  estimating   sulphuric 
acid  in  acid  solution  by  means  of  barium  chlorid.     Barium  sulphate  is 
deposited  rapidly  from  a  hot  solution  containing  excess  of  sulphuric 
acid,  leaving  a  clear  solution,  but  as  the  excess  of  acid  grows  smaller 
it  settles  more  slowly,  until  near  and  beyond  the  neutral  point  we  must 
either  wait  a  long  time  for  the  precipitate  to  settle  or  by  some  means 
filter  a  small  portion  of  the  solution  and  test  the  filtrate  to  see  whether 
enough  barium  chlorid  has  been  added.     For  the  filtration,  Wilden- 
stein employs  the  following  special  apparatus,  shown  in  Fig.  77.     A  is 
a   bottle   of   white   glass    whose  bottom  has  been 
removed,  and  which  will    contain    about    900   cc. 
An    ordinary    quart    bottle    answers    the   purpose. 
The  bent  funnel-tube  B  is  covered  in  the  follow- 
ing way:    A  piece  of  muslin  is  first  laid  over  the 
aperture,  then  two  pieces   of   fine   Swedish    filter- 
paper,  and  finally  another  piece  of  muslin.      The 
whole  is  then  fastened  tightly  over  the  mouth   of 
the    tube    by    means    of   waxed   thread,  and   the 
projecting  edges  neatly  trimmed.      In  binding  over 
the  aperture  care   must  be  taken  not  to  break  the 
filter-paper.      Fill  the  vessel  with  hot  water  above 
FIG.  77.          the  funnel-tube,  and  by  opening  the  pinch-cock  the 
funnel-tube  is  completely  filled.     Add  the  solution 
of  the  sulphate,  which  is  to  be  estimated,  and  acidify  with  a  few  drops 
of  hydrochloric  acid,  and  then  add  the  barium  chlorid  from  the  burette 
gradually,  after  each  addition  opening  the  pinch-cock  and  allowing 
a  volume  of  fluid  corresponding  to  the  contents  of  the  tube  to  flow  into 
a  small  beaker;    this  is  then  returned  to  the  beaker,  another  portion 
run  out  into  the  same  beaker  without  rinsing  it,  and  this  tested  by  a 
drop  of  barium  chlorid  from  the  burette.     If  a  precipitate  forms,  the 
contents  of  the  beaker  are  returned  to  the  bottle,  a  little  more  barium 
chlorid  added,  the  contents  of  the  tube  emptied  and  returned  to  the 
beaker,  and  then  a  second  portion  run  into  the  beaker  and  tested, 
and  so  on  until  the  solution  fails  to  give  a  perceptible  precipitate  with 
a  drop  of  barium  chlorid  after  a  lapse  of  two  minutes.     It  is  neces- 
sary to  empty  the  contents  of  the  tube  each  time  before  testing,  since 
the  barium  chlorid  which  is  added  to  the  solution  in  the  bottle  does 
not  enter  the  funnel  tube  until  the  pinch-cock  is   opened.     In  the 
precipitation  of  sulphuric  acid  by  barium  chlorid  a  point  occurs  where 
both  barium  chlorid  and  sulphuric  acid  produce  a  precipitate.     This 


SULPHURIC   ACID   AND  SULPHATES  333 

point  marks  the  time  when  barium  chlorid  and  sulphuric  acid  are 
present  in  solution  in  precisely  equivalent  quantities  and  therefore 
this  point  should  be  taken  as  the  end  of  an  analysis.  After  a  little 
practice  it  is  easy  to  hit  this  point  and  great  accuracy  may  be  attained. 
With  Barium  Chlorid  and  Potassium  Bichromate.*  Add  an 

N 

excess  of  —  barium  chlorid  solution  and  heat  to  boiling,  then   add 
10 

some  ammonia  water  and  titrate  the  excess  of  barium  chlorid  with 
decinormal  potassium  dichromate.  The  latter  is  added  in  small 
portions,  boiling  after  each  addition  until  the  fluid  above  the  precipitate 
is  of  a  faint  yellow  color.  The  decinormal  potassium  dichromate 
solution  is  made  by  dissolving  7.307  gms.  of  the  salt  in  sufficient  water 
to  make  1000  cc.  The  reactions  are  as  follows: 

(a)  K2SO4+BaCl2=BaSO4 


N 

Each  cc.  of  —  barium  chlorid  solution  represents 
10 

0.0086535  gm-  °f  K2SO4; 
0.0048675    "    "  H2SO4. 

In  order  to  obtain  satisfactory  results  by  this  method  the  solution 
must  be  neutral  or  slightly  alkaline  and  must  contain  no  carbonate 
or  other  acid  besides  sulphuric,  capable  of  precipitating  barium  from 
a  neutral  solution. 

If  the  solution  under  analysis  contains  carbonates,  a  slight  excess 
of  hydrochloric  acid  is  added,  and  the  solution  boiled  until  all  carbon 
dioxid  is  driven  off,  and  then  ammonia  (which  must  be  free  from 
carbonate)  is  added  to  alkaline  reaction.  The  solution  is  heated  to 
boiling  and  then  barium  chlorid  solution  added  in  excess,  more  ammonia 
is  added  and  then  potassium  dichromate  by  small  portions,  boiling 
after  each  addition  until  the  supernatant  liquid  is  faintly  yellow. 

By  Precipitation  as  Lead  Sulphate.  A  decinormal  solution  of 
lead  nitrate  is  prepared  by  dissolving  16.4245  gms.  of  pure  dry  lead 
nitrate  in  sufficient  water  to  make  1000  cc.  The  sulphate  is  dissolved 
in  water  and  titrated  with  the  lead  nitrate  solution  until  precipitation 
is  complete.  A  solution  of  potassium  iodid  may  be  used  as  indicator. 
The  reaction  is  known  to  be  completed  when  a  drop  of  the  solution 

*  Wildenstein,  Fres.  Zeit.,  I,  323. 


334  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

brought  in  contact  with  a  drop  of  the  indicator  on  a  porcelain  slab 
gives  a  yellow  color,  due  to  the  formation  of  lead  iodid.  The  reaction 
is: 

K2SO4  +  Pb(NO3)2  =  PbSO4  +  2KNO3 

2)173.07         2)328.49 

10)86.535      10)164.245 

8.6535  16.4245  =  1000  cc.  _  V.  S. 

IO 

Each  cc.= 0.0086535  gm.  of  K2SO4; 
=0.0048675    «    "  H2SO4. 


CHAPTER  XXV 

ALUMINUM 

Alum  and  Aluminum  Salts.  The  salt  is  dissolved  in  water, 
phenolphthalein  added,  and  then  a  measured  excess  of  —  sodium 
hydroxid.  This  makes  the  solution  red. 

)3  +  6NaOH=Al2(OH)6+3Na2S04. 


The  A12(OH)6  dissolves  in  the  excess  of  NaOH. 

Normal  acid  solution  is  now  added  until  the  red  color  disappears, 
the  quantity  of  the  acid  solution  used  is  deducted  from  the  alkali  added, 
and  the  remainder  multiplied  by  the  factor. 

Each  cc.X  0.0169  gm. 
X  0.05664  g 

The  Ph.  Germ,  directs  the  following  procedure  for  estimating 
aluminum  sulphate: 

One  gram  of  the  salt  is  dissolved  in  10  cc.  of  water  and  1.2  gm. 
of  barium  chlorid  added.  Then  a  few  drops  of  phenolphthalein  T.  S. 

N 
are  added  and  the  mixture  titrated  with  —  potassium  hydroxid  until 

red  color  appears. 

The  process  depends  upon  the  fact  that  the  acid  combined  with 
aluminum  behaves  toward  the  indicator  as  though  it  were  in  a  free 
state.  The  red  color  does  not  appear  until  the  aluminum  is  com- 
pletely precipitated.  In  the  case  of  sulphate  of  aluminum,  however, 
the  addition  of  alkali  hydroxid  solution  is  apt  to  cause  the  precipita- 
tion of  basic  sulphate  of  aluminum. 

Hence  in  this  process  barium  chlorid  is  added  in  order  to  convert 
the  sulphate  into  chlorid  of  aluminum,  which  can  be  accurately  titrated 
with  the  alkali  solution. 

335 


336  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

The  reactions  are: 


then 

A12C16  +  6KOH=A12(OH)6 

Aluminum  sulphate  is  apt  to  have  some  free  acid,  and  this  is  of 
course  included  in  the  calculation  together  with  the  combined  acid. 

N 
Hence  not  more  than  8.7  cc.  of  —  KOH  V.  S.  should  be  used;  any 

quantity  above  that  would  indicate  free  acid. 

N 
Each  cc.  of  —  alkali=  0.05664  gm.  of  A12(SO4)3.     The  free  acid 

may  be  estimated  by  the  use  of  tropaeolin  O.  O.,  which  reacts  only 
with  free  acid. 

Titration  with  Standard  Barium  Hydroxid  (A.  H.  White).* 
This  method  depends  upon  precipitating  the  free  and  combined  sul- 
phuric acid  of  alum,  by  titrating  with  standard  barium  hydroxid  in 
the  presence  of  Rochelle  salt;  and  then  titrating  another  portion  of 
the  sample  with  barium  hydroxid  in  the  presence  of  sodium  citrate. 
The  second  titration  gives  the  free  acid  plus  two  thirds  of  the  acid 
combined  with  alumina.  There  is  no  precipitation  of  aluminum 
hydroxid  in  either  case,  and  hence  the  end-reaction  with  phenolphthalein 
is  very  sharply  defined.  The  difference  between  the  two  titrations 
upon  the  same  quantities  of  material  represents  one  third  of  the  sul- 
phuric acid  combined  with  alumina,  and  hence  one  third  of  the  alumina. 

The  Process.  Dissolve  3  gms.  of  alum  in  sufficient  water  to  make  TOO 
cc.  Take  25  cc.  sample,  add  50  cc.  strictly  neutral  10  per  cent  potassium 
sodium  tartrate,  and  titrate  with  fifth  -normal  barium  hydroxid,  using 
phenolphthalein  as  indicator.  This  is  equivalent  to  the  sulphuric 
acid  combined  with  alumina,  plus  the  free  acid.  Evaporate  a  dupli- 
cate 25  cc.  sample  to  dryness  on  the  water-bath,  dissolve  in  50  cc. 
strictly  neutral  10  per  cent  sodium  citrate  solution,  allow  to  stand 
ten  minutes  and  titrate  with  barium  hydroxid  using  phenolphthalein 
indicator  as  before.  The  difference  between  these  results  is  equiva- 
lent to  one  third  of  the  sulphuric  acid  combined  with  the  alumina, 
and  hence  to  one  third  of  the  alumina.  The  barium  hydroxid  solution 
should  be  standardized  by  a  blank  determination  upon  a  solution  of 
sulphuric  acid  in  which  approximately  enough  precipitated  aluminum 
hydroxid  has  been  dissolved  to  correspond  to  aluminum  sulphate. 
The  aluminum  hydroxid  may  be  best  made  by  precipitation  of  the 

*J.  A.  C.  S.,  XXIV  (1902),  457- 


ALUMINUM  SALTS  337 

chlcrid  to  insure  absence  of  sulphate.  Caustic  soda,  even  when  freed 
from  carbon  dioxid  by  barium  hydroxid,  does  not  give  such  satisfactory 
results  as  the  barium  hydroxid. 

lodometric  Method  (Stock).*  When  a  mixture  of  potassium  iodid 
and  iodate  acts  upon  an  aluminum  salt  in  solution,  a  precipitate  of 
aluminum  hydroxid  is  formed,  and  a  corresponding  quantity  of  iodin 
is  liberated,  according  to  the  equation: 


This  is  a  reaction  which  begins  very  rapidly  in  the  cold,  but  which 
is  not  complete  for  several  days.  The  velocity  of  the  reaction  can, 
however  be  accelerated  if  the  iodin  liberated  during  the  reaction  is 
removed  by  means  of  sodium  thiosulphate,  or  by  allowing  the  pre- 
cipitation to  take  place  when  hot.  By  working  on  a  water-bath,  the 
reaction  is  complete  in  a  few  minutes,  and  by  removing  the  iodin  at 
the  same  time  by  means  of  standard  thiosulphate  solution,  an  accurate 
method  of  estimating  alumina  can  be  devised.  The  precipitation  is 
complete  even  in  dilute  solutions. 

*  Alfred  Stock,  Chem.  News,  Feb.  16,  1900,  83,  from  Compt.  rend.,  130, 
Jan.  22,  1900,  No.  4. 


CHAPTER  XXVI 
AMMONIA  AND  AMMONIUM  SALTS 

Ammonium  Salts  may  be  estimated  by  distilling  them  with  potas- 
sium or  sodium  hydroxid,  and  receiving  the  ammonia  (NHs)  which 
distils  over,  in  a  known  volume  or  normal  or  decinormal  acid.  After 
the  distillation  is  completed,  the  quantity  of  the  ammonia  is  found  by 
titrating  back  with  normal  or  decinormal  alkali.  The  apparatus 
illustrated  in  Fig.  67  may  be  used  for  this  purpose. 

The  ammonium  salt  in  solution  is  put  into  flask  A',  b  contains 
strong  solution  of  sodium  hydroxid.  The  receiving-flask  B  contains  a 
measured  quantity  of  normal  hydrochloric  acid,  which  is  poured  in 
through  the  tube  c,  containing  fragments  of  glass. 

The  pinch-cock  upon  b  is  opened,  which  allows  the  sodium  hydroxid 
solution  to  run  into  the  flask,  and  the  solution  is  then  gently  boiled 
until  all  the  ammonia  is  driven  over  and  absorbed  by  the  normal  acid. 

Care  must  be  taken  not  to  heat  too  strongly,  or  some  of  the  fixed 
alkali  may  be  projected  up  into  the  connecting-tube  and  carried  over 
into  the  acid  in  flask  B. 

After  the  distillation  is  completed  the  acid  adhering  to  the  broken 
glass  in  c  is  washed  into  the  flask,  phenolphthalein  added,  and  the 
excess  of  acid  found  by  titrating  with  normal  alkali.  The  amount  of 
normal  alkali  used  is  deducted  from  the  quantity  of  normal  acid  added, 
and  the  remainder  is  the  acid  which  combined  with  the  ammonia. 

NH3  +  HC1=NH4C1.      N 

16.93       36.18       =  1000  cc.  —  V.  S. 

i 

N 
.01693     .03618=       i  cc.  —  V.  S. 

N 
Thus  i  cc.  —  HC1=  0.01393  gm.  N; 


=0.01693  gm. 

=  0.01793  gm.  NH4; 

=  0.05311  gm.  NH^Cl. 

This  method  may  be  employed  for  ammonia  in  all  of  its  salts  and 
compounds;   either  sodium  or  potassium  hydroxid  or  lime  may  be 

338  ' 


AMMONIUM    SALTS  339 

used  for  liberating,  except  in  the  case  of  substances  containing  nitro- 
genous organic  compounds,  in  which  case  the  use  of  freshly  ignited 
magnesia  is  preferred,  because  the  alkali  and  alkali  earth  hydroxids 
convert  organic  nitrogen  into  ammonia. 

Indirect  Method.  In  the  case  of  pure  ammoniacal  salts  or  solu- 
tions free  from  acid,  the  following  method  may  be  employed. 

To  a  weighed  quantity  of  the  salt  a  measured  quantity  of  normal 
sodium  hydroxid  is  added,  and  the  mixture  boiled  in  an  open  vessel 
until  all  the  ammonia  is  expelled. 

The  residual  alkali  in  the  flask  is  then  titrated  with  normal  acid, 
and  the  difference  between  the  normal  acid  used  and  the  normal  soda 
added,  gives  the  quantity  of  the  latter  which  reacted  with  the  ammonium 
salt. 

The  reaction  is  thus  expressed:. 

NH4Cl+NaOH=NaCl+H2O  +  NH3; 

N 
each  cc.  of  —  NaOH= 0.05311  gm.  NH4C1; 

=  0.0656     "     (NH4)2S04. 

The  estimation  of  ammonium  carbonate  is  described  on  page  79. 
The  carbonic  acid  in  this  may  be  determined  by  adding  to  the  hot 
solution  of  the  salt  sufficient  barium  chlorid  to  precipitate  the  carbonate 
as  barium  carbonate.  The  precipitate  is  then  well  washed,  and  dis- 
solved in  excess  of  normal  acid  and  retitrated  with  normal  alkali,  the 
number  of  cc.  of  normal  acid  taken  up,  multiplied  by  0.021835,  gives 
the  weight  of  carbon  dioxid  in  the  sample.  See  page  244. 

By  Means  of  Formaldhyde  (Ronchese).  This  method  depends 
upon  the  fact  that  a  neutral  solution  of  formaldehyde  added  in  excess 
to  a  neutral  solution  of  any  ammonium  salt,  will  react  with  formation 
of  hexamethylenamin  and  liberation  of  the  corresponding  acid. 

N 

The  liberated  acid  is  titrated  with  —  sodium  hydroxid,  using  phenol - 

10 

phthalein  as  indicator.  Cochineal  or  methyl  orange  cannot  be  em- 
ployed, as  hexamethylenamin  reacts  alkaline  with  these  indicators. 
The  solution  of  ammonium  salt  is  diluted  to  100  cc.  with  recently  boiled 
distilled  water,  a  few  drops  of  phenolphthalein  added,  and  then  a 
large  excess  of  a  neutral  20  per  cent  solution  of  formaldehyde.  The 
solution  is  then  carefully  titrated  with  decinormal  alkali.  In  case  the 
sample  originally  contained  free  acid,  it  is  divided  into  two  equal 
portions,  in  one  the  acidity  is  determined  as  above,  in  the  other  the 
total  acidity,  using  an  indicator  not  effected  by  ammonium  salts,  as 
rosolic  acid  or  fluorescein. 


CHAPTER  XXVII 
ANTIMONY 

Oxidation  by  lodin  in  Alkaline  Solution  (Mohr).  Antimonous 
Oxid,  or  any  of  its  compounds,  is  estimated  in  the  manner  described 
on  page  195. 

Solution  of  the  oxid  is  first  effected  by  means  of  tartaric  acid,  and 
any  excess  of  the  latter  neutralized  by  sodium  carbonate.  Then  for 
every  o.i  gm.  of  80203,  10  cc.  of  a  cold  saturated  solution  of  sodium 
bicarbonate  are  added,  then  starch  solution,  and  finally  titrated  with 

N  .   ,. 
—  lodin. 

10  N 

Each  cc.  of  —  iodin  V.  S.  =  0.005965  gm.  Sb; 
10 

=  0.007156  gm.  Sb2C>3. 

One  gram  of  the  antimonous  oxid  is  weighed  out  into  a  250  cc. 
flask.  About  2  gms.  of  tartaric  acid  and  25  cc.  of  water  are  added 
and  the  mixture  shaken  until  the  antimonous  oxid  is  dissolved.  The 
solution  is  then  just  neutralized  with  sodium  carbonate  and  diluted 
with  water  to  make  250  cc.  50  cc.  of  this  solution  are  removed  by 
means  of  a  pipette,  transferred  to  a  beaker,  and  30  cc.  of  a  cold,  satu- 
rated solution  of  sodium  bicarbonate  added,  and  after  the  addition  of 
a  few  drops  of  starch  solution,  the  titration  with  decinormal  iodin  is 
begun  at  once. 

Example.  In  the  above  titration  26.8  cc.  of  decinormal  iodin 
solution  were  consumed.  Therefore  the.  i  gm.  taken  for  analysis 
would  require  26.8X5  =  134  cc. 

Antimonous  sulphid  may  be  dissolved  in  hydrochloric  acid  by  the 
aid  of  heat,  tartaric  acid  added,  and  then  the  solution  made  alkaline 
by  the  addition  of  sufficient  sodium  bicarbonate  and  titrated  with 
decinormal  iodin  as  above. 

Metallic  antimony  in  solution  (free  from  arsenic  and  tin),  in  fact 
all  antimony  compounds,  may  be  converted  into  sulphid  by  means 
of  hydrogen  sulphid.  The  precipitated  sulphid  is  thoroughly  washed 
and  then  dissolved  in  hydrochloric  acid,  the  solution  is  boiled  until 
all  traces  of  hydrogen  sulphid  have  been  removed,  and  after  diluting 

340 


ANTIMONY  341 

with  water  and  adding  tartaric  acid  and  then  sodium  bicarbonate  to 
alkalinity,  the  titration  is  conducted  with  decinormal  iodin  in  the  pres- 
ence of  starch,  as  before  described. 

When  assaying  antimonous  compounds  by  this  method,  the  titra- 
tion with  standard  iodin  must  be  begun,  and  completed  without  delay, 
as  otherwise  a  portion  of  the  antimony  will  be  precipitated  as  anti- 
monous hydrate,  upon  which  iodin  has  no  effect.  F.  H.  Alcock  (Ph. 
Jour.,  1900,  362),  recommends  the  following  modification  to  avoid 
this  precipitation,  and  thus  permit  of  working  with  less  haste. 

Weigh  off  i  gm.  of  tartar  emetic,  add  50  cc.  of  water  and  10  gms. 
of  Rochelle  salt,  and  after  solution  is  effected,  add  3  gms.  or  more  of 
sodium  bicarbonate  and  make  up  to  a  suitable  volume.  Of  this  solu- 
tion an  aliquot  is  titrated  with  the  iodin  solution. 

If  solution  of  antimonous  chlorid  is  to  be  assayed,  5  cc.  of  the  solu- 
tion are  taken  and  the  other  ingredients  added  in  the  same  quantities. 
If  antimonous  oxid  is  to  be  assayed,  i  gm.  is  converted  into  tartar 
emetic  by  the  aid  of  2  gms.  of  potassium  bitartrate,  10  gms.  of  Rochelle 
salt  are  added,  followed  by  an  excess  of  sodium  bicarbonate  and  water 
to  make  200  cc.;  an  aliquot  portion  of  the  solution  is  then  titrated 
with  iodin. 

Type  Metal  (containing  lead  and  antimony).  0.5  gm.  are  dissolved 
in  the  smallest  possible  quantity  of  aqua  regia.  and  then  an  excess  of 
aqua  ammonia  and  yellow  ammonium  sulphid  are  added  and  the 
mixture  allowed  to  digest  for  several  hours.  The  precipitated  lead 
sulphid  is  then  separated  by  filtration  and  thoroughly  washed.  The 
filtrate  and  washings  are  then  made  slightly  acid  with  diluted  sulphuric 
acid,  and  heated  to  drive  off  all  hydrogen  sulphid.  The  resulting 
precipitate  of  antimony  sulphid  is  then  dissolved  in  hydrochloric  acid, 
and  after  the  addition  of  tartaric  acid  and  sodium  bicarbonate,  as 

N 

above  described,  titration  with  —  iodin  is  begun,  using  starch  as  indi- 

10 

cator.  If  greater  accuracy  is  desired,  the  precipitate,  containing  lead 
sulphid,  is  dissolved  in  hydrochloric  acid  and  again  treated  with  am- 
monia and  ammonium  sulphid. 

Titration  with  Standard  Potassium  Bromate  in  Acid  Solution 
(Gyory).*  The  method  which  may  be  used  for  arsenic  as  well  as 
antimony,  is  based  upon  the  oxidation  of  arsenous  or  antimonous  oxid 
by  means  of  a  standard  solution  of  potassium  bromate,  in  the  presence 
of  hydrochloric  acid.  For  a  i  per  cent  solution  of  arsenous  acid  an 
equal  volume  of  diluted  hydrochloric  acid  should  be  taken.  In  the 
case  of  antimony,  however,  a  larger  quantity  of  the  acid  is  used ;  suffi- 

*  Zeitschr.  f.  analyt.  Chem.,  32,  416  (1893). 


342  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

cient  must  be  added  to  prevent  precipitation  of  the  antimony  during 
titration  as  a  result  of  the  increasing  dilution  of  the  solution;  thus  for 
0.33  gm.  of  tartar  emetic,  25  cc.  or  more  of  diluted  hydrochloric  acid 
should  be  employed.  The  reactions  with  arsenic  and  with  antimony- 
are  as  follows: 


and 


Methyl-orange  solution  (o.i  gm.  in  100  cc.  water)  is  added  to  the 
acid  solution  as  indicator.  The  slightest  excess  of  the  standard  bromate 
solution  completely  decolorizes  the  red  solution,  through  liberation  of 
bromin. 

The  decinormal  potassium  bromate  is  made  by  dissolving  ^  of 
the  molecular  weight  in  grams  of  pure  crystallized  potassium  bromate 
(KBrOs)  dried  at  110°  C.  in  sufficient  distilled  water  to  make  1000  cc. 

Each  cc.  of  this  solution  represents 

0.004911  gm.  of  As2C>3; 
0.007156  "  "  Sb203; 
0.016495  "  "  2K(SbO)C4H406H20. 

Titration  with  Standard  lodate  (Andrews).  The  determination 
of  antimony  by  this  method  is  precisely  like  that  of  arsenic,  which  see. 

Oxidation  by  Dichromate  or  Permanganate  in  Presence  of 
Hydrochloric  Acid  (Kessler).*  The  solutions  required  are: 

(a)  Standard  Arsenous  Oxid.     5  gms.  of  pure  arsenous  oxid  are 
dissolved  with  the  aid  of  sodium  hydroxid  solution,  hydrochloric  acid 
is  added  until  the  solution  is  slightly  acid,  and  then  100  cc.  of  pure 
hydrochloric  acid  (sp.gr.   1.12)  are   added  and  the  solution   diluted 
with  water  to  1000  cc. 

Each  cc.  of  this  solution  =  0.005  gm-  °f  arsenous  oxid,  which  corre- 
sponds to  0.007287  gm.  of  antimonous  oxid  (Sb2C>3). 

(b)  Solution  of  Potassium  Dichromate.     2.5   gms.   of  the  salt   in 
1000  cc. 

(c)  Solution  of  Ferrous  Sulphate.     Made  by  dissolving  i.i  gms.  of 
pure  iron  wire  in  20  cc,  of  diluted  sulphuric  acid  (1:4)  and  adding 
water  to  make  1000  cc. 

(d)  Solution  of  Potassium  Ferricyanid,  freshly  prepared  (Indicator). 
The  Relation  between  the  Dichromate  Solution  and  the 

Iron  Solution  is  found  as  follows  : 

*  Poggend.  Annal.  CXVIII,  17. 


ANTIMONY  343 

From  a  burette  10  cc.  of  the  dichromate  solution  are  run  into  a 
beaker,  5  cc.  of  hydrochloric  acid  and  50  cc.  of  water  are  added,  and 
then  the  iron  solution  delivered  into  the  mixture  from  another  burette, 
until  the  fluid  is  green.  Then  continue  adding  the  iron  solution, 
i  cc.  at  a  time,  testing  a  drop  of  the  fluid  after  each  addition  by  bringing 
it  in  contact  with  a  drop  of  the  ferricyanid  solution  on  a  white  slab, 
until  a  blue  color  is  obtained.  Then  add  0.5  cc.  more  of  the  dichromate 
solution,  and  again  the  iron  solution  in  drops  until  the  blue  color  just 
appears.  Now  read  off  both  burettes  and  calculate  how  much  dichro- 
mate solution  corresponds  to  10  cc.  of  the  iron  solution. 

If  the  solutions  are  made  as  above  described  10  cc.  of  the  iron 
solution  will  correspond  to  about  3.9  cc.  of  the  dichromate  solution,  i.e., 
each  cubic  centimeter  of  the  former  will  correspond  to  0.39  cc.  of  the 
dichromate  solution. 

The  Relation  between  the  Dichromate  Solution  and  the 
Arsenous  Oxid  Solution  is  now  to  be  ascertained. 

Ten  cubic  centimeters  of  the  arsenous  oxid  solution  are  introduced 
into  a  beaker,  together  with  20  cc.  of  hydrochloric  acid  (sp.gr.  1.12) 
and  from  80  to  100  cc.  of  water.* 

The  dichromate  solution  is  then  run  in  until  the  yellow  color  of  the 
fluid  indicates  it  to  be  in  excess.  The  mixture  is  allowed  to  react  a 
few  minutes  and  then  the  ferrous  sulphate  solution  added  until  a 
drop  of  the  solution  from  the  beaker  gives,  with  a  drop  of  the  ferri- 
cyanid solution,  a  blue  color.  The  end-point  is  more  accurately  deter- 
mined by  adding  0.5  cc.  of  the  dichromate  solution  and  again  titrating 
with  the  iron  solution  in  drops  until  the  precise  end-point  is  obtained. 
Then  deduct  from  the  total  quantity  of  dichromate  solution  used  the 
amount  corresponding  to  the  iron  solution  employed,  ie.,  about  0.39  cc. 
for  each  cc.  of  iron  solution.  This  will  give  the  quantity  of  dichromate 
solution  required  for  the  oxidation  of  10  cc.  of  the  arsenous  oxid  solu- 
tion; about  20.2  cc.  will  be  required. 

From  these  data,  the  quantity  of  antimony  corresponding  to  100  cc. 
of  the  dichromate  solution  is  easily  calculated. 

If  each  cubic  centimeter  of  the  arsenous  oxid  solution  corresponds 
to  0.007287  gm.  of  Sb2Os  and  100  cc.  of  the  dichromate  solution  rep- 
resents 49  cc.  of  the  arsenous  oxid  solution,  100  cc.  of  the  dichromate 
solution  will  represent  0.357663  gm.  of  Sb2Os,  or  each  cubic  centimeter 
0.00357663  gm.  Sb2Os. 

The  Actual  Analysis.  If  organic  matter,  heavy  metallic  oxids,  or 
other  oxidizable  bodies  are  absent,  the  antimonous  compound  is  dis- 

*  The  water  must  be  measured,  because  uniformity  of  action  is  insured  only 
if  the  volume  of  hydrochloric  acid  present  is  not  less  than  J  nor  more  than  $  of 
the  entire  volume  of  solution. 


344  A   MANUAL   OF    VOLUMETRIC   ANALYSIS 

solved  at  once  in  hydrochloric  acid  not  less  than  one  sixth  nor  more 
than  one  third  of  the  volume  of  the  solution  should  consist  of  hydro- 
chloric acid  (sp.gr.  1.12).  The  dichromate  solution  is  now  run  in, 
and  the  titration  carried  out  precisely  as  directed  for  the  determina- 
tion of  the  relationship  between  the  arsenous  oxid  solution  and  the 
dichromate  solution. 

The  use  of  more  than  one  third  of  hydrochloric  acid  will  interfere  with  a  nice 
and  precise  determination  of  the  end-reaction  with  potassium  ferricyanid.  Tar- 
taric  acid  is  inadmissible  here  as  a  solvent,  because  it  interferes  with  the  action 
of  chromic  acid  on  the  ferrous  salt.  If  the  direct  determination  of  antimony 
in  the  hydrochloric  acid  solution  is  not  practicable,  precipitate  it  with  hydrogen 
sulphid,  and  after  washing  the  precipitate,  dissolve  it  in  hydrochloric  acid  on  a 
water-b/ith,  and  after  removing  the  hydrogen  sulphid  by  the  addition  of  a  satu- 
rated solution  of  mercuric  chlorid  in  hydrochloric  acid,  proceed  as  directed. 

The  Titration  with  Potassitim  Permanganate.  The  same 
proportion  of  hydrochloric  acid  solution  is  necessary  as  in  the  foregoing. 
The  permanganate  solution  which  may  contain  about  1.5  gms.  of  the 
pure  crystallized  salt  in  the  liter  is  added,  till  the  rose  color  is  perma- 
nent. The  addition  of  a  small  quantity  of  magnesium  sulphate  pre- 
vents the  decomposition  of  permanganate  by  the  hydrochloric  acid. 
Tartaric  acid,  at  least  in  the  proportion  in  which  it  exists  in  tartar 
emetic,  does  not  interfere.  Hence  since  tartar  emetic  can  easily  be 
obtained  in  a  pure  state,  it  may  be  employed  for  standardizing  the 
permanganate  solution. 

Estimation  of  Antimonous  Sulphid  by  Oxidation  with  Ferric 
Sulphate,  and  Titration  of  the  Resultant  Ferrous  Sulphate  with 
Permanganate  Q.  Hanus).*  The  reaction  is  as  follows: 


The  sulphid  is  boiled  for  fifteen  minutes  in  a  beaker  with  an  excess  of 
ferric  sulphate,  allowed  to  cool,  and  after  adding  an  excess  of  sulphuric 
acid,  titrated  with  potassium  permanganate.  Sb2S3  corresponds  to 
loFe. 

Antimonic  Acid  and  its  Salts  f  are  dissolved  and  strongly  acidified 
with  hydrochloric  acid,  a  strong  solution  of  sodium  sulphite  is  then 
gradually  added,  the  mixture  boiled  to  drive  off  the  SC>2,  a  drop  of 
phenolphthalein  solution  added,  and  then  KOH  until  slightly  alkaline, 
as  shown  by  red  color.  Then  a  small  excess  of  tartaric  acid  is  added 
and  the  process  completed  as  for  antimonous  acid. 

N  . 
i  cc.  —  iodin  =  0.005965  gm.  Sb. 

*  Apoth.  Ztg.  Ang.  31,  1898,  613. 

f  According  to  Von  Knorre,  Zeitsch.  angew.  Chem.  (1888),  155. 


ANTIMONIC  ACID   AND   ITS  SALTS  345 

The  reduction  to  antimonous  oxid  may  also  be  made  by  means  of 
H2S.  See  page  196. 

Other  articles  on  the  titrimetric  estimation  of  antimony  which  may 
be  referred  to  with  profit  are: 

"  Estimation  of  Antimony  in  Alloys,  such  as  Babbitt  and  Type 
Metals."  H.  Yockey.  J.  A.  C.  S.  (1906),  page  1435. 

"Estimation  of  Antimony  in  the  Presence  of  Organic  Matter." 
Norton  and  Koch.  J.  A.  C.  S.  (1905),  page  1247. 

"Estimation  of  Antimony  and  Arsenic  in  Ores,"  etc.  A.  H.  Low. 
J.A.C.S.  (1906),  page  1715. 


CHAPTER  XXVIII 
ARSENICUM 

Oxidation  by  lodin  in  Alkaline  Solution  (Mohr).  The  estima- 
tion of  arsenous  compounds  by  this  method  is  fully  described  on  page 
192.  See  also  pages  188  and  190. 

Oxidation  by  Potassium  Bichromate  (Kessler).  This  method 
is  exactly  the  same  as  is  minutely  described  for  antimony.  See  page  342. 
In  its  simpler  form  it  is  as  follows: 

o.i  gm.  of  the  substance  is  dissolved  in  about  10  cc.  of  water  with 
the  aid  of  hydrochloric  acid.  Then  20  cc.  of  hydrochloric  acid  (sp.gr. 

N 
1.12)  and  80  cc.  of  water  are  added.     An  excess  of  -  •   potassium 

dichromate  (say  30  cc.)  is  now  introduced,  the  mixture  allowed  to 
react  for  a  few  minutes  and  then  retitrated  with  a  ferrous  sulphate 
solution  which  corresponds  in  strength  with  the  dichromate.  A  freshly 
prepared  solution  of  potassium  ferricyanid  is  used  as  the  indicator. 
The  difference  between  the  quantities  of  the  ferrous  sulphate  and 
dichromate  solutions  used  gives  the  quantity  of  the  latter  which  reacted 
with  the  arsenous  oxid.  In  order  to  find  the  end-reaction  more  accu- 
rately, it  is  advised  to  add  another  one  half  or  i  cc.  of  the  dichromate 
and  again  retitrate  with  the  ferrous  sulphate  solution.  The  reaction  is 
as  follows: 

3As2O3  +  2K2Cr2O7  +  i6HCl  =  4KC1  +  4CrCl3  +  3As2O5  +  8H2O; 

2)589.32  2)587.56 
6)294.66  6)293.76 
1 0)49- "  10)48.713 

4.911  gms.        4.8713    gms.=  iooo  cc.  —  dichromate. 

10 

N 
i  cc.  —  dichromate =0.0049 1 1  gm-  As2Os. 

The  reaction  between  the  ferrous  sulphate  and  the  dichromate  is 
shown  on  page  184. 

In  the  above  estimation  the  volume  of  hydrochloric  acid  must  not 
be  less  than  one  sixth  nor  more  than  one  third  that  of  the  solution. 

If  the  direct  titration  of  the  arsenous  salt  in  hydrochloric  acid  solu- 
tion is  not  practicable,  it  is  precipitated  by  H2S;  the  precipitate  is 

346 


T1TRATION  WITH   STANDARD  POTASSIUM  1ODATE     347 

washed  and  placed  in  a  stoppered  bottle  together  with  a  saturated 
solution  of  mercuric  chlorid  in  hydrochloric  acid  (sp.gr.  1.12)  and 
gently  heated  until  the  precipitate  is  white,  then  water  is  added  in 
such  amount  that  the  hydrochloric  acid  present  in  the  liquid  be  not 

N 

less  than  one  sixth  of  its  volume.     The  titration  with  the  —  dichromate 

10 

is  then  carried  out  as  above  described. 

By  Titration  with  Standard  Potassium  lodate.  Andrews 
(J.  A.  C.  S.,  25,  p.  759)  suggests  a  method  for  the  estimation  of  arsen- 
ous  oxid  or  chlorid  based  upon  the  following  equation  : 


The  process  is  conducted  in  the  same  manner  as  that  for  the  esti- 
mation of  iodids  by  means  of  standard  potassium  iodate  solution 
(see  page  263),  with  the  exception  that  too  great  a  concentration  of 
hydrochloric  acid  must  be  avoided.  The  amount  of  the  latter  added 
to  the  arsenous  solution  should  be  sufficient  to  make  the  hydrochloric 
acid  equal  to  about  20  per  cent  of  the  entire  mixture  at  the  end  of 
the  titration.  Chloroform  is  used  as  the  indicator,  5  cc.  being  usually 
taken,  and  the  standard  iodate  solution  run  in  from  a  burette,  at  first 
nearly  the  quantity  which  in  the  judgment  of  the  operator  the 
arsenous  solution  will  require,  then  the  liquid  is  shaken  and  the  titra- 
tion continued  cautiously  until  the  chloroform  is  decolorized. 

Example,  o.i  gm.  of  arsenous  oxid  is  dissolved  in  10  cc.  of  water, 
5  cc.  of  fuming  hydrochloric  acid  added,  and  then,  after  introducing 
5  cc.  of  chloroform,  the  titration  with  the  iodate  solution  is  begun. 
Nine  cubic  centimeters  are  added  at  once,  and  then  more  is  added, 
drop  by  drop  until  the  end-point  is  reached. 

N 
Each  cc.  of  -  -  KIO3=  0.00982  2  gm.  of  As2O3. 

Since  copper  does  not  interfere  in  the  least  with  the  application 
of  this  method,  it  is  possible,  for  example,  to  titrate  the  arsenic  in  Paris 
green  directly  without  preliminary  separation. 

0.5  gm.  of  Paris  green  are  dissolved  in  15  cc.  of  water  and  25  cc. 
of  fuming  hydrochloric  acid  and  directly  titrated  with  the  decinormal 
iodate  solution,  in  the  presence  of  chloroform  as  indicator. 

Arsenic  Oxid  by  Precipitation  with  Uranium  Solution. 
Arsenic  acid  forms  with  uranic  nitrate  or  acetate  a  precipitate  which  is 
analogous  in  composition  to  that  produced  by  phosphoric  acid.  The 
estimation  is  conducted  in  exactly  the  same  way  as  that  of  phosphoric 
acid  and  under  precisely  similar  conditions  as  to  quantity  of  fluid, 
the  amount  of  acetate  and  acetic  acid  added,  and  depth  of  color  obtained, 
with  indicator. 


348  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

The  arsenic  must  be  in  the  state  of  As2Os.  If  it  is  in  the  form  of 
As2Os  it  may  be  oxidized  to  As2Os  by  evaporation  with  strong  nitric 
acid,  neutralizing  with  an  alkali,  and  then  dissolving  in  acetic  acid. 

The  uranium  solution  may  be  standardized  by  means  of  pure 
sodium  arsenate  or  by  a  weighed  quantity  of  pure  arsenous  oxid  con- 
verted into  arsenic  oxid  by  evaporation  with  strong  nitric  acid,  neu- 
tralizing with  alkali,  and  then  dissolving  in  acetic  acid. 

Arsenic  Oxid  (As2O5).  This  may  be  estimated  by  iodin  as  directed 
for  As2C>3,  if  it  be  first  reduced  to  the  latter  form  by  boiling  with  potas- 
sium iodid  in  the  presence  of  hydrochloric  acid  in  large  excess  until 
the  iodin  vapors  are  entirely  dissipated.  It  is  then  cooled,  neutralized 
with  sodium  carbonate,  then  bicarbonate  added  in  excess  and 

N  . 

titrated  with  —  iodin  as  directed  for  As2C>3. 
10 

i  cc.  =  0.005705  gm.  As2Os. 

Arsenic  Oxid  and  Arsenates  may  also  be  estimated  by  means  of 
magnesia  mixture  in  exactly  the  same  way  as  described  for  phosphoric 
acid.  See  page  311. 

N 
Each  cc.  of  —  hydrochloric  acid  V.  S.  represents  0.005705  gm. 

As2O5. 

By  Distillation  with  Chromic  and  Hydrochloric  Acids  (Bunsen). 
When  potassium  dichromate  is  boiled  with  hydrochloric  acid,  chlorin 
is  given  off  in  accordance  with  the  following  reaction  : 


One  molecule  CrOs  gives  3  atoms  chlorin.  If  arsenous  oxid  be 
present,  it  is  oxidized  to  arsenic  oxid  at  the  expense  of  a  part  of  the 
chlorin,  and  less  is  therefore  given  off.  As2O3  +  4Cl-f-2H2O  =  As2O5+ 
4HC1.  The  excess  of  chlorin  evolved  from  a  measured  quantity  of 
dichromate  over  that  required  to  oxidize  the  arsenous  to  arsenic  oxid 
is  received  in  a  solution  of  potassium  iodid,  and  the  iodin  titrated 

N  N 

with  —  thiosulphate.     If  we  let  a  represent  the  cc.  of  —  dichromate 
10  N  10 

taken,  and  b  the  cc.  of  —  thiosulphate  used, 
10 

0—5X0.0019644=  As; 
0—6X0.004911  = 


The  distillation  may  be  conducted  in  one  of  the  apparatus  described 
under  distillation  methods,  page  214  and  shown  in  Figs.  60,  61,  and  62. 


HOUZEAN'S  METHOD  349 

By  Titration  with  Standard  Br ornate  (Gyory).  This  method  is 
described  under  antimony. 

ESTIMATION   OF   ARSENIC   IN   SMALL   QUANTITIES,   AS   IN  CASES   OF 

POISONING 

Houzean's  Method  (Comp.  rend.,  LXXV).  The  substance  contain- 
ing the  arsenic  is  placed  in  a  Marsh's  apparatus,  and  the  arseniureted 

N 
hydrogen  given  off  is  passed  into   a  measured  amount  of  -  -  silver 

nitrate  solution.     A  part  of  the  silver  nitrate  is  reduced  to  metallic 
silver,  which  may  be  separated  by  nitration  and  the  nitrate  titrated 

N 

with  —  sodium  chlorid.     The  loss  of  silver  corresponds  to  the  arsenic. 
10 

AsH3  +  6AgNO3  +  3H2O=6Ag+H3AsO3  +  6HNO3. 

6)75  6)1018.2 

10)12.5        10)160.7 

1000)1.25     1000)16.97 

.00125  .01697  =  1000  cc.  —  V.  S. 

10 

The  number  of  cc.  of  sodium  chlorid  deducted  from  the  number 

N 
of  cc,  of  —  AgNO3  solution  first  taken  gives  the  number  of  cc.  of  the 

latter  which  was  reduced  by  the  AsH3. 

Each  cc.  thus  reduced  represents  0.00125  gm.  of  As,  or  0.00408  gm. 
As203. 

R.  C.  Cowley  and  J.  P.  Catford,*  suggest  the  utilization  of  Reinsch's 
test  for  the  quantitative  estimation  of  arsenic  in  small  quantities.  The 
authors  assert  that  by  this  method  it  is  possible  to  definitely  measure 
to  the  -fa  of  a  milligram  or  even  to  -&irW  of  a  grain. 

The  method  is  as  follows: 

A  few  inches  of  fine  copper  wire  coiled  into  a  helix  by  twisting  it 
around  a  glass  tube,  is  immersed  in  10  cc.  of  the  liquid  to  be  tested,  to 
which  one  fifth  of  its  volume  of  hydrochloric  acid  has  been  added.  The 
liquid  and  acid  are  contained  in  a  test  tube,  which  is  supported  upright 
in  a  salt  water-bath  by  means  of  a  loop  of  wire  resting  on  the  edges 
of  the  bath.  The  coil  of  copper  wire  is  arranged  so  that  it  shall  reach 
from  the  bottom  of  the  arsenical  liquid  to  above  its  surface.  The 
test  tube  must  be  immersed  in  the  salt  water-bath  so  that  the  liquid 
it  contains  shall  be  below  the  level  of  the  liquid  in  the  bath;  the  bath 
is  to  be  kept  simmering,  without,  however,  reaching  the  boiling  point, 

*  Pharm.  Jour  ,  1904,  897. 


350  A    MANUAL  OF    VOLUMETRIC  ANALYSIS 

for  about  an  hour.  The  projecting  extremity  of  the  copper  is  novf 
to  be  pressed  down  below  the  surface  of  the  liquid,  and  if  it  remains 
bright,  after  continuing  the  application  of  heat  for  another  fifteen 
minutes,  the  arsenic  will  be  all  removed  from  the  liquid,  and  the  wire 
may  be  removed  to  a  small  dish,  rinsed  without  touching  it  with  the 
fingers,  and  the  deposit  then  dissolved  off  by  a  cubic  centimeter  of 
bromin  water  containing  a  little  hydrobromic  acid.  The  clean  wire 
is  lifted  out,  rinsed  with  water,  and  if  thought  necessary  may  be  returned 
to  the  acid  liquid  to  make  sure  that  all  the  arsenic  has  been  deposited 
from  it.  The  bromin  solution  now  contains  the  arsenic  as  arsenous 
acid.  To  it,  i  cc.  of  solution  of  potassium  hydroxid  is  added,  and  the 
liquid  is  boiled  until  the  light  green  copper  compounds  are  broken 
up.  During  this  treatment  the  cupric  oxid  formed  as  an  interme- 
diate compound  oxidizes  the  arsenic,  and  a  solution,  the  alkali 
arsenate  results,  which  is  filtered  from  the  copper  oxids.  An  aliquot 
part  of  the  filtrate  may  be  reserved  and  tested  for  arsenic  acid  by  the 
molybdate  reagent  after  evaporation;  the  remainder  is  reduced  again 

N 

to  arsenite  and  titrated  with  or  other  suitably  weak  solution  of 

100 

iodin. 

For  a  burette  the  authors  use  a  pipette  graduated  in  hundredths 
of  a  cubic  centimeter.  The  flow  is  controlled  by  slipping  over  the  top 
of  the  pipette  a  piece  of  rubber  tubing  compressed  by  a  screw  clamp. 

ESTIMATION    OF    ARSENIC    IN    PARIS    GREEN 

Smith's  Method.*  This  method,  which  is  generally  considered 
the  most  accurate,  depends  upon  precipitating  the  copper  as  cuprous 
oxid  by  boiling  with  sodium  hydroxid.  The  arsenic  being  present  as 
arsenite,  acts  as  a  reducing  agent  upon  the  copper.  The  filtrate, 
which  contains  some  arsenate,  is  concentrated  by  boiling,  and  after 
acidulation  with  hydrochloric  acid  the  arsenate  is  wholly  reduced  to 
arsenite  by  means  of  an  excess  of  potassium  iodid.  The  liberated 
iodin  is  then  taken  up  by  sodium  thiosulphate,  the  solution  neutralized 
with  sodium  carbonate,  and  after  the  addition  of  an  excess  of  sodium 

N 

bicarbonate  the  arsenite  is  titrated  with  —  iodin  solution  in  the  pres- 

10 

ence  of  starch  as  indicator. 

The  Process.  2  gms.  of  the  Paris  green  are  weighed  out,  and  about 
100  cc.  of  water  and  2  gms.  of  sodium  hydroxid  added.  The  solution 
is  brought  to  a  boil,  and  boiling  continued  for  a  few  minutes.  It  is 
then  allowed  to  cool  to  room  temperature  and  the  solution  made 

*J.A.  C.  S..XXXI  (1899X769 


THE   AVERY    AND   BEANS'    METHOD  351 

up  to  250  cc.  The  well-shaken  liquid  is  filtered  through  a  dry  filter 
and  50  cc.  taken  for  the  analysis.  This  portion,  equal  to  0.4  gm.  is 
concentrated  to  about  one  half  its  volume  and  allowed  to  cool  to  80°  C. 
An  equal  volume  of  strong  hydrochloric  acid  is  then  added,  accompanied 
by  3  gms.  of  potassium  iodid,  and  the  whole  allowed  to  stand  for  ten 
minutes  (longer  is  not  necessary).  The  deep  red  solution  is  slightly 
diluted  with  water  to  dissolve  the  precipitate  caused  by  the  potassium 
iodid,  and  a  dilute  solution  of  thiosulphate  added  until  the  color  just 
disappears.  This  solution  is  then  made  neutral  by  addition  of  dry 
sodium  carbonate  and  finally  an  excess  of  sodium  bicarbonate  is  added. 
Decinormal  iodin  solution  is  then  delivered  from  a  burette  and  the 
end-reaction  noted  by  starch  solution. 

The  Avery  and  Beans'  Method.*  This  method,  which  has  the 
advantage  of  consuming  very  little  time  is  as  follows:  Sample  the 
Paris  green  by  quartering  (as  one  would  an  ore  for  assaying)  down 
to  about  i  gm.  Pulverize  this  small  sample  in  an  agate  mortar  and 
weigh  out  0.2  to  0.3  gm.  into  a  beaker  of  about  300  cc.  capacity. 
Add  about  25  cc.  of  water,  and  to  the  Paris  green,  suspended  in 
water,  add,  with  constant  stirring,  concentrated  hydrochloric  acid  until 
solution  is  just  effected;  from  6  to  10  drops  are  usually  sufficient. 
Now  add  to  the  acid  solution  sodium  carbonate  solution  till  a  slight 
permanent  precipitate  is  formed,  and  at  this  point  add  2  or  3 
gms.  of  Rochelle  salt  in  solution.  The  tartrate  will  at  once  dis- 
solve the  precipitated  copper  and  prevent  further  precipitation  during 
the  subsequent  titration.  Dilute  to  about  200  cc.;  add  solid  sodium 

N 
bicarbonate  and  starch  solution,  and  titrate  with  —  iodin  solution  in 

the  usual  way. 

This  method,  it  will  be  seen,  rests  on  the  principle  that  arsenous 
acid  may  be  titrated  with  iodin  in  the  presence  of  cupric  salts,  pro- 
vided an  alkali  tartrate  be  present. 

By  this  method  most  excellent  results  are  obtained  if  the  Paris 
green  examined  is  pure,  but  as  Hay  wood  pointed  out,f  if  the  samples 
of  Paris  green  contain  considerable  free  arsenous  oxid  the  results  are 
always  low.  This  is  due  to  the  fact  that  the  free  arsenous  oxid  is 
not  readily  soluble  in  cold  hydrochloric  acid.  Avery  also  calls  atten- 
tion to  this,t  and  suggests  gentle  boiling,  and  if  solution  of  the  free 
arsenous  oxid  is  not  thereby  effected,  add  a  cold  saturated  solution 
of  sodium  acetate,  using  about  3  gms.  of  the  salt  for  each  o.i  gm. 
of  the  green.  After  solution  is  effected,  an  alkali  tartrate  is  added, 

*  J  A.  C.  S.,  XXIII  (1901),  485. 
tJ.A.C.S.,  XXV  (1903),  963. 
$  J.  A.  C.  S.,  XXV  (1903),  1096. 


352  A     MANUAL    OF    VOLUMETRIC    ANALYSIS 

and  solid  bicarbonate,  and  the  diluted  solution  titrated  in  the  usual 
manner. 

Haywood's  Modification  of  the  Avery  and  Beans'  Method* 
Sample  the  Paris  green  (as  one  would  an  ore  for  assaying)  down  to 
2  gms.  Pulverize  this  small  sample  in  a  mortar  and  place  from  0.3 
to  0.4  gm.  in  a  beaker.  Add  about  25  cc.  of  water  and,  while  con- 
stantly stirring,  add  concentrated  hydrochloric  acid,  a  drop  at  a  time, 
until  all  the  Paris  green  is  in  solution  and  the  free  arsenous  oxid 
remains  as  a  residue.  Filter,  and  wash  the  residue.  The  arsenous 
oxid  in  the  nitrate  is  determined  in  exactly  the  same  manner  as  in  the 
A  very-Bean's  method.  The  filter  and  contents  are  dropped  back 
into  the  beaker,  which  also  receives  the  water  used  in  washing  the 
funnel.  Five  grammes  of  sodium  bicarbonate  are  added  and  the 
solution  boiled  until  the  arsenous  oxid  is  completely  dissolved  (this 
takes  from  5  to  10  minutes).  The  resulting  solution  is  cooled  and 
acidified,  using  a  drop  of  methyl  orange  to  read  the  change.  It  is 
then  made  alkaline  again  with  bicarbonate,  starch  added,  and  titrated 
with  iodin  as  usual. 

Other  articles  on  the  volumetric  estimation  of  arsenic  are: 

Pierce 's  Method,  by  Standard  Silver.  Sutton's  ''Volumetric 
Analysis." 

J.  F.  Bennett's  ''Modification  of  Pierce's  Method."  J.  A.  C.  S., 
xxi  (1899),  431- 

Pattinson's  "Estimation  of  Arsenous  Sulphid  by  Iodin."  J.  S. 
C.  I.,  1898,  211. 

A.  H.  Low,  "  Estimation  of  Antimony  and  Arsenic  in  Ores," 
etc.  J.  A.  C.  S.  (1906),  1715. 

F.  A.  Norton  and  A.  E.  Koch,  "  A  Method  for  the  Estimation  of 
Arsenic  and  Antimony  in  the  Presence  of  Organic  Matter."  J.  A.  C. 
S.,  xxvn  (1905),  1247. 

*J.A.C.S.,XXV(i9o3),967. 


CHAPTER  XXIX 

BARIUM 

Barium  Oxid  or  Hydroxid  may  be  titrated  with  standard  acid, 
using  phenolphthalein  as  indicator.  But  since  the  hydroxid  frequently 
contains  carbonate  through  absorption  of  carbonic  acid  gas  from  the 
atmosphere,  it  is  usually  a  better  plan  to  add  an  excess  of  standard 
acid,  boil  to  expel  any  carbonic  acid,  and  then  retitrate  with  standard 
alkali. 

Barium  Carbonate  and  Organic  Salts  of  Barium  (the  latter 
after  ignition)  may  of  course  be  assayed  in  the  same  way.  See  Estima- 
tion of  the  Salts  of  the  Alkali  Earths,  page  91.  Under  the  same  head- 
ing will  also  be  found  a  method  for  the  estimation  of  soluble  salts  of 
barium,  such  as  chlorid  and  nitrate,  by  titration  with  a  standard 
solution  of  sodium  carbonate.  Barium  chlorid  in  the  absence  of 
other  chlorids  may  also  be  estimated  by  the  use  of  tenth-normal  silver 
nitrate  solution,  as  in  the  case  of  alkali  chlorids,  after  precipitating 
the  barium  by  means  of  potassium  sulphate.  The  precipitated  barium 
sulphate  may  be  separated  from  the  potassium  chlorid  formed  and  the 
filtrate  titrated  with  the  tenth-normal  silver  nitrate,  using  potassium 
chromate  as  indicator,  or  the  titration  may  be  done  without  first  filter- 
ing. The  precipitate  of  barium  sulphate  has  no  disturbing  effect. 

N 
Each  cc.  of  --  silver  solution =0.00682  gm.  of  Ba, 

=  0.010338  gm.  of  BaCl2. 

Soluble  barium  salts  may  also  be  estimated  by  precipitation  with 
sulphuric  acid.  The  process  is  the  converse  of  that  for  sulphuric  acid 
and  sulphate  by  means  of  barium  chlorid  (see  Chapter  XXIV). 

By  Titration  with  Potassium  Dichromate.  The  barium  salt 
in  alkaline  solution  is  titrated  with  standard  potassium  dichromate 
until  precipitation  is  complete  and  the  supernatant  liquid  shows  a 
slight  yellow  tint. 

The  dichromate  solution  used  for  this  purpose  is  a  decinormal 
solution,  but  it  differs  in  strength  from  that  used  as  an  oxidizing  solu- 
tion. 

It  is  made  by  dissolving  7.307  gms.  of  pure  potassium  dichromate 
in  water,  and  diluting  to  make  a  liter.  Each  cc.  of  this  decinormal 
potassium  dichromate  represents  .00682  gm.  of  Ba. 

353 


354  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

In  the  analysis  the  barium  compound  is  dissolved  in  water,  ammo- 
nia water  free  from  carbonate  is  added  to  alkaline  reaction,  the  mixture 
heated  to  70°  C.,  and  titrated  with  the  dichromate  solution,  allowing 
to  settle  after  each  addition  until  the  supernatant  fluid  shows  a  slight 
yellow  tint.  Lead  and  all  other  metals  which  form  chromate  insoluble 
in  ammoniacal  solution,  must  of  course  be  absent.  Calcium  and 
small  quantities  of  strontium  exercise  no  influence  upon  the  result. 


4)292.28 


10)126.78  I0)_73f7_  N 

12.678  gms.  7*307  gms.=  looocc.  —  V.  S. 


10 


The  Indirect  lodometric  Method.  This  mehod  depends  upon 
precipitating  the  barium  by  means  of  potassium  chromate,  and  then 
subjecting  the  precipitated  barium  chromate  to  the  digestion  process 
described  on  page  222. 

The  neutral  or  slightly  ammoniacal  solution  of  the  barium  salt  is 
treated  with  potassium  chromate  in  slight  excess  at  a  boiling  tem- 
perature, by  which  the  barium  is  completely  precipitated  as  chromate. 
The  solution  is  filtered,  the  precipitate  washed  with  hot  water  until 
the  filtrate  is  entirely  colorless,  and  while  still  moist  the  precipitate, 
together  with  the  filter,  is  pushed  into  the  stoppered  digestion  bottle 
(Fig.  64),  some  concentrated  hydrochloric  acid  is  added,  and  then  the 
air  in  the  bottle  is  displaced  by  carbon  dioxid.  (This  may  be  accom- 
plished by  throwing  in  some  sodium  bicarbonate  and  waiting  until 
effervescence  ceases).  An  excess  of  potassium  iodid  is  then  introduced, 
the  stopper  inserted  and  securely  fastened  in  the  frame;  the  bottle 
is  then  placed  in  a  water-bath  and  heated  for  about  half  an  hour.  After 
the  reaction  is  completed,  the  bottle  is  allowed  to  cool,  opened,  and 
the  free  iodin  found  by  titrating  with  decinormal  sodium  thiosulphate, 
using  starch  as  indicator. 

The  reaction  between  the  barium  chromate  and  potassium  iodid 
and  hydrochloric  acid  is  as  follows  : 


Thus  it  is  seen  that  one  molecule  of  barium  chromate  is  the  equivalent 
of  three  atoms  of  iodin,  i.e.,  one  third  of  an  atom  of  barium  corresponds 
to  one  atom  of  iodin,  or  4.546  gms.  of  barium  corresponds  to  12.59  gms. 

N 
of  iodin,  which  in  turn  corresponds  to  i  liter  of  —  sodium  thiosulphate. 

N 
Therefore  each  cc.  of  —    sodium  thiosulphate  =  0.004546  gm.  of  Ba. 


CHAPTER  XXX 

jr 

BISMUTH 

By  Precipitation  as  Oxalate  (Muir).  The  bismuth  in  nitric 
acid  solution  is  treated  with  a  strong  solution  of  oxalic  acid  in  consider- 
able excess,  and  the  mixture  shaken  up  and  then  set  aside  to  settle. 

The  supernatant  liquid  is  then  poured  off  and  the  precipitated 
oxalate  boiled  for  five  or  ten  minutes  with  successive  quantities  of 
about  50  cc.  of  water,  which  converts  it  into  the  basic  oxalate. 

So  soon  as  the  supernatant  liquid  ceases  to  show  an  acid  reaction 
the  transformation  is  complete. 

The  precipitate  is  then  dissolved  in  dilute  sulphuric  acid  and  titrated 

N 

with  —  potassium  permanganate. 
20 

The  original  bismuth  solution  must  be  free  from  hydrochloric  acid 
and  must  contain  just  sufficient  nitric  acid  to  prevent  the  precipitation 
of  basic  nitrate  before  the  oxalic  acid  solution  is  added. 

One  molecule  of  oxalic  acid  corresponds  to  one  atom  of  bismuth, 
or  125.1  =  206.9.  The  reaction  may  be  represented  as  follows: 

Bi(N03)3+H2C204+H20=Bi(OH)C204-r-3HN03. 

N 
Each  cc.  of  the  — permanganate  solution  represents  0.005172  gm. 

ofBi. 

The  results  by  this  method  are  fairly  accurate,  though  usually  a 
little  too  high.  This  is  doubtless  due  to  incomplete  conversion  to  the 
basic  oxalate. 

By  Precipitation  as  Chroinate  (Mohr).  The  metal  must  be  in 
in  the  form  of  nitrate,  and  must  be  free  from  chlorin.  If  not  in 
solution  it  is  dissolved  in  nitric  acid,  then  ammonia  water  is  added 
until  a  slight  precipitate  forms,  then  some  dilute  nitric  acid  to  dissolve 
the  precipitate.  An  excess  of  potassium  dichromate  is  now  added 
and  the  solution  heated  until  the  precipitate  subsides. 

The  clear  liquid  should  be  tested  with  ammonia  water  and  potas- 
sium dichromate,  and  if  neither  of  these  produce  a  precipitate  the 
entire  solution  is  poured  upon  an  asbestos  filter,  and  the  precipitate 
on  the  filter  washed  with  hot  water.  The  precipitate,  together  with 

355 


356  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

the  filter,  is  then  placed  in  a  flask,  a  weighed  amount  of  ferrous  ammo- 
nium sulphate  (Mohr's  salt)  added,  followed  by  some  sulphuric  acid, 
and  whilst  a  stream  of  carbonic  acid  gas  or  hydrogen  is  passed  through 
the  flask,  it  is  heated  gently  until  reaction  is  complete. 

The  unchanged  ferrous  salt  is  then  found  by  titration  with  deci- 
normal  potassium  permanganate  V.  S. 

This  method  is  for  obvious  reasons  not  a  very  exact  one,  the  follow- 
ing method  gives  much  better  results.  See  also  page  174. 

lodometric  Estimation  of  the  Chromate*  Rupp  and  Shaumann 
suggest  the  following  procedure : 

The  solution  of  bismuth  containing  the  smallest  possible  quantity 
of  free  acid,  is  poured  into  a  known  volume  of  standard  potassium 
dichromate.  The  solution  is  then  diluted,  shaken  energetically,  and 
after  ten  minutes  filtered.  After  making  certain  that  the  whole  of  the 
bismuth  is  precipitated,  the  excess  of  dichromate  is  found  in  an  aliquot ' 
portion  of  the  filtrate,  by  adding  a  slight  excess  of  potassium  iodid, 
acidulating  with  sulphuric  acid,  and  titrating  the  liberated  iodin  by 
means  of  standard  sodium  thiosulphate.  The  reactions  are  explained 
on  page  204. 

By  Precipitation  as  Phosphate  (Muir).  This  method  depends 
upon  the  complete  precipitation  of  bismuth  from  its  solution,  by  means 
of  sodium  phosphate,  in  the  presence  of  free  acetic  acid,  and  then 
determining  the  excess  of  sodium  phosphate  by  means  of  a  standard 
uranium  solution. 

The  standard  sodium  phosphate  solution  used  is  prepared  by 
dissolving  35.56  gms.  of  crystallized  sodium  phosphate  (Na2HPO4-f 
I2H2O)  in  water  to  make  1000  cc.  Each  cc.  of  this  solution  repre- 
sents 0.007047  gm.  of  P2O5.  The  standard  uranium  solution  is  made 
by  dissolving  38.5  gms.  of  uranium  acetate,  or  an  equivalent  quantity 
of  yellow  uranium  oxid,  and  about  50  cc.  of  glacial  acetic  acid  in  about 
1000  cc.  of  water,  and  then  so  adjusting  the  solution  that  it  and  the 
standard  phosphate  will  correspond,  volume  for  volume. 

The  titration  should  be  conducted  in  the  presence  of  an  approxi- 
mately equal  amount  of  sodium  acetate  and  free  acetic  acid. 

The  solution  containing  bismuth,  which  must  be  free  from  hydro- 
chloric or  sulphuric  acid,  is  mixed  with  a  considerable  excess  of  sodium 
acetate;  the  liquid  is  heated  to  boiling,  and  a  measured  volume  (excess) 
of  standard  sodium  phosphate  solution  run  in.  After  boiling  a  few 
minutes  the  liquid  is  filtered  into  a  measuring  flask,  the  precipitate 
thoroughly  washed  with  hot  water,  and  the  mixed  filtrate  and  wash- 
ings made  up  to  the  mark.  An  aliquot  portion  of  the  solution  is  then 
titrated  for  excess  of  the  phosphate,  by  the  inverted  uranium  process 

*  Zeitsch.  anofg.  Chem.,  XXXII,  359. 


BISM  UTH  357 


(see  p.  314.)     The  precipitate  of  bismuth  phosphate    has    the   com- 
position BiPC>4.     The  reaction  may  be  expressed  as  follows: 


Thus  each  cc.  of  the  phosphate  solution  represents  0.02069  gm.  of  Bi. 

By  Precipitation  as  Molybdate.*  To  20  cc.  of  a  solution  of 
bismuth  nitrate  (containing  about  o.i  gm.  of  Bi)  in  a  beaker  add 
one  cc.  of  nitric  acid  and  an  excess  of  ammonium  molybdate  reagent 
(U.  S.  P.),  about  three  or  four  times  the  theoretical  amount  —  in  this  case 
25  cc.  The  whole  is  then  carefully  neutralized  with  ammonia  water, 
using  methyl  orange  to  determine  the  neutral  point,  and  2  or  3  drops 
of  30  per  cent  nitric  acid  introduced. 

The  bismuth  is  then  completely  precipitated  as  molybdate.  The 
composition  of  this  precipitate  is  probably  BiNH^MoO^.  The 
mixture  is  then  warmed,  without  boiling,  by  placing  on  a  thick  asbestos 
pad  over  a  low  flame,  until  the  fine  flocculent  precipitate  has  collected. 
The  precipitate  is  then  stirred  with  a  glass  rod  until  it  is  broken  up, 
and  finally  allowed  to  settle,  which  it  does  rapidly,  forming  a  compact 
sediment  which  is  easily  washed. 

The  supernatant  liquid,  which  must  be  perfectly  clear,  is  decanted 
through  a  plain  filter  paper,  and  the  precipitate  washed  twice  by 
decantation  with  a  three  per  cent  solution  of  ammonium  sulphate, 
after  which  it  is  washed  on  the  filter  with  the  ammonium  sulphate 
solution.  It  is  then  dissolved  in  diluted  sulphuric  acid  and  run  through 
a  Jones'  reductor  with  suction  ;•)•  after  this  it  is  strongly  acidified  with 
sulphuric  acid  and  immediately  titrated  with  standard  potassium 
permanganate.  The  degree  of  reduction  of  the  molybdenum  is  believed 
to  correspond  to  Mo24O37  and  the  ratio  of  bismuth  to  molybdenum  is 
close  to  1:24 

The  color  of  the  molybdate  precipitate  is  pure  white,  but  if  the 
conditions  are  not  followed  exactly,  a  slightly  yellowish  compound 
results  which  gives  varying  but  always  lower  results.  This  yellow 
molybdate  precipitate  can,  however,  be  easily  changed  to  the  white, 
by  first  making  the  solution  alkaline  with  ammonia,  to  throw  down 
bismuth  oxyhydrate,  and  then  dissolving  the  latter  in  nitric  acid,  all 
this  being  done  in  the  whole  mixture  of  precipitate  and  solution.  The 

*  Herman  S.  Riederer,  J.  A.  C.  S.,  XXV  (1903),  907. 

f  A  reductor  with  a  column  of  zinc  40  to  50  cm.  in  length,  in  a  tube  1.25  cm. 
in  diameter  is  recommended,  in  order  to  avoid  the  necessity  of  more  than  one 
passage  through  the  zinc.  The  suction  flank  should  be  large  enough  to  admit 
the  titration  being  made  in  it  directly,  thus  air  has  less  chance  of  reoxidizing  the 
molybdous  oxid  which  is  to  be  determined  by  the  permanganate. 

|  See  Miller  and  Frank,  J.  A.  C.  S.,  (1903),  919. 


358  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

clear  solution  is  now  re-treated  as  a  new  solution,  except  that  it  is 
not  always  necessary  to  add  more  ammonium  molybdate. 

The  important  conditions  in  this  work  are,  that  the  temperature  of 
the  solution  passed  through  the  reductor  should  be  70  to  75°  C. 
the  time  of  passage  seven  to  ten  minutes,  volume  of  solution  200  cc. 
and  the  acidity  10  cc.  concentrated  sulphuric  acid.  After  the  passage 
of  the  molybdate  solution,  100  cc.  of  hot  water  should  be  passed  through 
the  reductor,  and  the  titration  begun  at  once.  The  reduction  is  theo- 
retically as  follows: 


N 
Each  cc.  of  —  permanganate  solution  will  represent  0.003544  gm. 


of  Bi. 


CHAPTER  XXXI 
CALCIUM 

THE  best  methods  for  the  quantitative  estimation  of  calcium 
depend  upon  its  precipitation  as  oxalate.  Any  calcium  salt  soluble 
in  water  or  acetic  acid  may  be  estimated  as  follows : 

Dissolve  in  water  and  add  ammonia  water  until  the  solution  is 
alkaline,  and  then  ammonium  chlorid  and  oxalate.  Heat  gently  until 
the  calcium  oxalate  has  completely  separated,  collect  on  a  filter,  wash 
thoroughly,  dissolve  in  warm  dilute  sulphuric  acid,  and  estimate  the 

N 
oxalic  acid  liberated  by  means  of  —  permanganate.     If  the  calcium 

salt  is  insoluble  in  water  but  soluble  in  dilute  acetic  acid,  it  is  to  be 
dissolved  in  the  smallest  possible  quantity  of  the  latter,  and  the  calcium 
precipitated  as  oxalate  with  sulphuric  acid;  the  excess  of  oxalate  is 

N 
determined  by  titration  with  —  permanganate. 

Rupp  and  Bergdoldt  *  give  the  following  directions  for  estimating 
calcium  and  other  alkali  earths: 

Ten  to  twenty  centimeters  of  the  solution  of  a  calcium  salt  are 
placed  in  a  flask  together  with  i  or  2  gms.  of  ammonium  chlorid,  and 
heated  to  boiling;  ammonia  water  is  then  added  in  slight  excess  fol- 

N 
lowed  by  a  known  volume  of  —  oxalic  acid  or  of  ammonium  oxalate 

i 

of  known  titer,  and  the  heating  continued  for  several  minutes.  The 
flask  with  contents  is  then  placed  in  cold  wrater,  and  when  cool  brought 
to  a  specific  volume  with  water  and  filtered.  The  first  few  cc.  of 
filtrate  are  rejected  and  then  50  or  100  cc.  collected,  diluted  to  150  or 
200  cc.  with  hot  water,  and  10  cc.  of  dilute  sulphuric  acid,  without 
previous  addition  of  ammonia.  The  excess  of  oxalic  acid  is  then 
found  by  titration  with  permanganate. 

N 
Each  cc.  of  —  permanganate  =  0.00199    gm.  Ca; 

=0.004968  "     CaCO3; 
=  0.002784  "     CaO; 

*  Arch.  d.  Ph.,  242,  VI,  Aug.  1904. 

359 


360  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Even  difficultly  soluble  calcium  salts,  such  as  the  sulphate,  phos- 
phate, tartrate,  and  citrate,  may  be  converted  into  oxalates  by  longer 
treatment  with  oxalic  acid,  or  ammonium  oxalate. 

The  following  method  may  be  employed  if  the  solution  of  a  calcium 
salt,  to  be  estimated,  contains  no  other  substance  capable  of  reducing 
permanganate.  To  the  solution  of  the  calcium  salt  a  measured  volume 
in  excess  of  a  solution  of  ammonium  oxalate  is  added,  the  mixture 
warmed,  and  then  diluted  to  200  cc.  The  precipitate  is  allowed  to 
settle,  and  50  or  100  cc.  of  the  clear  supernatant  liquid  removed  by 
means  of  a  pipette,  and  after  acidulating,  2  to  3  gms.  of  manganous 

N 
sulphate   are  added,  and   the  oxalic  acid   titrated  with  —  perman- 

TO 

ganate. 

N 
Each  cc.  —  oxalic  acid =0.00199    Sm'  Ca; 

=  0.002784  "     CaO; 
=0.005508  "     CaCl2. 
(See  also  page  170.) 

The  alkalimetric  methods  for  the  estimation  of  calcium  salts  and 
other  alkali  earths  are  described  on  page  91. 

The  above  described  oxalic  acid  method  may  be  employed  in  the 
estimation  of  calcium  even  in  the  presence  of  iron,  aluminum,  or  mag- 
nesium. As  for  instance,  in  dolomite  or  limestone,  as  follows: 

Ten  grams  of  the  pulverized  limestone  are  dissolved  in  dilute  hydro- 
chloric acid,  the  insoluble  residue  separated  by  nitration,  and  the 
solution  diluted  to  1000  cc.  50  cc.  of  the  clear  solution  are  intro- 
duced into  a  beaker,  ammonia  added  to  slight  alkalinity,  and  then  a 
sufficient  quantity  of  acetic  acid  is  added  to  redissolve  the  precipitated 
ferric  hydroxid.  The  solution  is  then  heated  to  boiling,  and  sufficient 
ammonium  oxalate  added  to  precipitate  all  the  calcium  as  oxalate. 
If  the  precipitation  is  done  in  a  boiling  solution,  and  an  excess  of 
ammonium  oxalate  is  used  the  precipitate  settles  rapidly,  and  is  of 
a  crystalline  character,  in  which  state  filtration  and  washing  are  easily 
accomplished.  The  mixture  is  then  filtered  through  a  plain  filter,  the 
entire  precipitate  being  transferred  to  the  filter  by  using  a  spritz  bottle 
and  a  glass  rod  tipped  with  a  piece  of  rubber  tubing,  and  washed  with 
hot  water  until  the  washings  no  longer  react  acid.  The  filter  is  then 
placed  over  a  porcelain  capsule,  and  the  precipitate  dissolved  by  mois- 
tening it  with  dilute  sulphuric  acid,  and  washed  into  the  capsule  with 
hot  water,  any  occurrence  of  calcium  sulphate  will  not  in  the  least 
interfere. 

N 
The  solution  in  the  capsule  is  then  titrated  with  —  permanganate. 


'  CALCIUM  361 

Instead  of  dissolving  the  precipitate  on  the  filter,  it  may.be  intro- 
duced directly  into  a  beaker,  by  making  a  hole  in  the  bottom  of  the 
filter  and  washing  the  precipitate  through  it,  then  treating  with  dilute 
sulphuric  acid  and  titrating  as  above  under  constant  agitation. 

Another  method  is:  Take  a  sample  of  dolomite  (0.5  gm.),  dissolve 
in  hydrochloric  acid,  and  dilute  to  250  cc.  with  water;  to  this  add 
about  five  drops  of  nitric  acid  and  boil  for  a  few  minutes.  Then 
add  an  excess  of  ammonia,  and  filter  to  remove  the  precipitate  of 
aluminum  and  ferric  hydroxids,  wash  the  precipitate  thoroughly  with 
hot  water,  and  to  the  mixed  filtrate  and  washings  add  oxalic  acid 
solution  until  the  calcium  is  completely  precipitated  as  oxalate.  The 
precipitate  is  collected,  washed,  and  redissolved  in  water  by  aid  of 
hydrochloric  acid,  and  after  making  alkaline  with  ammonia,  the  cal- 
cium is  again  precipitated  by  oxalic  acid.  This  precipitate  is  then 
thoroughly  washed,  mixed  with  about  100  cc.  of  water  and  sufficient 

N 
dilute  sulphuric  acid,  and  finally  titrated  with  j—  permanganate. 


CHAPTER  XXXII 
COPPER 

By  Precipitation  as  Cuprous  Oxid.  Copper  may  be  precipi- 
tated as  cuprous  oxid  (Cu2O)  by  means  of  grape-sugar,  from  an  alka- 
line solution  of  the  metal  containing  tartaric  acid. 

To  the  copper  solution  tartaric  acid  or  an  alkali  tartrate  is  added, 
and  then  an  excess  of  sodium  or  potassium  hydroxid.  A  clear,  deep, 
blue  colored  liquid  should  be  obtained,  if  not,  more  tartaric  acid  must 
be  added.  A  considerable  quantity  of  grape-sugar  is  now  added,  and 
the  liquid  heated  on  a  water-bath  (though  not  above  90°  C.)  until  the 
precipitate  becomes  bright  red  in  color.  This  precipitate  is  separated 
by  nitration  through  a  plug  of  asbestos,  and  washed  with  hot  water 
until  the  washings  are  perfectly  clear  and  colorless.  The  precipitate, 
together  with  the  asbestos,  is  then  transferred  to  a  flask  containing  an 
acid  solution  of  a  ferric  salt  and  heated  until  the  cuprous  oxid  is  com- 
pletely dissolved.  The  reaction  is  as  follows: 

Cu2O+Fe2Cl6+2HCl=2CuCl2+2FeCl2-|-H2O. 

The  resulting  ferrous  salt  is  then  titrated  with  permanganate.  It 
is  advisable  to  pass  a  stream  of  carbon  dioxid  or  hydrogen  through  the 
flask  whilst  the  cuprous  oxid  is  dissolving,  to  prevent  oxidation  of 
ferrous  salt. 

As  shown  by  the  above  equation,  for  every  two  equivalents  of  copper, 
two  equivalents  of  iron  are  reduced,  i.e.,  63.1  gins,  of  copper  represent 

N 
55.5  gms.  of  iron.     Therefore  each  cc.  of  —  potassium  permanganate 

represents  0.00555  gm.  of  Fe,  which  is  equivalent  to  0.00631  gm.  of  Cu. 
Estimation  as  Sulphid.  This  method  depends  upon  precipi- 
tating copper  out  of  alkaline  solutions  by  means  of  sodium  sulphid. 
The  composition  of  the  precipitate  so  obtained  varies,  according  to  the 
temperature  at  which  the  precipitation  is  done.  If  the  precipitation 
is  carried  out  at  the  ordinary  temperature  of  the  atmosphere  the  pre- 
cipitate will  consist  of  simple  copper  sulphid  (CuS),  which,  however, 
settles  so  slowly  in  the  alkaline  solution  that  the  determination  of  the 
end-point  becomes  a  very  difficult  matter,  and  hence  the  titration  at 
this  temperature  is  quite  impracticable.  Working  at  higher  tem- 

362 


BY    PRECIPITATION    AS    METALLIC    COPPER  363 

peratures  the  composition  of  the  precipitate  is  quite  different,  being  of 
the  nature  of  an  oxysulphid,  but  its  advantage  lies  in  the  fact  that  the 
precipitate  settles  rapidly  and  hence  admits  of  a  rapid  and  accurate 
determination  of  the  end-point.  The  method  of  Pelouze  depends  upon 
working  at  a  temperature  of  from  60°  to  80°  C.  The  precipitate  is 
then  an  oxysulphid  of  the  composition  CuO .  CuS.  % 

The  end-point  is  indicated  by  the  discharge  of  the  blue  color  of  the 
alkaline  copper  solution,  and  by  noting  the  point  at  Which  the  further 
addition  of  the  standard  sulphid  solution  fails  to  produce  a  precipitate. 

The  sodium  sulphid  is  standardized  against  a  solution  of  pure 
copper  sulphate  of  known  strength,  which  is  best  made  by  dissolving 
39.279  gms.  of  pure  copper  sulphate  (CuSO4 .  5H2O)  in  sufficient 
water  to  make  i  liter.  Each  cc.  of  this  solution  will  contain  o.ci  gm. 
of  copper. 

Precipitation  in  Acid  Solution.  The  estimation  of  copper  by 
means  of  sodium  sulphid  is  comparatively  simpler  if  the  precipitation  is 
carried  out  in  acid  solution  instead  of  in  an  ammoniacal  solution.  The 
precipitate  in  this  case  does  not  vary  in  composition  with  the  tem- 
perature at  which  the  operation  is  conducted.  Its  composition  is 
invariably  CuS. 

The  copper  solution  is  diluted  to  about  200  cc.  with  hot  water  in  a 
stoppered  flask,  acidulated  with  hydrochloric  acid,  and  the  standard 
sulphid  solution  added  little  by  little,  replacing  the  stopper  and  shaking 
after  each  addition  until  the  end-point  is  reached.  The  CuS  formed  here 
subsides  quickly.  The  reaction  is  thus  illustrated: 

CuSO4 + Na2S = CuS + Na2SO4. 

By  Precipitation  as  Metallic  Copper,  and  Subsequently  Titration 
with  Ferric  Chlorid  and  Permanganate.  Into  the  copper  solution, 
which  must  contain  no  nitric  acid  or  other  metal  precipitable  by  zinc 
(as  bismuth  or  lead),  is  placed  a  rod  of  clean  pure  zinc.  The 'copper 
deposits  in  the  metallic  state.  As  soon  as  the  deposition  of  the  copper 
is  complete,  which  is  known  by  the  solution  being  colorless,  the  zinc 
is  removed,  and  the  precipitated  spongy  copper  collected  carefully 
on  a  filter,  upon  which  it  is  thoroughly  washed.  The  precipitate, 
together  with  the  filter,  is  then  transferred  to  a  flask  containing  a 
mixture  of  ferric  chlorid  and  hydrochloric  acid,  and  gentle  heat  applied. 
The  copper  is  thus  dissolved  according  to  the  equation 

Cu + Fe2Cl6  =  CuCl2 + 2  FeCl2. 

In  order  to  prevent  oxidation  of  the  resulting  ferrous  chlorid  a 
stream  of  hydrogen  is  passed  through  the  flask  whilst  the  copper  is 
dissolving,  or  a  small  quantity  of  sodium  carbonate  may  be  added 


364  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

to  the  contents  of  the  flask,  which,  by  disengaging  carbon  dioxid,  expels 
the  atmospheric  air  present  and  thus  prevents  oxidation.  When  the 
copper  is  completely  dissolved  the  solution  is  titrated  with  perman- 

N 
ganate,  55.5  Fe=3i.55  Cu.     Each  cc.  of  —  permanganate  represents 

0.00555  gm.  of  Fe,  which  is  the  equivalent  of  0.003155  gm.  of  Cu. 

By  Means  of  Potassium  Cyanid.  This  method  originated  with 
Parkes.*  When  potassium  cyanid  is  added  to  the  blue  ammoniacal 
solution  of  a  copper  salt,  the  solution  loses  its  blue  color,  and  copper 
and  ammonium  cyanid  is  formed.  Cyanogen  is  also  liberated,  which 
reacts  on  the  free  ammonia,  producing  urea,  urea  oxalate,  ammonium 
cyanid,  and  formate  (Liebig).  The  reaction  varies  with  the  amount 
of  free  ammonia,  ammoniacal  salts,  the  concentration  of  the  solution, 
and  the  temperature.  The  process  therefore  is  worthless,  unless  all 
analyses  made  by  it  are  done  under  the  same  conditions.  Manganese, 
nickel  cobalt,  mercury,  silver,  and  larger  quantities  of  zinc  (over  3  per 
cent)  must  first  be  removed.  Cadmium  materially  increases  the 
quantity  of  potassium  cyanid  used.  Arsenous,  antimonous  and  stan- 
nous  salts  have  a  deleterious  effect,  but  in  the  oxidized  state  have  no 
influence  upon  the  result. 

The  titration  should  be  conducted  at  or  near  the  boiling  tem- 
perature. The  lower  the  temperature  the  greater  the  quantity  of 
potassium  cyanid  solution  used.  The  results  also  appear  to  vary  with 
the  speed  at  which  the  titration  is  conducted. 

In  order  to  avoid  the  irregularities  occasioned  by  the  presence  of 
varying  quantities  of  ammonia  or  its  salts,  J.  L.  Davies  f  substituted 
sodium  carbonate  for  the  ammonia,  the  value  of  which  modification  is 
confirmed  by  Fessenden^  and  by  Fernekes  and  Koch,§  as  well  as  by 
Sutton  and  others. 

The  acid  copper  solution  is  neutralized  with  sodium  carbonate, 
and  then  an  excess  of  the  latter  is  added,  to  nearly  redissolve  the  pre- 
cipitate. It  is  not  necessary  to  add  sufficient  sodium  carbonate  to 
completely  dissolve  the  precipitate,  as  the  latter  dissolves  readily  upon 
the  addition  of  the  standard  cyanid  solution. 

The  cyanid  solution  is  then  run  in  until  the  blue  color  of  the  solution 
is  just  discharged.  Sutton  ||  neutralizes  the  copper  solution  with  sodium 
carbonate,  and  adds  a  trifling  excess,  and  then  i  cc.  of  ammonia, 
sp.gr.  0.960.  This  addition  of  ammonia  is,  however,  entirely  unneces- 
sary. 


*  Mining  Journal,  1851.  §  J.  A.  C.  S.,  XXVII,  1225. 

f  Chem.  News,  58,  131.  ||  Handbook  of  Volumetric  Analysis. 

J  Chem.  News,  61,  253-283. 


STEINBECK'S    PROCESS    FOR    ORES  365 

Sutton  further  states  that  the  presence  of  iron  in  the  solution  does 
not  interfere,  in  fact  it  is  rather  an  advantage,  acting  as  an  indicator. 
Lead  and  aluminum  have  no  influence  upon  the  accuracy  of  the 
titration. 

The  reaction  between  potassium  cyanid  and  the  copper  salt  may 
be  expressed  by  the  equation 

CuS04  +  4KCN=K2Cu(CN)4+K2S04. 

In  standardizing  the  cyanid  solution  it  is  advisable  to  do  so  under 
the  same  conditions,  as  in  the  actual  assay.  Commercial  potassium 
cyanid  is  never  entirely  pure,  and  therefore  the  requisite  quantity 
required  for  making  a  strictly  normal  solution  varies  greatly.  l£  is 
the  custom  to  weigh  off  7  gms.  of  commercial  potassium  cyanid, 
dissolve  it  in  1000  cc.  of  water,  and  then  standardize  it  against  pure 
metallic  copper  dissolved  in  nitric  acid,  and  transferred  into  sulphate 
by  evaporating  with  sulphuric  acid.  Or  against  pure  recrystallized 
copper  sulphate. 

As  seen  in  the  above  equation  four  molecules  of  KCN  are  the 
equivalent  of  one  atom  of  Cu.  This  standard  solution  does  not  keep 
its  titer  very  long,  hence  freshly  prepared  solutions  only  should  be 
used.  This  process  reversed  serves  for  the  estimation  of  cyanids 
(see  page  275). 

Steinbeck's  Process  for  Ores.*  5  gms.  of  finely  powdered  ore 
are  put  into  a  flask  with  40  to  50  cc.  crude  hydrochloric  acid  (sp.gr. 
1.16)  and  6  cc.  nitric  acid  (prepared  by  mixing  nitric  acid  (sp.gr.  1.12), 
with  its  own  bulk  of  water).  Ores  containing  much  sulphur  or  bitu- 
men must  be  roasted  before  being  treated  with  acid.  The  flask  is 
heated  on  a  sand-bath  for  half  an  hour,  and  then  boiled  for  fifteen 
minutes;  then  filtered  into  a  large  beaker  and  the  residue  washed  with 
water.  A  rod  of  zinc,  weighing  about  50  gms.  and  as  free  from  lead 
as  possible,  is  surrounded  with  a  stout  piece  of  platinum  foil  and 
immersed  in  the  copper  solution.  The  precipitation  is  usually  com- 
plete in  about  half  an  hour.  The  precipitate  of  metallic  copper  is 
collected  together  with  the  platinum  foil  upon  which  a  part  has  de- 
posited, and,  if  it  amounts  to  6  per  cent  of  the  ore  (known  approximately 
from  its  bulk),  dissolved  in  8  cc.  of  the  nitric  acid  before  mentioned. 
If  more  than  6  per  cent,  it  is  dissolved  in  16  cc.  nitric  acid,  diluted  to 
100  cc.  and  one  half  taken.  To  the  solution  of  copper  10  cc.  ammonia 
water  (prepared  by  mixing  one  volume  sp.gr.  0.93  with  two  volumes 
of  water)  is  added,  and  the  fluid  titrated  with  potassium  cyanid  until 
the  blue  color  has  disappeared. 

*  Zeitschr.  f.  Analyt.  Chem.,  VIII,  i,  and  Chem.  News,  XIX,  181. 


366  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Dulin's  Process  for  Ores.*  The  copper  ore  is  treated  in  the 
usual  manner  so  as  to  obtain  a  solution  of  copper  partially  free  from 
lead  and  silver.  (See  Low's  process.)  This  solution  is  boiled  with 
strips  of  aluminum  foil,  which  results  in  the  complete  precipitation  of 
the  copper,  together  with  any  silver  which  may  be  present,  and  which 
is  always  so  small  as  to  be  negligible.  If  cadmium  is  present  it  is  only 
partially  precipitated,  beginning  after  the  copper  is  thrown  down.  If 
the  boiling  be  stopped  immediately  after  the  copper  is  precipitated, 
which  a  practiced  eye  will  readily  detect,  the  amount  of  cadmium 
precipitated  is  so  small  as  not  to  cause  a  sensible  error.  The  liquid  is 
then  decanted  off  from  the  aluminum  foil  and  copper  quickly  washed 
several  times  with  hot  water,  care  being  taken  not  to  wash  away  any 
particles  of  copper.  Three  cc.  of  nitric  acid  are  then  added  to  the 
flask  and  boiled  until  the  copper  is  dissolved.  The  solution  is  then 
treated  with  ammonia  or  with  sodium  carbonate,  as  described  under 
Estimation  by  Means  of  Potassium  Cyanid,  and  titrated  with  the 
cyanid  solution  in  the  usual  way. 

ESTIMATIONS  WITH  POTASSIUM   IODID  AND   SODIUM  THIOSULPHATE 

lodometric  Method.  This  method  f  is  based  upon  the  reaction 
between  potassium  iodid  and  copper  sulphate,  in  acid  solution.  The 
reaction  is  expressed  as  follows  : 


Cuprous  iodid  is  precipitated  as  a  dirty  white  powder,  while  iodin  is 
set  free.  The  latter  is  then  immediately  titrated  with  sodium  thiosul- 
phate  and  starch,  and  thus  the  corresponding  quantity  of  copper 
found. 

The  copper  should  be  in  the  form  of  sulphate,  and  the  solution 
must  contain  no  nitric  or  hydrochloric  acid,  though  acetic  acid  may  be 
present. 

If  nitric  acid  is  present,  the  solution  must  be  evaporated  with  sul- 
phuric acid  until  the  former  is  expelled.  Other  salts  of  copper  may 
be  converted  into  sulphate  by  precipitating  with  H2S,  dissolving  the 
resulting  sulphid  in  nitric  acid,  and  then  evaporating  the  solution  to 
dryness  after  adding  some  sulphuric  acid  and  sodium  sulphate. 

N 
Each  cc.  of  —  thiosulphate=  0.0063  1  gm-  Cu.     In  titrating  copper 

by  this  method  it  is  of  course  necessary  to  remove  all  other  substances 

*  Dulin,  J.  A.  C.  S.,  XVII,  346. 

f  De  Haen,  Annal.  d.  Chem.  u.  Pharm.,  XCI,  237,  and  Brown,  Quart.  Jour. 
of  Chem.  Soc.,  X,  65. 


1ODOMETRIC    METHODS  367 

which  cause  a  liberation  of  iodin  from  potassium  iodid,  such  as  ferric 
salts,  free  chlorin,  or  nitrites.  This  can  readily  be  accomplished  by 
placing  into  the  solution  a  rod  of  clean  pure  zinc.  Metallic  copper  then 
quickly  and  completely  separates  as  a  spongy  precipitate,  which  may 
be  collected  upon  a  filter,  and  after  thoroughly  washing,  dissolved  in 
nitric  acid.  This  nitric  acid  solution  is  evaporated  to  dryness  after 
the  addition  of  sulphuric  acid,  and  the  resulting  copper  sulphate  dis- 
solved in  water  and  treated  with  potassium  iodid,  as  above  described. 

In  alloys  containing  besides  copper,  zinc,  tin,  or  lead,  i  or  2  gms. 
of  the  alloy  in  coarse  powder  are  dissolved  in  concentrated  nitric  acid, 
and  the  solution  diluted  with  water  to  100  cc.  After  the  metastannic 
acid  has  settled  to  the  bottom,  the  clear  solution  is  poured  off,  and  a 
measured  or  weighed  quantity  of  the  latter  transferred  to  a  porcelain 
capsule.  The  excess  of  nitric  acid  is  neutralized  by  means  of  calcium 
carbonate,  and  any  excess  of  the  latter  removed  by  the  addition  of  a 
few  drops  of  diluted  hydrochloric  acid.  The  now  weakly  acid  solution 
is  then  treated  with  potassium  iodid  and  the  liberated  iodin  titrated 
in  the  usual  manner. 

In  copper-nickel  alloys  the  separation  of  the  nickel  must  be  brought 
about.  For  this  purpose  the  method  of  Dewilde  may  be  employed. 
The  alloy  is  dissolved  in  hydrochloric  acid  containing  some  nitric  acid. 
The  excess  of  acid  is  removed  by  evaporating  or  by  neutralizing  with 
sodium  hydroxid.  The  resulting  chlorid  is  dissolved  in  about  50  cc. 
of  water,  and  to  this  is  added  twice  as  much  potassium  bitartrate  as 
was  taken  of  the  alloy  for  analysis,  and  the  mixture  warmed.  An 
alcoholic  solution  of  potassium  hydroxid  is  then  added  in  small  por- 
tions at  a  time  until  the  precipitate  which  at  first  forms  is  redissolved. 
The  solution  is  then  allowed  to  cool  and  some  pure  grape-sugar  in 
solution  added  and  the  liquid  boiled  for  one  or  two  minutes. 

The  copper  is  then  completely  precipitated  as  Cu2O.  This  pre- 
cipitate is  collected  on  a  filter,  washed  thoroughly,  dissolved  in  dilute 
nitric  acid,  and  the  resulting  copper  nitrate  solution  titrated  as  above 
described. 

lodometric  Methods  for  the  Technical  Assay  of  Copper  Ores. 
J.  W.  Westmoreland  *  recommends  the  following  for  copper  in  its 
various  ores: 

The  ore  is  dissolved  in  nitric  acid,  to  which  hydrochloric  acid  is 
also  added  later  on.  The  solution  is  then  evaporated  to  dryness,  with 
excess  of  sulphuric  acid.  Thus  the  bases  are  converted  into  sulphates. 
The  residue  is  then  treated  with  warm  water  and  any  lead  sulphate  or 
other  insoluble  matters,  separated  by  filtration.  The  filtrate  is  heated 
to  boiling  and  precipitated  with  sodium  thiosulphate;  the  copper  is 

*J.S.C.I.,V,5i. 


368  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

thereby  completely  precipitated  as  subsulphid  mixed  with  sulphur. 
This  can  be  readily  filtered  off  and  washed.  Arsenic  and  anlimony, 
if  present,  are  also  precipitated,  but  tin,  zinc,  iron,  nickel,  cobalt,  and 
manganese  are  not  precipitated. 

The  precipitate  is  washed  thoroughly  with  hot  water,  dried  and 
roasted,*  and  the  resulting  copper  oxid  and  sulphid  dissolved  in  nitric 
acid.  The  excess  of  acid  is  then  removed  by  evaporation  and  sodium 
carbonate  added  to  precipitate  part  of  the  copper  and  to  neutralize 
any  remaining  mineral  acid.  Sufficient  acetic  acid  is  now  added  to 
produce  a  clear  solution,  and  then  about  ten  parts  of  potassium  iodid 
to  one  of  copper,  supposed  to  be  present,  are  added,  and  the  titration 
of  the  liberated  iodin  carried  out  by  means  of  sodium  thiosulphate,  in 
the  usual  manner. 

About  5  gms.  of  the  ore  should  be  taken  for  the  assay;  for  high 
percentage  ores  relatively  smaller  quantities  are  to  be  taken. 

Estimation  of  Copper  in  Ores  Containing  Iron.  (Andrew  M. 
Fairlie.f)  For  the  precipitation  of  copper  and  its  separation  from 
iron,  zinc  or  aluminum,  thiocyanate  is  used  in  preference  to  sodium 
thiosulphate.  In  the  following  process  the  thiocyanate  is  used. 

The  following  solutions  are  required: 

(a)  Sodium  thiosulphate,  19.5  gms.  per  liter. 

(b)  Ammonium  thiocyanate,  100  gms.  per  liter. 

(c)  Potassium  iodid,  50  per  cent. 

(d)  Starch  solution. 

To  Standardize  the  Solutions.  0.2  gm.  of  copper  are  dissolved  in 
5  cc.  of  diluted  nitric  acid  (1:1)  and  after  the  addition  of  50  cc.  of 
water  boiled  for  five  or  ten  minutes.  A  slight  excess  of  ammonia  is 
then  added  and  the  liquid  acidified  with  acetic  acid  and  cooled.  6  cc. 
of  the  potassium  iodid  solution  are  then  added,  and  the  liberated 
iodin  titrated  with  the  standard  sodium  thiosulphate. 

The  Process.  Of  the  ore  to  be  examined  a  sufficient  quantity  is 
taken  to  make  a  solution  containing  0.2  to  0.24  gm.  of  copper.  2  gms. 
of  potassium  chlorate  are  added  together  with  10  cc.  of  nitric  acid 
(sp.gr.  1.42),  and  the  vessel  covered  and  shaken  for  a  short  time. 
Heat  is  then  applied  and  the  chlorin  and  nitrous  vapors  driven  off. 
The  solution  is  then  cooled  quickly,  10  cc.  of  hydrochloric  acid  (sp.gr. 
1.2)  are  added,  and  again  boiled  for  one  or  two  minutes.  By  this 
procedure  all  sulphur  present  is  oxidized,  while  the  copper  dissolves  and 
a  white  sand-like  residue  remains.  The  solution  is  then  diluted  (with 
out  filtering)  with  50  cc.  of  water,  an  excess  of  ammonia  is  added,  and 
after  acidifying  the  liquid  with  sulphuric  acid  and  heating  nearly  to 

*  The  roasting  causes  most  of  the  arsenic  and  the  sulphur  to  be  expelled, 
t  Chem.  Ztg.,  1904,  Rep.  385. 


COPPER    IN    ORES    CONTAINING    IRON  369 

boiling,  10  cc.  of  sulphurous  acid  are  added  to  reduce  the  iron  present. 
The  copper  is  then  precipitated  by  adding  5  cc.  of  the  thiocyanate 
solution,  filtered,  and  washed  with  hot  water. 

The  precipitate,  together  with  the  filter,  is  then  introduced  into  a 
suitable  flask  and  dissolved  in  concentrated  nitric  acid,  driving  off  the 
acid  vapors  by  boiling;  ammonia  water  is  then  added,  and  finally  acetic 
acid  to  faint  acid  reaction,  and  when  the  liquid  is  cool,  6  cc.  of  the 
potassium  iodid  solution  are  added,  and  the  titration  with  thiosulphate 
begun.  Upon  the  addition  of  the  sulphurous  acid  the  solution  should  be 
neutral,  and  during  the  titration  large  quantities  of  ammonium  aoetate 
must  not  be  present,  since  this  hinders  to  some  extent  the  reaction 
between  copper  and  the  iodid. 

Low's  Process.  The  following  modification  by  A.  H.  Low  * 
gives  excellent  results: 

The  sodium  thiosulphate  solution  is  standardized  as  follows:  19  gms. 
of  the  salt  are  dissolved  in  i  liter  of  water.  0.2  gm.  of  pure  copper 
foil  are  placed  in  a  250  cc.  flask,  and  5  cc.  of  a  mixture  of  equal  parts 
of  nitric  acid  (sp.gr.  1.42)  and  water  are  added  and  the  mixture  boiled 
until  red  fumes  cease  to  be  given  off.  Now  remove  the  flame  and  add 
6  or  7  gms.  of  crystallized  zinc  acetate  and  about  15  cc.  of  water,  or 
about  20  cc.  of  a  cold  saturated  solution  of  the  zinc  salt.  Heat  to 
boiling,  cool,  and  dilute  to  about  50  cc.  Now  add  3  gms.  of  potassium 
iodid  and  shake  gently  until  dissolved.  Then  titrate  at  once  with 
the  thiosulphate  solution,  until  the  brown  tinge  has  become  weak,  and 
then  add  sufficient  starch  solution  to  produce  a  blue  color,  and  con- 
tinue the  titration  cautiously  until  the  blue  color  vanishes,  i  cc. 
of  the  thiosulphate  solution  should  correspond  to  about  0.005  gm-  °f 
copper.  In  assaying  ores,  etc.,  when  half  a  gram  is  taken,  i  cc.  of  the 
standard  solution  will  be  equal  to  one  per  cent  of  copper.  The  use  of 
the  zinc  acetate  is  to  counteract  the  nitric  acid  used  in  dissolving  the 
copper. I 

The  Process.  Treat  half  a  gram  of  the  ore  in  a  flask  of  250  cc. 
capacity  with  5  or  6  cc.  of  strong  nitric  acid,  and  boil  gently  until  nearly 
dry.  Then  add  5  cc.  of  strong  hydrochloric  acid  and  boil  again. 

*  J.  A.  C.  S.,  XVIII,  458,  and  XXIV,  1082. 

f  In  a  later  modification  of  this  process,  Low  uses  instead  of  zinc  acetate,  5  cc. 
of  strong  bromin  water.  The  mixture  is  boiled  until  the  bromin  is  expelled; 
this  is  to  insure  complete  destruction  and  removal  of  the  red  fumes.  It  is  then 
removed  from  the  heat,  and  a  slight  excess  of  strong  ammonia  water  added, 
ordinarily  7  cc.  of  ammonia  water  0.90  sp.gr.;  then  boiled  to  expel  excess  of  am- 
monia, which  is  shown  by  a  change  of  color  of  the  liquid  and  a  partial  precipita- 
tion of  the  copper.  Now  strong  acetic  acid  is  added  (3  or  4  cc.  of  the  80  per  cent) 
and  boil  if  necessary  to  redissolve  the  copper;  cool,  add  the  potassium  iodid, 
and  proceed  as  above  described. 


370  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

As  soon  as  the  incrusted  matter  has  dissolved,  add  6  cc.  of  strong  sul- 
phuric acid  and  heat  strongly  over  a  naked  flame,  until  more  volatile 
acids  are  expelled  and  the  fumes  of  sulphuric  acid  are  coming  off  freely. 
Allow  to  cool  and  then  add  25  cc.  of  cold  water,  and  heat  to  boiling, 
to  dissolve  any  anhydrous  sulphates  of  iron,  etc.  Now  filter,  to  remove 
more  especially  any  lead  sulphate,  and  receive  the  filtrate  in  a  beaker 
about  2\  inches  in  diameter.  Wash  the  flask  and  filter  with  hot  water, 
and  endeavor  to  keep  the  volume  of  the  filtrate  down  to  50  or  60  cc. 

Place  in  the  beaker  two  pieces  of  sheet  aluminum,  which,  for  the 
sake  of  convenience  in  subsequent  washing,  may  be  prepared  as 
follows:  Stout  sheet  aluminum,  say  about  one  sixteenth  of  an  inch  in 
thickness,  is  cut  into  two  pieces  an  inch  and  a  half  square,  and  then  the 
four  corners  are  bent  for  about  a  quarter  of  an  inch,  alternately  up  and 
down  at  right  angles.  This  scheme  prevents  the  pieces  from  lying 
flat  against  each  other  or  upon  the  bottom  of  the  beaker,  and  their 
washing  is  thus  facilitated.  Add  one  drop  of  strong  hydrochloric 
acid,  cover  the  beaker  and  heat  to  boiling  for  about  seven  minutes.* 

Transfer  the  solution  back  to  the  original  flask,  and  by  means  of  a 
wash-bottle  of  hot  water,  rinse  in  also  as  much  of  the  copper  as  possible, 
leaving  the  aluminum  behind.  Drain  the  beaker  as  completely  as 
possible,  and  temporarily  set  it  aside  with  the  aluminum,  which  may 
still  retain  a  little  copper.  Allow  the  copper  in  the  flask  to  settle  and 
then  decant  the  liquid  through  a  filter.  Again  wash  the  copper  simi- 
larly two  or  three  times  with  hot  water,  retaining  it  as  completely  as 
possible  in  the  flask.  Now  pour  upon  the  aluminum  in  the  beaker 
5  cc.  of  a  mixture  of  equal  volumes  of  strong  sulphuric  acid  and  water, 
and  warm  the  beaker  gently  but  do  not  boil.  See  that  any  copper 

*  In  Low's  later  modification  of  this  method  the  directions  from  here  on  are 
as  follows:  There  is  danger  of  the  finely  divided  copper  being  slightly  oxidized 
in  the  subsequent  washing.  To  prevent  this,  and  at  the  same  time,  precipitate 
any  traces  of  copper  still  remaining  in  solution,  add  about  15  cc.  of  strong  hydrogen 
sulphid  water.  If  the  copper  in  the  ore  does  not  exceed  much  over  20  per  cent, 
proceed  as  follows:  Decant  the  supernatant  liquid  through  a  p-cm.  filter,  add 
10  cc.  of  strong  hydrogen  suplhid  water  to  the  residue  in  th.e  beaker,  and  then 
transfer  the  liquid  and  precipitate  to  the  filter.  Wash  thoroughly  with  cold 
water  without  delay,  to  prevent  oxidation.  Now  place  the  clean  original  flask 
under  the  funnel.  Pour  over  the  aluminum  in  the  beaker  5  cc.  of  a  mixture  of 
equal  parts  of  strong  nitric  acid  (sp.gr.  1.42)  and  water.  This  dissolves  any 
adhering  particles  of  copper.  Heat  to  boiling  and  pour  over  the  precipitate  on  the 
filter  so  as  to  dissolve  all  the  copper.  Now  without  washing,  pour  5  cc.  of  a 
cold  saturated  aqueous  solution  of  bromin  into  the  filter,  and  then  wash  the 
beaker  and  filter  with  hot  water.  Boil  the  filtrate,  which  usually  does  not  exceed 
75  cc.  in  bulk,  to  thoroughly  expel  the  excess  of  bromin.  Then  remove  from 
the  heat  and  add  ammonia  water  in  slight  excess  (about  7  cc.  of  strong  ammonia 
water).  Boil  off  the  excess  of  ammonia,  acidify  with  acetic  acid,  and  finish  the 
process  as  described  under  the  Standardization  of  Sodium  Thiosulphate  Solution. 


COPPER    IN    ORES    CONTAINING    IRON  371 

present  is  dissolved  and  pour  the  warm  solution  through  the  filter  last 
used,  thus  dissolving  any  retained  particles  of  copper,  and  receive  the 
filtrate  in  the  flask  containing  the  main  portion  of  the  copper.  At  this 
stage  do  not  wash  either  the  aluminum  or  the  filter,  but  simply  remove 
the  flask  and  set  the  beaker  in  its  place.  Heat  the  mixture  in  the 
flask  to  boiling  and  see  that  all  the  copper  is  dissolved.  Then  add 
about  half  a  gram  of  potassium  chlorate  and  again  boil  for  a  moment. 
This  is  to  oxidize  any  arsenic  present  to  arsenic  acid  and  is  a  very 
important  point.  Remove  the  flask  from  the  lamp  and  again  place  it 
under  the  funnel  and  wash  the  beaker,  aluminum  and  filter  with  as 
little  hot  water  as  possible,  again  boil  sufficiently  to  remove  every  trace 
of  red  fumes.  All  the  copper  is  now  in  the  flask  as  nitrate.  Add 
the  zinc  acetate  and  proceed  from  this  point  precisely  as  described  with 
the  original  nitrate  of  copper  solution  in  the  standardization  of  the 
thiosulphate;  finally  calculate  the  precentage  of  copper  present  from 
the  amount  of  standard  thiosulphate  required. 

A  sufficient  excess  of  potassium  iodid  should  be  taken,  not  less 
than  3  gms.  —  an  excess  does  no  harm.  Silver  does  not  interfere.  Lead 
and  bismuth  are  without  effect,  except  that  by  forming  yellow  iodids, 
they  may  mark  the  end-point  before  the  starch  is  added.  Arsenic, 
when  oxidized  as  described,  has  no  effect.  The  return  of  the  blue 
tinge  in  the  liquid  by  long  standing  after  titration,  is  of  no  significance, 
but  a  quick  return  of  the  color,  which  an  additional  drop  or  two  of  the 
thiosulphate  does  not  permanently  destroy,  may  indicate  either  incom- 
plete combination  of  all  the  nitric  acid  with  the  zinc,  or  a  failure  in 
completely  boiling  off  the  red  fumes. 

Note  —  0.5  gm.  of  copper  requires  theoretically  ^.65  gms.  of  potassium  iodid, 
yet  when  only  this  proportion  is  used  the  reaction  is  slow.  It  is  therefore  better 
in  practice  to  use  an  excess,  the  quantity  used  being  governed  by  the  amount 
of  copper  present,  which  can  always  be  approximately  estimated.  Low  suggests 
to  use  about  i  gm.  of  potassium  iodid  for  every  15  per  cent  of  copper. 

By  Preciptiation  with  Thiocyanate  (Volhard).  The  estima- 
tion of  copper  by  means  of  thiocyanate  solution  depends  upon  the  fact 
that  in  the  presence  of  a  reducing  agent  thiocyanates  completely  pre- 
cipitate copper  out  of  its  hot  solution,  as  cuprous  thiocyanate: 


The  reducing  agent  best  suited  for  this  purpose  is  sulphurous  acid. 
The  precipitate  of  cuprous  thiocyanate  is  white  in  color  and  quite 
insoluble.  The  estimation  is  best  carried  out  by  residual  titration; 

N 

an  accurately  measured  quantity  (excess)  of  —  potassium  thiocvanate 

10 

is  added  to  the  copper  solution,  and  the  uncombined  excess  deter- 


372  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

N 

mined  by  titrating  with  —  silver  nitrate.  Ferric  alum  is  used  as  indi- 
cator. This  process  is  not  available  in  the  presence  of  silver,  mercury 
cyanids,  halogens,  or  arsenic  in  the  oxidized  state. 

The  Process,  A  sample  of  metallic  copper  or  an  alloy,  say  0.5  gm., 
is  dissolved  in  a  small  quantity  of  concentrated  nitric  acid  (sp.gr.  1.42), 
and  the  solution  heated  until  excess  of  acid  and  nitrous  fumes  are 
expelled.  About  100  cc.  of  water  are  added  and  the  solution  neu- 
tralized carefully  with  sodium  carbonate,  until  a  slight  precipitate 
begins  to  form.  A  sufficient  quantity  of  sulphurous  acid  is  now  added 
to  gfve  the  solution  a  strong  odor  of  SC>2.  The  solution  is  then  heated 

N 

to  boiling  and  an  excess  of  —  potassium  thiocvanate  slowly  added. 

10 

The  greenish  coloration  which  is  produced  upon  each  addition  of  the 
thiocyanate  is  due  to  the  formation  of  cupric  thiocyanate;  this  upon 
agitation  is  reduced  to  the  white  cuprous  thiocyanate  which  settles 
rapidly.  The  addition  of  the  standard  solution  is  continued  until  no 
further  coloration  is  produced  and  the  copper  is  completely  precipi- 
tated. 

The  quantity  of  thiocyanate  solution  used  is  noted,  and  after  the 
solution  has  cooled  down  it  is  diluted  with  distilled  water  to  exactly 
500  cc.  This  solution  is  then  thoroughly  shaken  and  filtered,  and  100  cc. 
(representing  o.i  gm.  of  the  metal)  is  removed  to  a  beaker,  10  cc.  of 
ammonio-ferric  sulphate  solution  added  as  indicator,  then  10  cc.  of 

N 

nitric  acid,  and  the  titration  with  —  silver  nitrate  begun  and  continued 

10 

until  the  solution  is  colorless.  The  quantity  of  silver  solution  con- 
sumed, deducted  from  the  quantity  of  thiocyanate  solution,  gives  the 
quantity  of  the  latter  which  reacted  with  and  hence  represents  the 
copper.  A  more  accurate  determination  of  the  end-point  may  be 
obtained  by  retitrating  with  the  thiocyanate,  after  the  silver  nitrate,  in 
which  case  the  quantity  of  silver  used  is  deducted  from  the  total  quan- 
tity of  thiocyanate. 

Precipitation  as  Cuprous  Thiocyanate  and  the  Acidimetric 
Determination  of  the  Combined  Thiocyanic  Acid  (W.  E.  Gar- 
rigues).*  This  method  depends  upon  precipitating  the  copper  as 
cuprous  thiocyanate  (CuSCN)  in  the  presence  of  sulphurous  acid, 
according  to  the  method  of  Rivot,  and  then  acidimetrically  determining 
the  combined  thiocyanic  acid  in  the  precipitate. 

The  solution  of  copper  sulphate,  slightly  acidulated  with  sulphuric 
acid,  is  warmed  and  treated  with  a  sufficient  excess  of  sulphurous  acid 


*  J.  A.  C.  S.,  XIX,  940. 


PRECIPITATION    WITH    TH1OCYANATE  373 

to  impart  a  distinct  odor  to  the  solution.  An  excess  of  ammonium 
thiocyanate  solution  is  now  added,  and  the  precipitate  allowed  to  settle 
(the  settling  is  facilitated  by  warming).  The  fluid  is  then  filtered, 
preferably  through  a  Gooch  crucible,  and  the  precipitate  well  washed 
with  water.  The  filter  and  contained  precipitate  are  then  transferred  to 
a  beaker  and  boiled  with  a  measured  excess  of  standard  caustic  alkali, 
for  a  few  minutes,  then  cooled  and  diluted  to  a  convenient  volume 
(say  200  cc.).  The  reaction  which  occurs  is  as  follows: 

2CuSCN-f  2NaOH=  Cu2(OH)2  +  sNaSCN. 

The  cuprous  hydroxid  is  then  filtered  off  and  the  filtrate  titrated  to 
neutrality  with  normal  hydrochloric  acid,  using  methyl  orange  as  indi- 

N 
cat  or.     One  cc.  of  —  NaOH= 0.0631  gm.  Cu. 

The  results  obtained  by  this  process  compare  very  favorably  with 
those  obtained  by  the  iodometric  method.  Garrigues  claims  even 
greater  accuracy  than  the  iodometric  method  is  capable  of.  It  is 
especially  useful  in  alloy  assaying. 

Precipitation  as  Cuprous  Thiocyanate  and  Subsequent  Titra- 
tion  of  the  Thiocyanic  Acid  by  Means  of  Potassium  Permanga- 
nate (H.  A.  Guess).*  The  first  steps  of  this  method  are  the  same 
as  in  the  foregoing.  The  precipitated  cuprous  thiocyanate  is  washed 
by  decantation  with  hot  water,  and  then  transferred  by  washing  onto  a 
filter. 

The  funnel  with  its  contents  is  then  placed  in  the  original  precipi- 
tation flask  and  a  boiling  ten  per  cent  solution  of  caustic  soda  poured 
through.  The  operation  is  repeated  with  a  further  portion  of  the 
caustic  soda  solution,  the  filter  being  filled  full  each  time.  The  cuprous 
thiocyanate  is  decomposed,  and  cuprous  hydroxid  remains  on  the 
filter,  while  sodium  thiocyanate  passes  through.  The  filter  and 
contents  are  well  washed  with  hot  water,  the  alkaline  filtrate  is  made 
decidedly  acid  with  dilute  sulphuric  acid,  and  the  liberated  thiocyanic 
acid  titrated  with  standard  potassium  permanganate  while  still  warm. 
According  to  the  equation 

ioHCNS+i2KMnO4+8H2SO4=6K2SO4+i2MnSO4+ioHCN+8H2O, 

the  iron  factor  multiplied  by  0.1892  should  give  the  theoretical  amount 
of  copper.  Guess  found  that  by  using  0.192  as  the  factor  greater 
accuracy  was  obtained. 

This  factor  overcomes  the  loss  occasioned  by  the  slight  solubility 

*  J.  A.  C.  S.,  XXIV,  708. 


374  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

of  the  cuprous  thiocyanates  and  the  consequent  incomplete  precipita- 
tion. 

The  permanganate  solution  used  for  this  purpose  is  best  standard- 
ized against  pure  copper,  under  the  same  conditions  as  the  regular 
assays. 

TITRATION  OF  THE  IRON  EQUIVALENT  OF  COPPER   BY  MEANS  OF 
POTASSIUM  PERMANGANATE 

Meade's  Process.*  The  copper  is  brought  into  solution  as  a 
sulphate,  either  by  dissolving  it  in  sulphuric  acid,  or  by  evaporation  of 
its  solution  with  sulphuric  acid.  The  greater  part  of  the  free  acid  is 
neutralized  by  ammonia,  the  solution  warmed,  sulphurous  acid  added 
until  the  solution  smells  strongly  of  the  reagent,  and  then  a  slight 
excess  of  ammonium  or  potassium  thiocyanate  added.  The  copper  is 
immediately  precipitated  as  cuprous  thiocyanate.  Stirring  and  warming 
renders  the  precipitate  heavy  and  easily  handled.  The  solution  is 
filtered  through  asbestos,  using  the  pump,  and  well  washed.  The 
precipitate  and  filter  are  thrown  into  the  beaker  in  which  the  precipita- 
tion was  made,  and  heated  with  a  solution  of  caustic  soda  or  potash. 
The  resulting  cuprous  hydroxid  is  filtered  on  asbestos  and  washed  well 
with  hot  water. 

The  precipitate  and  filter  are  again  placed  in  the  same  beaker, 
and  an  excess  of  ferric  chlorid  or  ferric  sulphate  (free  from  nitric  acid, 
free  chlorin  or  ferrous  salts),  together  with  a  little  dilute  sulphuric 
acid  added.  The  cuprous  oxid  reduces  a  corresponding  amount  of 
iron  from  the  ferric  to  the  ferrous  state. 


The  beaker  is  warmed  and  stirred  until  all  the  copper  oxid  is  dis- 
solved. The  solution  is  then  poured  through  a  perforated  platinum 
disk  and  the  asbestos,  which  stays  behind,  washed  with  water  to  which 
has  been  added  a  little  sulphuric  acid  and  a  little  ferric  chlorid  or 
sulphate.  The  solution  is  then  titrated  with  permanganate.  The 
iron  equivalent  to  the  permanganate  used,  multiplied  by  1.125,  gives 
the  weight  of  copper  in  the  sample.  Instead  of  sulphurous  acid, 
ammonium  or  sodium  bisulphate  may  be  used.  Copper  is  the  only 
metal  precipitated  by  an  alkaline  thiocyanate  out  of  an  acid  solution. 
Therefore  the  presence  of  arsenic,  antimony,  bismuth,  and  zinc  will 
not  interfere  with  the  accuracy  of  the  results.  The  filtration  may  be 
done  through  a  Gooch  crucible. 

*  J.  A.  C.  S.,  XX,  610. 


PRECIPITATION    WITH    THIOCYANATE  375 

S.  W.  Parr's  Process.*  This  method  depends  upon  precipitat- 
ing the  copper  as  cuprous  thiocyanate,  as  in  the  preceding  processes; 
then  decomposing  the  precipitate  by  heating  with  sodium  hydroxid, 
and  titrating  the  alkaline  solution  at  once  with  potassium  perman- 
ganate until  a  green  tint  is  imparted  to  the  liquid  portion  of  the  mixture. 
The  reaction  to  this  point  is  as  follows; 


The  appearance  of  the  green  tint  indicates  that  the  copper  has  been 
completely  oxidized,  and  that  the  permanganate  is  acting  upon  the 
sodium  thiocyanate  with  formation  of  the  green  manganates,  as  per 
the  reaction: 

8KMnO4+NaCNS+ioNaOH 


The  alkaline  solution  is  then  treated  with  diluted  sulphuric  acid 
(1:5)  slowly  added  with  constant  stirring  until  the  solution  becomes 
clear;  a  divided  excess  of  the  diluted  acid  is  then  added,  and  the  whole 
warmed  to  60°  or  70°  C.,  and  the  titration  with  the  permanganate 
continued  to  the  usual  pink-end  reaction.  This  is  really  a  titration 
of  cuprous  thiocyanate  by  permanganate.  For  further  details,  the 
articles  themselves  are  to  be  read. 

By  Precipitation  with  Potassium  Ferrocyanid  (Spica). 
When  a  solution  of  a  copper  salt  is  treated  with  potassium  ferrocyanid 
a  brown  precipitate  of  copper  ferrocyanid  forms: 

2CuS04+K4Fe(CN)6=Cu2Fe(CN)6+2K2SO4. 

This  reaction  is  the  basis  of  a  method  proposed  by  M.  Spica  f  for 
the  estimation  of  copper.  The  end-point,  i.e.,  the  point  at  which  a 
slight  excess  of  the  ferrocyanid  is  present,  is  determined  by  means  of 
ferric  chlorid.  A  drop  of  the  solution  being  titrated  is  removed  on  a 
glass  rod  and  brought  in  contact  with  a  piece  of  bibulous  paper,  which 
has  been  previously  impregnated  with  a  solution  of  ferric  chlorid  and 
dried.  The  production  of  a  blue  color  on  this  test  paper  indicates 
that  an  excess  of  the  ferrocyanid  has  been  added,  and  hence  that  the 
copper  is  completely  precipitated. 


*  J.  A.  C.  S.,  XXII,  685,  and  XXIV,  580. 

t  Matteo  Spica,  Chem.  Centrabl.,  65,  815  (1894). 


376  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  standard  solution  used  is  made  by  dissolving  0.84  gm.  of  pure 
crystallized  potassium  ferrocyanid  in  water  to  make  1000  cc.  Each  cc. 
of  this  solution  represents  o.ooi  gm.  of  CuSO4+5H2O. 

The  copper  solution  to  be  titrated  and  which  should  be  a  0.5  to 
i  per  cent  solution  is  slightly  acidified  with  hydrochloric  acid  before 
titrating.  The  titration  should  be  conducted  hot. 

This  method  can  be  used  only  for  pure  copper  solutions,  since 
under  similar  conditions  many  other  metals  are  precipitated.  For 
instance,  lead,  silver,  mercury,  cadmium,  bismuth,  zinc,  iron,  man- 
ganese, cobalt,  and  nickel.  It  must  be  admitted,  however,  that  with 
pure  copper  solutions  very  satisfactory  results  are  obtained. 

Estimation  by  Means  of  Stannous  Chlorid  (Fr.  Weil).*  This 
process  depends  upon  the  fact  that  a  hot  solution  of  a  cupric  salt  in 
the  presence  of  considerable  excess  of  hydrochloric  acid,  shows  a  dis- 
tinct greenish  yellow  color.  The  addition  of  the  minutest  excess  of 
stannous  chlorid  solution  decolorizes  the  copper  solution  instantly,  and 
thus  affords  a  sharp  easily  distinguishable  end-reaction.  The  reaction 
is  as  follows: 

2CuCl2 + SnCl2=  Cu2Cl2 + SnCU. 

The  further  certainty  that  the  reaction  is  complete  and  that  the 
stannous  chlorid  is  in  slight  excess,  is  afforded  on  adding  a  few  drops 
of  mercuric  chlorid  solution  to  a  small  portion  of  the  cooled  liquid; 
any  precipitate  of  calomel  indicates  the  presence  of  stannous  chlorid. 

The  stannous  chlorid  may  be  prepared  as  described  on  page  232. 
It  should  be  standardized  against  a  solution  of  copper  sulphate  as 
follows : 

Twenty-five  cc.  of  a  copper  sulphate  solution  (containing  o.i  gm. 
Cu)  are  introduced  into  a  loo-cc.  flask.  5  cc.  of  pure  concentrated 
hydrochloric  acid  are  added,  and  the  liquid  heated  to  boiling.  The 
stannous  chlorid  solution  is  added  to  the  boiling  liquid,  rapidly  at  first, 
but  towards  the  end  by  drops,  until  the  liquid  is  colorless.  5  cc.  more 
of  hydrochloric  acid  are  added,  if  a  slight  color  develops  more  stannous 
chlorid  solution  is  added  until  the  color  is  again  discharged,  and  the 
end-point  affirmed  by  testing  with  mercuric  chlorid  solution,  as  above 
described.  In  titrating  any  cupric  salt,  the  procedure  is  the  same; 
nitric  acid  and  ferric  salts  must  be  absent,  as  must  also  mercuric  salts, 
and  all  other  substances  which  react  with  stannous  chlorid. 


*  Zeitschr.  f.  analyt.  Chem.,  IX,  297. 


BIBLIOGRAPHY  377 


BIBLIOGRAPHY 


Steinbeck,  Chem.  News,  xix,  181. 

Davies,  Chem.  News,  LVIII,  131. 

Fessenden,  Chem.  News,  LXI,  253-283. 

J.  J.  and  C.  Beringer,  Chem.  News,  XLIX,  3. 

Spica,  Chem.  Centrbl.,  LXV,  815  (1894). 

Parkes,  Mining  Jour.,  (1851). 

Dulin,  J.  A.  C.  S.,  xvii,  346. 

Low,  J.  A.  C.  S.,  xviii,  458,  and  xxiv,  708. 

Garrigues,  J.  A.  C.  S.,  xix,  940. 

Meade,  J.  A.  C.  S.,  xx,  610. 

Shengel  and  Smith,  J.  A.  C.  S.?  xxi,  932. 

Parr,  J.  A.  C.  S.,  xxn,  685,  and  xxiv,  580. 

Guess,  J.  A.  C.  S.,  xxiv,    708. 

Fernekes  and  Koch,  J.  A.  C.  S.,  xxvii,  1224. 

Griggi,  Boll.  Chim.  Farm.,  XLII,  392. 

Westmoreland,  J.  S.  C.  I.,  v,  51. 

Liebig,  Ann.  Chem.,  95,  118. 

Mohr,  Ann.  Chem.,  94,  148. 

Weil,  Zeitschr.  f.  analyt.  Chem.,  ix,  397. 


CHAPTER  XXXIII 

GOLD 

THE  gold  must  be  in  the  form  of  chlorid  (AuCla). 

N 
To  the  solution  of  gold  chlorid  a  measured  excess  of  —  oxalic  acid 

solution  is  added  and  the  mixture  set  aside  for  twenty-four  hours. 

The  solution  is  then  made  up  to  a  definite  volume  (say  300  cc.). 
Then  by  means  of  a  pipette  100  cc.  are  removed,  and  the  excess  of 

N 
oxalic  acid  found  by  titrating  with  —  permanganate  in  the  presence 

of  sulphuric  acid.     The  reaction  is: 


N 
Each  cc.  of  —  oxalic  acid  solution  =0.065  23  gm-  of  Au,  or  0.1004 


gm.  of 

378 


CHAPTER  XXXIV 

IRON 

Ferrous  Salts  are  estimated  by  direct  titration  with  potassium 
permanganate  or  dichromate,  as  described  in  Chapter  XII. 

Ferric  Salts  may  be  estimated  by  means  of  potassium  perman- 
ganate or  dichromate,  but  they  must  first  be  reduced  to  the  ferrous 
state.  The  reduction  may  be  accomplished  in  various  ways,  as  follows: 

The  ferric  salt  in  solution  may  be  percolated  through  a  column  of 
zinc  dust,  which  reduces  it  to  the  ferrous  state,  and  the  iron  may  then 
be  estimated  in  the  usual  way  with  permanganate. 

Another  way  for  reducing  ferric  salts  is  to  treat  the  solution  with 
small  pieces  of  metallic  zinc  or  magnesium  and  a  little  sulphuric  acid. 
The  reduction  is  known  to  be  complete  when  the  solution  ceases  to 
give  a  red  color  with  potassium  sulphocyanate.  The  change  of  color 
of  the  ferric  solution  from  dark  to  light  is  also  an  indication  of  reduc- 
tion. 

When  zinc  or  magnesium  are  used  the  metal  must  be  entirely 
dissolved  before  titration  is  begun. 

For  full  details  of  the  method  for  estimating  ferric  salts  after  reduc- 
tion, see  page  162. 

The  zinc  used  must  be  free  from  iron,  or  if  any  is  present  its  exact 
quantity  should  be  known. 

The  iron  solution  to  be  reduced  by  zinc  should  not  contain  more 
than  0.12  in  200  cc.,  and  for  this  quantity  about  10  gms.  of  zinc  and 
20  cc.  of  sulphuric  acid  should  be  employed. 

Stannous  chlorid  solution  is  also  employed  for  reducing  ferric 
salts.  This  solution  is  made  by  dissolving  10  gms.  of  pure  tin  in 
200  cc.  of  strong  hydrochloric  acid  and  diluting  to  1000  cc.  It  should 
be  freshly  made  when  needed. 

After  reduction  by  stannous  chlorid  the  titration  may  be  performed 
with  dichromate. 

When  stannous  chlorid  is  used,  sufficient  must  be  added  to 
completely  reduce  the  iron,  but  an  excess,  even  the  slightest,  must 
be  avoided.  A  practiced  eye  can  very  closely  determine  the  point  of 
complete  reduction  by  the  discharge  of  the  color  in  the  ferric  solution, 
provided  the  solution  is  tolerably  concentrated.  A  surer  way  is  to  use 

379 


380  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

a  concentrated  aqueous  solution  of  mercuric  chlorid;  this  converts  any 
excess  of  stannous  chlorid  present  to  stannic  chlorid,  while  a  turbidity 
due  to  the  precipitation  of  mercurous  chlorid  results. 

If  the  excess  of  stannous  chlorid  were  not  thus  destroyed,  it  would 
consume  the  dichromate,  and  thus  lead  to  inaccurate  results.  The 
presence  of  the  mercurous  chlorid  precipitate  does  not  in  the  least 
interfere  with  the  titration,  which  should  be  begun  with  potassium 
dichromate  at  once. 

Other  reducing  agents  for  ferric  salts  which  are  used  in  technical 
analyses,  are  sodium  sulphite,  ammonium  bisulphite,  and  sulphurous 
acid.  When  these  are  used,  the  ferric  solution  should  be  dilute  and 
not  too  acid.  The  sulphite  is  added  to  the  ferric  solution  and  heated 
in  a  flask  until  the  color  is  discharged.  It  is  then  boiled  until  all 
sulphur  dioxid  is  driven  off,  quickly  cooled  and  titrated  with  the  standard 
dichromate. 

After  reduction  by  any  method,  the  titration  should  be  started 
without  delay,  because  the  iron  is  rapidly  re-oxidized  upon  exposure. 

Ferric  salts  may  also  be  estimated  direct,  i.e.,  without  previous 
reduction,  but  the  first  mentioned  methods  are  by  far  the  best. 

The  Direct  Titration  by  Stannous  Chlorid  is  explained  on 
page  231. 

The  Direct  Titration  by  Sodium  Thiosulphate.  When  a 
ferric  salt  is  treated  under  suitable  conditions  with  a  solution  of  sodium 
thiosulphate,  the  following  reaction  occurs  and  a  fairly  accurate  estima- 
tion of  the  ferric  salt  may  be  obtained: 


Sodium  thiosulphate  was  first  suggested  for  use  for  this  purpose  by 
Scherer  in  1859,  afterwards  by  Kremer  and  Landolt  (Zeitschr.  f.  analyt 
Chem.,   I,   214).     Later   Oudemans   in    the  same  journal   (VI,   129) 
described  the  method  briefly  given  below. 

Oudemans'  Process.  A  small  quantity  of  a  solution  of  pure  copper 
sulphate  (i  cc.  of  a  i  per  cent  solution)  is  added  to  the  ferric  salt,  and 
then  a  few  drops  of  a  solution  of  potassium  sulphocyanate,  which 
colors  the  fluid  intensely  red.  Sodium  thiosulphate  is  then  nin  in 
slowly  from  the  burette.  A  few  minutes  should  elapse  after  each 
addition  of  thiosulphate.  When  the  color  communicated  by  the  sulpho- 
cyanate has  disappeared,  a  milkiness  makes  its  appearance,  due  to 
cuprous  sulphocyanate,  which  is  formed.  This  marks  the  completion 
of  the  reaction.  The  operator  is  very  apt  to  overstep  the  mark,  and 
the  best  plan  is  to  add  thiosulphate  in  very  slight  excess,  and  find  the 
amount  in  excess  by  adding  starch,  and  titrating  back  with  iodin. 


DIRECT    TIT  RATION    BY    SODIUM    THIOSULPHATE     381 

The  method  is  tolerably  good  and  sufficiently  accurate  for  technical 
estimations,  but  not  entirely  satisfactory  where  great  exactness  is 
required.  The  copper  sulphate  is  supposed  by  Oudemans,  who  first 
used  it,  to  act  by  transferring  oxygen  from  the  ferric  salt  to  the  thio- 
sulphate.  The  estimation  may  be  made  without  using  copper  sulphate, 
if  the  solution  be  heated  to  40°  C. 

N 
i  cc.  —  thiosulphate= 0.00555  g111-  Fe. 

Kerner  gives  the  following  as  being  a  satisfactory  procedure: 
The  iron  solution,  in  which  the  metal  must  be  in  complete  oxida- 
tion (best  obtained  by  using  hydrogen  dioxid  in  excess  and  boiling 
off  the  excess),  is  acidified  with  hydrochloric  acid.  Sodium  acetate  is 
added  until  the  mixture  is  red,  and  then  dilute  hydrochloric  acid  until 
the  red  color  is  discharged;  the  solution  is  then  diluted  until  the  iron 

N 
amounts  to  0.25  per  cent.    An  excess  of  —  sodium  thiosulphate  solu- 

10  N 

tion  is  then  added,  and  the  excess  determined  by  titration  with  — 

iodin  solution  and  starch. 

J.  T.  Norton  (Am.  J.  Sci.,  CLvm,  25-30)  made  careful  experiments 
with  this  thiosulphate  method,  and  found  that  the  best  results  are 
obtained  under  the  following  conditions:  The  concentration  of  the 
iron  solution  must  not  be  greater  than  o.i  gm.  ferric  oxid  in  400  cc. ; 
the  amount  of  acid  present  should  not  exceed  i  cc.  of  strong  hydro- 
chloric acid  in  the  same  volume;  the  time  of  reaction  must  be  as  short 
as  possible  to  prevent  progressive  oxidation  by  air,  and  the  excess  of 
thiosulphate  used  should,  for  the  same  weight  of  iron,  be  not  less  than 
15  cc.  of  a  decinormal  solution. 

Moreau  *  describes  the  following  modification  for  the  titration  of 
solution  of  ferric  chlorid: 

Five  gms.  of  the  solution  is  treated  with  2  cc.  of  pure  hydrochloric 
acid  and  diluted  with  water  to  80  cc.  10  cc.  of  this  is  measured  off 
off  and  diluted  with  20  to  30  cc.  of  water,  about  o.i  gm.  of  sodium 
salicylate  is  then  added  as  an  indicator,  followed  by  10  cc.  of  a  10  per 

N 
cent  solution  of  cupric  sulphate.  The  mixture  is  then  titrated  with  — 

10 

sodium  thiosulphate,  added,  drop  by  drop,  until  the  liquid  has  lost  its 
violet  color,  or  shows  only  the  blue  color  of  the  copper.  The  interval 
between  drops  should  be,  near  the  end,  five  seconds.  The  reaction 
between  the  ferric  salt  and  the  thiosulphate,  which  is  shown  on 

*  Bull,  de  Sciences  Pharm.,  from  Pharm.  Jour.,  1904,  744. 


382  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

page  380,  takes  place  only  in  warm  solutions,  but  in  the  presence  of 
copper  it  proceeds  in  the  cold. 

The  best  method,  however,  for  simple  ferric  solutions  is  the  iodo- 
metric  method,  in  which  the  ferric  salt  is  reduced  by  digestion  with 
potassium  iodid,  and  the  liberated  iodin  which  is  set  free  in  this  reac- 
tion, titrated  with  standard  sodium  thiosulphate  solution  (see  page 
224). 

Titration  by  Permanganate  after  Precipitation  as  Ferrous 
Sulphid  (N.  Matolcsy,  Ph.  Post.,  1903,  41).  This  method  depends 
upon  the  fact  that  ferric  salts,  as  well  as  ferrous  salts,  are  precipitated 
by  ammonium  sulphid  in  the  form  of  ferrous  sulphid: 

Fe2Cl6 + 3  (NH4)2S = 2FeS + 6NBUC1+ S, 
FeCl2  +  (NH4)2S = FeS + 2NH4C1, 

and  that  the  precipitate  dissolves  in  sulphuric  acid,  forming  ferrous  sul- 
phate, which  may  be  titrated  with  permanganate  solution  in  the  usual 
manner,  after  the  expulsion  of  hydrogen  sulphid. 

The  Process.  To  the  aqueous  solution  of  the  iron  salt,  acidulated 
with  hydrochloric  acid,  an  excess  of  H2S  solution  is  added  and  then 
ammonia  water  sufficient  to  make  the  solution  alkaline;  this  causes 
the  precipitation  of  the  iron  as  ferrous  sulphid. 

In  order  to  bring  about  the  agglutination  of  the  precipitate,  some 
ammonium  chlorid  is  added  and  the  solution  slightly  warmed.  The 
precipitate  is  then  collected,  washed  with  ammonium  sulphid  solution, 
and  then  dissolved  in  dilute  sulphuric  acid.  The  resulting  solution 
of  FeSO4  is  then  boiled  until  the  H2S  is  completely  driven  off,  i.e., 
until  the  steam  evolved  no  longer  blackens  silver  nitrate  paper.  The 
addition  to  the  boiling  solution  of  a  small  piece  of  calcite,  or  pure 
marble,  facilitates  the  dispersion  of  the  H2S  and  insures  an  even  dis- 
tribution of  the  heat.  The  solution  is  then  quickly  cooled  and  titrated 
with  decinormal  potassium  permanganate  solution,  in  the  usual  manner. 

Titration  of  Ferric  Salts  by  Permanganate  after  Reduction 
with  Stannous  Chlorid.*  The  following  solutions  are  required : 

(a)  A  Five  Per  Cent  Solution  of  Stannous  Chlorid.     This  solution 
need  not  be  freshly  prepared,  but  it  must  be  clear  and  should  give  no 
precipitate  when  diluted  with  twenty  times  its  volume  of  water  and 
boiled.     It  may  be  kept  in  good  condition  by  acidifying  with  hydro- 
chloric acid  and  putting  a  few  fragments  of  tin  into  the  bottle. 

(b)  A  Mercuric  Sulphate  Solution.    This  is  made  by  adding  to 
200  gins,  of  mercuric  sulphate,  80  cc.  of  sulphuric  acid  (cone.),  and 

*  Cady  and  Ruediger,  J.  A.  C.  S.,  XIX,  577,  describe  this  method  for  titra- 
tion  with  permanganate,  in  the  presence  of  hydrochloric  acid. 


METALLIC    IRON   IN    REDUCED    IRON  383 

adding  to  this  paste,  800  cc.  of  water.  If  a  yellow  precipitate  of  basic 
mercuric  sulphate  is  formed,  add  more  sulphuric  acid.  Add  to  this 
100  gms.  of  orthophosphoric  acid  and  dilute  to  one  liter. 

The  Process,  i  gm.  of  the  substance  is  dissolved  in  15  cc.  of  hydro- 
chloric acid  (sp.gr.  i.io)  in  a  small  Erlenmeyer  flask,  2  cc.  of  the 
mercuric  sulphate  solution  (b)  is  added,  and  the  whole  heated  to  boiling 
and  the  stannous  chlorid  solution  added  in  small  quantities  at  a  time. 
The  precipitate  that  may  be  temporarily  produced  as  each  drop  of 
stannous  chlorid  is  added,  will  dissolve  after  boiling  a  few  seconds, 
until  all  the  iron  is  reduced,  when  the  solution  becomes  colorless,  and 
the  addition  of  a  single  drop  of  stannous  chlorid  produces  a  perceptible 
turbidity  or  a  precipitate  which  does  not  redissolve  on  boiling.  This 
shows  the  end  of  the  reduction.  The  contents  of  the  flask  is  rinsed 
into  a  beaker  and  diluted  to  about  300  cc.  50  cc.  of  dilute  sulphuric 
acid  are  added,  and  then  45  cc.  of  the  mercuric  sulphate  solution, 
and  the  titration  begun  at  once  with  standard  permanganate. 

The  other  method  proposed  by  the  same  authors  is  as  follows: 
Dissolve  in  water  with  a  varying  quantity  of  hydrochloric  acid, 
dilute  to  100  cc.,  and  heat  to  boiling.  Reduce  with  stannous  chlorid, 
adding  small  portions  at  a  time  until  the  solution  becomes  colorless, 
and  a  droplet  gives  no  red  color  with  potassium  thiocyanate.  Any 
excess  of  stannous  chlorid  that  may  have  been  used  is  oxidized  by 
adding  potassium  permanganate,  drop  by  drop,  until  a  droplet  of  the 
iron  solution  gives  a  faint  red  color  with  thiocyanate.  One  or  two  drops 
of  stannous  chlorid  solution  are  now  added,  the  whole  process  being 
carried  on  at  a  boiling  heat.  The  solution  is  then  cooled.  50  cc.  of 
dilute  sulphuric  acid  are  added,  and  for  every  10  cc.  of  hydrochloric 
acid  that  has  been  used  add  35  cc.  of  the  mercuric  sulphate  solution; 
dilute  to  400  cc.  and  titrate  with  permanganate. 

ESTIMATION  OF  METALLIC  IRON  IN  REDUCED  IRON 

Professor  E.  Schmidt,  of  Marburg,  recommends  *  the  following 
process  : 

Weigh  accurately  0.4  gm.  of  reduced  iron,  and  place  in  a  loo-cc. 
flask  with  10  cc.  of  water,  and  add  2  gms.  of  pure  dry  iodin. 

The  iodin  combines  with  metallic  iron,  but  does  not  react  with  any 
ferric  oxid  which  may  be  present. 


Now  rinse  down  the  iodin  left  in  the  neck  of  the  flask  with  some 
water,  and  add  i  gm.  of  potassium  iodid;    when  all  of  the  iodin  has 

*  Proc.  Soc.  German  Naturalists  and  Physicians,  Sept.,  1897 


384  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

dissolved,  add  sufficient  water  to  make  100  cc.     Shake  the  flask  and 
allow  to  stand  for  several  hours. 

Then  measure  off  50  cc.  of  the  clear  liquid  and  titrate  the  free  iodin 
with  decinormal  sodium  thiosulphate,  using  starch  as  an  indicator. 
The  reaction  is  thus  expressed  : 


I2  +  2Na2S2O3,  5H2O  =  Na2S4O6+2NaI+ioH2O. 
2)251.8          2)492.92 
10)125.9         10)246.46 

12.59  gms.       24.646  gms.       or  1000  cc.  —  V.  S. 

10 

.01259  gm.      .024646  gm.  "         i  cc.  "      " 

Example.  Assuming  that  9  cc.  of  the  decinormal  solution  were 
employed  in  titrating  the  50  cc.,  then  18  cc.  would  be  required  by  the 
entire  quantity. 

As  seen  in  the  above  equation,  each  cc.  of  the  decinormal  solution 
represents  0.01259  gm.  of  iodin;  hence  if  18  cc.  are  employed  we  have 
18X0.01259  gm.==  0.22662  gm.,  the  quantity  of  free  iodin. 

Then  by  subtracting  this  amount  from  the  quantity  of  iodin  taken 
(2  gms.)  we  ascertain  the  quantity  which  went  into  combination  with 
the  iron,  namely,  1.7733  gms.  All  that  is  now  necessary  is  to  ascertain 
by  calculation  the  quantity  of  metallic  iron  represented  by  the  above 
weight  of  iodin. 

Fe  +  I2  =  FeI2; 
55-5     251.8 

1-7733X55-5 


Thus  the  0.40  gm.  of  reduced  iron  taken  contained  0.3511+  gm.  of 
metallic  iron,  or  87  per  cent. 

This  process,  very  slightly  modified,  is  official  in  the  U.  S.  P.  and 
the  Pharm.  G.  In  both  of  these  standards  the  iron  is  dissolved  in  a 
solution  of  iodin  in  potassium  iodid. 

The  Estimation  of  Metallic  Iron  in  Reduced  Iron  may  also  be 
made  by  the  process  of  digestion  with  mercuric  chlorid.  This  con- 
verts all  the  iron  present  in  a  free  state  to  ferrous  chlorid,  which  is  then 
titrated  by  potassium  permanganate  (see  page  152). 

To  confirm  the  assay,  add  a  few  drops  of  alcohol  to  decolorize  (or 
decompose)  the  excess  of  permangante,  then  add  i  gm.  of  potassium 
iodid,  and  digest  for  half  an  hour  at  a  temperature  of  40°  C.  (104°  F.). 

Fe2(SO4)3+2KI=2FeSO4  +  I2  +  K2SO4. 
2)111.  2)251.80 

io)55-5 


5-55  I2-59 


TOTAL    IRON    CONTENT    OF    FERRUM    REDUCTUM       385 

The  cooled  solution  is  mixed  with  a  few  drops  of  starch  test  solu- 
tion, which  gives  it  a  dark  blue  color,  because  of  the  formation  of  iodid 
of  starch.  Then  add  carefully,  from  a  burette,  decinormal  sodium 
thiosulphate  until  the  blue  color  is  discharged. 

I2  +2(Na2S2O3  • 

2)251.8  2)492.92 

10)125.9 


12.59  gms.  24.646  gms.  or  1000  cc.  —  Na2S2O3  V.  S. 

10 

Thus  each  cc.  of  the  standard  thiosulphate  represents  0.01259  gm. 
of  iodin,  or  0.00555  gm.  of  metallic  iron. 

In  both  of  these  estimations  the  quantity  of  standard  solution  used 
should  be  the  same. 

Total  Iron  Content  of  Ferrum  Reductum.  Reduced  iron 
should  contain  at  least  90  per  cent  of  metallic  iron.  But  there  is 
always  more  or  less  oxidation  of  the  iron  upon  exposure  and  long 
standing,  so  that  for  this  reason  and  through  imperfect  reduction  in 
its  manufacture  there  is  always  present  some  ferric  oxid  and  more  or 
less  sulphid,  arsenic,  and  copper,  as  well  as  insoluble  matter.  • 

The  total  iron  content,  i.e.,  free  and  combined,  may  be  estimated 
as  follows:  *  o.i  gm.  of  the  sample  is  digested  upon  a  water-bath  with 
hydrochloric  acid  to  complete  solution.  10  cc.  of  sulphuric  acid  are 
now  added,  and  the  hydrochloric  acid  evaporated  off.  The  whole  is 
then  rinsed  into  an  Erlenmeyer  flask,  diluted  to  100  cc.,  2  gms.  of 
pure  zinc  added  and  the  flask  set  aside  in  a  warm  place  for  about  one 
hour,  after  which  time  the  contents  are  filtered  through  a  glass  plug 
and  immediately  titrated  with  standard  permanganate  in  the  usual 
way. 

Another  o.i  gm.  of  the  same  sample  is  then  assayed  for  free  iron 
by  either  of  the  foregoing  methods.  The  quantity  of  free  iron  found 
subtracted  from  the  total  iron  gives  the  amount  of  iron  present  in  a 
combined  state,  principally  as  ferric  oxid. 

Estimation  of  the  Iron  in  Liquor  Ferri  Albuminata.  20  cc. 
of  the  solution  are  mixed  in  a  loo-cc.  flask  with  20  cc.  of  water  and 
3  cc.  of  diluted  sulphuric  acid  (1:5)  the  mixture  is  heated  on  a  water- 
bath  until  the  brown  precipitate  at  first  produced  becomes  whitish. 

The  contents  of  the  flask  are  then  allowed  to  cool,  diluted  with  water 
to  100  cc.  and  filtered.  20  cc.  of  this  solution  are  then  measured  off, 
and  potassium  permanganate  solution  added  drop  by  drop  until  the 
whole  quantity  of  solution  is  faintly  red,  the  color  being,  however, 

*  G.  C.  Steventon,  Proc.  A.  Ph.  A.,  1894,  172. 


386  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

transient,  and  when  it  disappears  and  leaves  the  solution  colorless 
(the  addition  of  a  few  drops  of  alcohol  will  facilitate  the  discharge  of 
the  color),  2.5  to  3  gms.  of  potassium  iodid  are  added,  the  flask  stoppered 
securely,  and  set  aside  for  one.  hour  at  the  ordinary  temperature.  At 

N 

the  end  of  this  the  liberated  iodin  is  titrated  with  —  sodium  thio- 

10 

sulphate  and  the  calculation  made  in  the  usual  manner. 

Estimation  of  Iron  in  Iron  Ores.  It  is  usually  required  in 
estimating  the  quantity  of  iron  in  iron  ores  to  determine  the  ferrous 
and  the  ferric  oxid.  This  may  be  done  as  follows : 

0.5  gm.  of  the  ore  in  fine  powder  is  dissolved  with  careful  exclusion 
of  air  *  in  20  cc.  of  a  mixture  of  equal  parts  of  strong  hydrochloric 
acid,  and  water,  and  the  resulting  solution  titrated  with  potassium 
dichromate  or  permanganate,  for  the  quantity  of  ferrous  oxid. 

N 

Each  cc.  of  —  dichromate  =  0.005 5 5    gm.  Fe; 
10 

=  0.007138  "     FeO; 
=  0.007932   "     Fe2O3. 

Anotner  0.5  gm.  of  the  finely  powdered  ore  is  then  dissolved  as 
above,  and  when  complete  solution  is  effected,  the  liquid  is  diluted 
with  water,  15  cc.  of  strong  sulphuric  acid  are  added,  a  few  fragments 
of  pure  zinc  are  introduced,  and  the  ferric  oxid  present  thus  reduced 
to  the  ferrous  state;  air  is  excluded  during  the  reduction  to  prevent 
reoxidation,  and  the  titration  with  permanganate  immediately  begun. 
The  total  iron  content  is  thus  obtained,  and  by  subtracting  the  ferrous 
oxid  found  from  the  total  iron  in  the  ore,  the  amount  of  iron  in  the 
ferric  state  is  ascertained. 

If  any  doubt  exists  as  to  the  complete  reduction  of  the  iron,  a  drop 
of  the  solution  is  withdrawn  on  a  glass  rod  and  tested  with  potassium 
thiocyanate.  A  red  color  will  indicate  the  presence  of  unreduced  iron. 
In  this  event  it  will  be  necessary  to  add  more  inc  and  possibly  more 
acid  in  order  to  completely  reduce  the  remaining  ferric  oxid. 

Note.  If  permanganate  is  used  as  the  titrating  reagent,  the  amount 
of  hydrochloric  acid  must  be  small,  and  the  solution  cold  and  very 
dilute,  otherwise  a  vitiating  reaction  will  take  place  between  the  hydro- 
chloric acid  and  the  permanganate  (see  page  140). 

In  estimating  total  iron,  the  reduction  to  the  ferrous  state  may  be 
made  either  by  means  of  sodium  sulphite  or  stannous  chlorid.  If 
these  reducing  agents  are  used,  the  titration  should  be  done  by  means 
of  potassium  dichromate. 

*  The  apparatus  shown  in  Fig.  57  may  be  used  for  this  purpose. 


ESTIMATION    OF    IRON    IN    IRON    ORES  387 

If  the  ore  under  examination  is  not  readily  soluble  in  hydrochloric 
acid,  solution  may  be  effected  by  the  addition  of  some  potassium  chlorate, 
and  the  application  of  heat.  The  solution  is  then  evaporated  to  dryness, 
and  the  residue  dissolved  in  hydrochloric  acid.  Any  residue  which 
may  be  left  after  treatment  with  hydrochloric  acid,  should  be  separated 
by  nitration  and  fused  with  sodium  carbonate;  this  will  render  all  the 
iron  soluble.  Ores  which  contain  organic  matter  or  pyrites  should 
first  be  roasted  in  an  open  platinum  crucible. 


CHAPTER  XXXV 

X 

LEAD 

Lead  Oxid  or  Carbonate  is  estimated  by  dissolving  in  a 
measured  excess  of  normal  nitric  acid  solution,  and  then  titrating  back 
with  normal  sodium  carbonate  or  hydroxid  solution  until  a  faint 
milkiness  appears,  or  phenolphthalein  which  has  been  added  turns 
red.  The  quantity  of  normal  alkali  solution  used  is  deducted  from 
the  quantity  of  normal  acid  added,  and  the  remainder  multiplied  by 
the  factor  for  the  lead  salt  examined. 

Pb         =0.102675  gm.; 
PbO     =o.  11061  gm.; 
PbCOs=o.  13245  gm. 

Soluble  lead  salts  may  be  converted  into  carbonates  by  adding  an 
excess  of  ammonium  carbonate.  The  precipitated  lead  carbonate  is 
then  washed  with  hot  water  and  estimated  as  above. 

Before  titrating  the  nitric  acid  solution  with  the  standard  alkali, 
Mohr  adds  enough  neutral  solution  of  sodium  sulphate  to  precipitate 
the  lead  as  sulphate.  The  excess  of  nitric  acid  is  then  determined  by 
means  of  the  standard  alkali,  without  previous  filtration. 

By  Titration  with  Standard  Dichromate.  Lead  salts  may 
also  be  estimated  by  precipitation  with  a  standard  solution  of  potas- 
sium dichromate.  The  end-point  is  reached  when  a  further  addition 
of  a  drop  of  the  dichromate  solution  fails  to  produce  a  precipitate. 
Beale's  filter  may  here  be  employed,  or  a  neutral  solution  of  silver 
nitrate  may  be  used  as  an  indicator,  by  contact  on  a  porcelain  slab. 
The  dichromate  solution  used  for  precipitating  lead  is  the  same  as  that 
used  for  the  estimation  of  barium  containing  7.307  gms.  in  a  liter. 
The  reaction  is  as  follows: 


The  lead  solution  must  not  contain  much  free  acid.  Sufficient 
sodium  acetate  is  added  to  saturate  the  mineral  acid  and  set  free  an 
equivalent  of  acetic  acid. 

388 


BY    TITRATION    WITH    STANDARD    DICHROMATE       389 

N 

Each  cc.  of  —  dichromate=o.oio2675  gm.  Pb; 
10 

=0.011061  gm.  PbO. 

J.  H.  Wainwright  (J.  A.  C.  S.,  1897,  389)  modifies  this  method 
as  follows: 

One  gm.  of  litharge,  for  example,  is  dissolved  in  10  to  15  cc.  of  nitric 
acid  (sp.gr.  1.20),  the  solution  is  neutralized  with  ammonia  in  excess, 
and  a  considerable  excess  of  acetic  acid  is  added.  The  solution  is 
then  boiled  and  the  standard  potassium  dichromate  solution  run  in. 
When  nearly  enough  of  the  latter  has  been  used  to  completely  precipi- 
tate the  lead,  the  mixture  is  again  boiled  until  the  precipitate  of  lead 
chromate  has  become  orange  colored.  The  titration  is  now  continued, 
one-half  cc.  or  less  at  a  time,  stirring  after  each  addition,  until  the 
reaction  is  nearly  complete,  which  is  known  by  the  sudden  clearing 
up  of  the  solution,  the  lead  chromate  settling  promptly  to  the  bottom 
of  the  beaker.  This  will  occur  if  the  solution  is  hot,  usually  within 
one  cc.  of  the  end  of  the  reaction. 

The  dichromate  should  now  be  added,  drop  by  drop,  allowing 
settling  after  each  addition,  and  testing  by  bringing  a  drop  or  two  of 
the  solution  in  contact  with  a  drop  of  silver  nitrate  solution  on  a  white 
porcelain  tile,  until  a  distinct  red  color  is  produced. 

The  dichromate  solution  should  be  made  of  such  strength  that  one  cc. 
will  equal  about  o.oi  gm.  of  lead,  and  should  be  standardized  either 
by  means  of  pure  metallic  lead  or  pure  "white  lead,"  in  which  the 
metal  has  been  accurately  determined  gravimetrically. 

This  method  is  particularly  adapted  to  the  assay  of  such  substances 
as  litharge,  white  lead,  red  lead,  pig  lead,  etc.,  and  for  lead  ores  in 
which  the  lead  exists  as  carbonate.  "White  lead"  may  be  dissolved 
directly  in  dilute  acetic  acid  and  the  resulting  solution  titrated  without 
nitration.  In  the  case  of  "red  lead  "  solution  should  be  effected  by 
means  of  nitric  acid,  boiling,  and  then  adding,  drop  by  drop,  a  dilute 
solution  of  oxalic  acid  until  the  lead  oxid  formed  is  completely  dissolved. 
If  organic  matter  is  present,  the  solution  should  be  filtered. 

The  essential  points  to  be  observed  in  order  to  attain  success 
are: 

First.  The  lead  solution  should  be  concentrated  and  decidedly 
acid  with  acetic  acid. 

Second.  Other  metals  should  be  absent,  especially  such  as  may 
exist  in  the  lower  forms  of  oxidation;  antimony  and  tin  are  especially 
to  be  avoided,  unless  they  are  thoroughly  oxidized  by  repeated  evap- 
oration with  fuming  nitric  acid.  Bismuth  also  should  be  absent. 

Third.  The  solution  should  be  kept  as  near  the  boiling-point  as 
possible  during  the  titration. 


390  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Precipitation  as  Chromate  and  Digestion  of  the  Precipitate 
with  a  Ferrous  Salt.  If  the  lead  salt  is  acid,  sodium  acetate  is 
added  in  sufficient  quantity  to  saturate  the  mineral  acid  and  set  free 
an  equivalent  of  acetic  acid.  The  solution  is  warmed  and  potassium 
dichromate  is  added  in  sufficient  quantity  to  precipitate  all  of  the  lead. 

The  precipitate  is  then  collected  on  an  asbestos  filter  and  washed 
thoroughly  with  warm  water;  and  then  together  with  the  asbestos  is 
placed  in  a  flask  containing  a  weighed  quantity  of  ferrous  ammonium 
sulphate  and  the  solution  warmed  until  the  chromate  is  completely 
decomposed.  A  current  of  hydrogen  or  carbon  dioxid  is  passed  through 
the  flask  during  the  reaction  (see  Fig.  57).  The  resulting  solution 
contains  ferric  and  chromic  sulphate,  together  with  lead  sulphate  and 
some  unoxidized  ferrous  salt. 

N 
The  quantity  of  the  latter  is  found  by  —  permanganate,  after 

acidulation  with  H2SO4  (see  p.  174),  and  deducted  from  the  original 
weight,  the  remainder  multiplied  by  -0.1757  gm.  will  give  the  weight 
of  lead.  The  reactions  which  occur  in  this  process  are: 


(a) 

(6)     2PbCrO4+6FeSO4  .  (NH4)2SO4+6H2O  +  8H2SO4=  Cr2(SO4)3 


1168.02  gms.  of  the  ferrous-ammonium  sulphate  =205  .3  5  gms.  of  Pb. 
Thus  i  gm.  of  ferrous  ammonium  sulphate  =0.175  7  gm-  Pb. 
i  gm.  of  Fe=i.23i3  gms.  of  Pb. 

By  Titrating  with  Excess  of  Dichromate  and  Determining  the 
Excess  of  the  latter  by  Titration  with  Standard  Solution  of 
Ammonio-ferrous  Sulphate.  This  modification  of  the  foregoing 
was  suggested  by  A.  S.  Cushman  and  J.  Hayes-Campbell,*  and  con- 
sists in  adding  a  measured  quantity  (excess)  of  standard  potassium 
dichromate  solution  to  the  lead  solution  containing  some  sodium 
acetate.  The  precipitated  lead  chromate  is  separated  by  filtration, 
the  precipitate  thoroughly  washed  with  hot  water.  The  mixed  filtrate 
and  washings  are  then  titrated  with  a  standardized  solution  of  ammonio- 
ferrous  sulphate,  using  a  freshly  prepared  solution  of  potassium  ferri- 
cyanid  as  an  outside  indicator.  The  standardization  of  the  iron 
solution  is  effected  by  titrating  against  a  decinormal  solution  of  potas- 
sium dichromate.  The  standard  ferrous  solution  should  be  preserved 
under  a  layer  of  paraffin  oil,  in  a  stock  bottle  provided  with  a  siphon 
tube  and  pinch  -cock;  this  arrangement  being  used  to  prevent  too 
rapid  oxidation  of  the  standard  solution. 

*  J.  A.  C.  S.,  XVII,  901. 


LEAD  391 

The  Assay  of  a  Lead  Ore  by  this  method  is  carried  out  as  follows: 

"One  gm.  of  finely  pulverized  ore  is  digested  in  a  casserole,  or 
evaporat ing-dish,  with  15  cc.  of  a  mixture  of  two  parts  of  nitric  acid 
and  one  part  of  sulphuric  acid  until  decomposition  is  complete.  10  cc. 
or  more  of  sulphuric  acid  are  now  added,  and  the  liquid  evaporated 
until  it  fumes  freely.  Cool,  dilute  with  10  cc.  of  dilute  sulphuric  acid 
(i :  10),  and  then  add  gradually  40  cc.  of  water.  Heat  to  boiling,  filter, 
and  wash  by  decantation  with  dilute  sulphuric  acid  (1:10),  getting  as 
little  of  the  lead  sulphate  on  the  filter  as  possible.  To  the  residue  in 
the  dish  add  20  cc.  of  strong  ammonia,  then  make  slightly  acid  with 
acetic  acid.  Boil  until  the  lead  sulphate  is  dissolved,  then  pour  the 
liquid  through  the  filter,  having  first  moistened  the  paper  with  ammonia. 
Wash  the  filter  with  water  containing  ammonium  acetate,  and  finally 
once  or  twice  with  hot  water.  Cool  the  filtrate  and  run  in  from  a 
burette  an  excess  of  standard  dichromate  solution,  stirring  until  the 
precipitate  settles  rapidly  and  the  supernatant  liquid  has  a  yellow  color. 
Allow  to  settle  for  a  few  minutes,  then  filter,  under  pressure  if  possible, 
wash  a  few  times  and  titrate  the  filtrate  with  the  standard  ammonio- 
ferrous  sulphate  solution." 

"  In  case  the  ore  is  known  to  be  free  from  bismuth  and  antimony, 
it  may  be  dissolved  in  nitric  acid  alone,  and  the  solution  neutralized 
with  an  excess  of  ammonia  and  then  made  acid  with  acetic  acid." 
This  solution  is  then  immediately  titrated  as  above.  The  results  by 
this  method  are  a  trifle  low. 

The  Technical  Assay  of  Lead  Ores  by  the  Conversion  of  the 
Precipitated  Chromate  into  Lead  Oxalate,  and  Titration  of  the 
latter  by  Means  of  Permanganate.  The  following  method  is 
described  by  A.  H.  Low,*  who  considers  it  the  best  for  technical  work: 

Take  0.5  gm.  of  the  ore  and  treat  it  in  a  6-ounce  flask  by  the  usual 
methods  to  obtain  the  washed  lead  sulphate,  etc.,  on  a  p-cm.  filter. 

The  lead  sulphate  on  the  filter  is  dissolved  by  treatment  with  hot 
sodium  acetate  solution.  The  sodium  acetate  solution  is  made  by 
diluting  a  cold  saturated  solution  of  sodium  acetate  with  an  equal 
bulk  of  water  and  adding  40  cc.  of  80  per  cent  glacial  acetic  acid  per 
liter. 

The  completeness  of  the  extraction  of  the  lead  sulphate  is  ascertained 
by  passing  a  small  quantity  of  the  sodium  acetate  solution  through  the 
filter  and  testing  with  potassium  dichromate.  A  yellow  precipitate 
indicates  incomplete  extraction  of  the  lead  sulphate.  The  solution  of 
lead  sulphate  so  obtained  is  shaken,  and  heated,  if  necessary,  to  re- 
dissolve  any  separated  precipitate;  and  10  cc  of  a  5  per  cent  solution 
of  commercial  potassium  dichromate  are  added  and  the  mixture  heated 

*  J.  A.  C.  S.,  XXX,  587. 


392  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

to  boiling,  until  the  precipitate  is  basic,  which  is  known  by  its  becoming 
reddish  yellow.  Then  filter  hot,  and  wash  the  precipitate  only  once, 
simply  to  clear  the  upper  edge  of  the  filter.  The  precipitate  is  then 
washed  off  the  filter  into  a  flask  by  means  of  a  jet  of  hot  oxalic  acid 
solution,  using  25  to  40  cc.,  and  subsequently  with  hot  water.  The 
oxalic  acid  solution  consists  of  a  cold  saturated  solution  of  oxalic  acid 
one  part  and  water  three  parts. 

To  the  mixture  in  the  flask  add  grain  alcohol  and  boil  until  all  the 
chromic  acid  is  reduced  and  the  lead  converted  into  oxalate.  Then 
add  30  cc.  of  cold  water  and  cool  the  solution;  filter,  wash  out  the  flask, 
and  then  wash  the  precipitate  ten  times  with  cold  water. 

Place  5  cc.  of  strong  sulphuric  acid  in  a  flask,  dilute  with  hot  water 
to  about  125  cc.;  add  the  filter  and  precipitate,  and  titrate  with  standard 
permanganate  to  the  usual  pink  tinge. 

Neither  calcium  nor  antimony  will  interfere  with  this  method, 
but  the  presence  of  a  large  quantity  of  bismuth  will  materially  raise 
the  result.  Most  of  the  bismuth  is,  however,  removed  as  sulphate  and 
chromate.  This  method  is  a  good  technical  one,  though  an  error  of 
o.oi  to  0.05  may  be  expected. 

By  Precipitating  with  Excess  of  Potassium  Chromate  and 
Determining  the  Excess  lodometrically.  The  reactions  are: 

(a)     Pb(NO3)2+K2Cr04=PbCr04+2KNO3; 


The  potassium  chromate  is  added  in  excess,  the  mixture  heated, 
filtered,  the  precipitate  washed,  and  then  the  potassium  chromate 
estimated  in  the  filtrate  by  adding  potassium  iodid  in  excess  and 
titrating  for  free  iodin  with  sodium  thiosulphate  in  the  usual  way. 

Titration  with  Standard  Sulphate  Solution.  A  better  method 
is  the  converse  of  that  described  for  sulphates  (p.  333).  In  this  the 
lead  salt  in  solution  is  titrated  with  a  decinormal  solution  of  potassium 
sulphate  until  precipitation  is  complete  or  until  a  drop  of  the  solution 
ceases  to  produce  a  yellow  spot  upon  paper  impregnated  with  potas- 
sium iodid  and  sodium  thiosulphate. 

The  lead  should  be  in  the  form  of  nitrate.  If  any  free  nitric  acid 
is  present  this  should  be  neutralized  by  means  of  ammonia  water. 
Before  applying  a  drop  of  the  solution  to  the  test  paper  it  is  important 
to  allow  the  suspended  lead  sulphate  to  settle,  because  this  will  also 
react  with  the  potassium  iodid.  The  decinormal  factors  are  the  same 
as  those  given  above.  The  reaction  is  as  follows: 

Pb(N03)2+  K2S04=  PbS04+  2KNO3. 


LEAD  393 

N 
The   —  solution   of  potassium  sulphate   is   made   by  dissolving 

8.6535  gms.  of  pure  anhydrous  potassium  sulphate  in  sufficient  distilled 
water  to  make  one  liter. 

By  Precipitating  as  Oxalate.  Lead  may  also  be  estimated  by 
precipitating  as  oxalate,  and  then  titrating  the  oxalate  with  per- 
manganate, or  by  adding  an  excess  of  oxalic  acid,  and  then  retitrating 
with  permanganate  in  an  aliquot  portion  of  the  supernatant  liquid. 
The  lead  should  be  in  the  form  of  a  soluble  salt,  such  as  acetate 
or  nitrate. 

The  Distillation  Method.  Lead  in  the  form  of  peroxid  (PbO2) 
may  be  assayed  by  the  distillation  method  described  on  page  214. 

A  weighed  quantity  of  the  lead  peroxid  is  boiled  with  concentrated 
hydrochloric  acid,  and  the  chlorin  set  free  is  conveyed  into  a  solution 
of  potassium  iodid,  from  which  it  liberates  iodin.  The  iodin  is  then 
titrated  with  sodium  thiosulphate  in  the  usual  way. 


CHAPTER  XXXVI 

MAGNESIUM 

MOST  magnesium  compounds  may  be  converted  into  the  sulphate 
by  evaporating,  treating  with  concentrated  sulphuric  acid,  evaporating 
to  dryness  and  heating  to  dull  redness  to  drive  off  the  excess  of  acid. 
The  heat  must  not  be  raised  higher  than  dull  redness,  otherwise  some 
of  the  sulphate  is  apt  to  be  decomposed.  Dissolve  the  residue  in 
water,  add  a  few  drops  of  hydrochloric  acid,  and  determine  the  sul- 

N 

phuric  acid  by  means  of  —  barium  chlorid. 
10 

N 

Each  cc.  of  —  barium  chlorid =0.001209  Sm-  Mg; 
10 

=  0.0020       "    MgO; 
=  0.004186   "     MgCO3; 
=  0.005967   "    MgSC>4. 

Estimation  as  Phosphate  (Stolba).  Magnesium  salts  may  be 
precipitated  as  ammonio-magnesium  phosphate  and  the  precipitate 

N 

then  titrated  with  —  hydrochloric  acid,  or  with  uranium  solution  as 
10 

directed  under  estimation  of  phosphoric  acid. 

The  magnesium  in  the  form  of  a  soluble  salt  is  dissolved  in  a  small 
quantity  of  water,  at  least  twice  the  quantity  of  ammonium  chlorid  is 
added,  and  then  ammonia  water  to  make  strongly  alkaline.  Then 
sodium  phosphate  solution  is  added  in  excess,  and  the  mixture  allowed 
to  stand  twelve  hours.  The  magnesium  is  thus  completely  precipi- 
tated as  ammonio-magnesium  phosphate. 

This  precipitate  is  separated  by  nitration  and  washed,  first  with  a 
mixture  of  water,  three  parts,  and  ammonia  water,  one  part,  and  then 
with  50  or  60  per  cent  alcohol  to  remove  the  ammonia. 

N 

The  precipitate  is  then  dissolved  in  a  measured  excess  of  — r  hydro- 
chloric acid,  a  few  drops  of  methyl  orange  added,  and  the  excess  of 

N 
acid  found  by  retitrating  with  —  potassium  hydroxid.     The  difference 

394 


MAGNESIUM  395 

between  the  quantities  of  acid  and  alkali  solutions  used  is  the  quantity 
of  the  former  which  reacted  with  the  ammonio-magnesium  phosphate. 

N 
Each  cc.  of  —  hydrochloric  acid  =0.001209  gm.  Mg; 

=  0.0020       "     MgO. 

Or,  the  precipitate  of  ammonio-magnesium  phosphate  may  be 
dissolved  in  acetic  acid  and  estimated  by  uranium  solution,  as  directed 
under  Phosphoric  Acid. 

Each  cc.  of  uranium  solution  =0.001695  Mg; 

=  0.002-817  MgO. 

Stolba's  method  gives  fairly  accurate  results  in  the  assaying  of 
cements,  limestone,  and  with  'soluble  magnesium  salts  generally.  Its 
objectionable  feature  lies  in  the  difficulty  encountered  and  the  time 
consumed  in  the  removal  of  the  ammonia  by  alcohol. 

Meade's  Method  (J.  A.  C.  S.,  1899,  746)  has  the  great  advantage 
over  the  foregoing  in  that  it  can  be  worked  much  more  rapidly. 

It  consists  essentially  in  using  sodium  arsenate  instead  of  sodium 
phosphate.  The  precipitate  obtained  is  then  magnesium-ammonium 
arsenate  (Mg2(NH4)2As2Os+H2O)  instead  of  magnesium  -ammonium 
phosphate.  The  arsenic  in  the  precipitate  is  then  estimated  by  William- 
son's process  (J.  Soc.  Dyers  and  Colorists,  May,  1896),  which  depends 
upon  the  reaction  between  arsenic  oxid  and  potassium  iodid  in  the 
presence  of  hydrochloric  or  sulphuric  acid.  The  arsenic  is  reduced 
as  per  the  equation,  and  an  equivalent  of  iodin  is  set  free. 


For  every  molecule  of  arsenic  oxid  reduced,  corresponding  to  two 
atoms  of  magnesium,  four  atoms  of  iodin  are  liberated.  The  latter 
is  then  titrated  with  standard  sodium  thiosulphate. 

The  Standard  Sodium  Arsenate  solution  is  made  by  dissolving 
12.29  gms.  of  pure  arsenous  oxid  in  nitric  acid,  evaporating  to  dry- 
ness  on  a  water-bath,  neutralizing  with  sodium  carbonate,  and  making 
up  to  1000  cc.  with  water.  Each  cc.  of  this  solution  is  equivalent  to 
0.005  gm-  °f  Mg. 

The   Standard    Sodium    Thiosulphate    solution  is   made  to  corre- 

spond with  the  above  by  direct  titration,  or  made  to  correspond  with 

a  standard  iodin  solution  containing  52.24  gms.  of   pure  resublimed 

Jodin  and  75  gms.  of  potassium  iodid  in  a  liter.     Each  cc.  =  0.005  gm- 

Mg. 

The  Process.  Pour  the  magnesium  solution,  which  should  not 
contain  too  great  an  excess  of  ammonium  chlorid  or  oxalate,  into  a 
large  Erlenmeyer  flask  or  gas  -bottle  of  sufficient  size.  Add  one  third 
the  volume  of  the  solution  of  strong  ammonia  and  50  cc.  of  sodium 


396  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

arsenate.  Cork  up  tightly  and  shake  vigorously  for  ten  minutes. 
Allow  the  precipitate  to  settle  somewhat,  and  wash  with  a  mixture  of 
three  parts  of  water  and  one  part  of  strong  ammonia  until  the  washings 
cease  to  react  for  arsenic.  Avoid  using  an  excess  of  the  washing  fluid, 
however.  Dissolve  the  precipitate  in  dilute  hydrochloric  acid  (1:1), 
allowing  the  acid  solution  to  run  into  the  flask  in  which  the  precipita- 
tion was  made,  and  wash  the  filter-paper  with  the  dilute  acid  until  the 
washings  and  solution  measure  75  or  100  cc.  Cool,  if  not  already  so, 
and  add  from  3  to  5  gms.  of  potassium  iodid  free  from  iodate.  Allow 
the  solution  to  stand  a  few  minutes,  and  then  run  in  the  standard 
sodium  thiosulphate  until  the  color  of  the  liberated  iodin  fades  to 
a  pale  straw  color.  Add  starch  solution  and  titrate  until  the  blue  is 
discharged.  If  preferred,  an  excess  of  the  thiosulphate  may  be  added, 
then  starch  and  standard  iodin  until  the  blue  color  is  produced.  On 
adding  the  iodid  of  potassium  to  the  acid  solution,  a  brown  precipitate 
forms,  which,  however,  dissolves  when  the  thiosulphate  is  added. 

G.  B.  Frankforter  and  Lillian  Cohn  (J.  A.  C.  S.,  1907,  p.  1464) 
avoid  the  difficulty  usually  experienced  with  the  starch  indicator  in  the 
presence  of  strong  hydrochloric  acid  by  omitting  its  employment 
altogether.  They  assert  that  a  much  sharper  end-point  can  be  obtained 
if  the  starch  indicator  is  not  used.  They  employ  the  following  modifi- 
cation for  the  determination  of  magnesium  in  .water. 

Measure  out  500  cc.  of  the  water  to  be  examined,  precipitate  out 
the  iron  group  and  the  calcium  by  the  ordinary  methods.  Acidify  and 
evaporate  until  the  salts  start  to  crystallize  out.  Make  up  to  100  cc., 
transfer  to  an  Erlenmeyer  flask,  add  one  third  of  the  volume  of  con- 
centrated ammonia  and  25  cc.  of  a  10  per  cent  solution  of  sodium 
arsenate,  cork,  and  shake  vigorously  for  ten  minutes.  After  the  pre- 
cipitate has  settled,  filter  and  wash  with  dilute  ammonia  until  the 
washings  are  free  from  arsenic,  using  as  small  a  quantity  of  ammonia 
as  possible.  Now  add  10  cc.  of  sulphuric  acid  (1:4),  and  allow  it  to 
run  into  the  flask  in  which  the  precipitation  was  made.  Wash  the 
filter-paper  with  hot  water  until  the  total  solution  measures  about 
100  cc.  Now  add  10  cc.  of  sulphuric  acid  (i :  i\  cool,  and  add  3.5  gms. 
of  pure  potassium  iodid.  Allow  the  solution  to  stand  for  five  minutes 
and  titrate  with  thiosulphate  to  a  pale  straw  color.  Then  add  the  thio- 
sulphate cautiously,  drop  by  drop,  until  the  yellow  solution  becomes 
colorless. 

The  commercial  magnesium  carbonate  and  magnesium  oxid  may  be 

.examined  readily,  by  dissolving  a  weighed  quantity  (recently  ignited 

and  cooled)  in  a  measured  excess  of  normal  sulphuric  acid,  and  then 

retitrating  with  normal  alkali  in  the  presence  of  methyl  orange  as 

indicator. 


CHAPTER  XXXVII 

MANGANESE 

Estimation    of   the  Available   Oxygen   in  Manganese   Ores. 

The  oxids  of  manganese  are  as  follows:  MnO,  Mn3O4,  Mn2C>3,  MnO2j 
MnO3  (anhydrid  of  H2MnO4);  and  Mn2O7  (anhydrid  of  H2Mn2O8 
or  2HMnO4). 

With  the  exception  of  the  first  or  protoxid  all  the  others  will,  when 
distilled  with  hydrochloric  acid,  cause  a  liberation  of  chlorin.  The 
amount  of  chlorin  liberated  is  proportional  to  the  available  oxygen, 
i.e.,  the  excess  of  oxygen  over  that  necessary  to  form  protoxid.  This 
is  shown  in  the  following  equations: 


K2MnO4+8HCl=2KCl+MnCl2 
2KMnO4+i6HCl=2KCl+2MnCl2 

The  assay  may  be  carried  out  in  several  ways:  (a)  The  chlorin 
evolved  may  be  conducted  by  means  of  a  suitable  apparatus  into  a 
solution  of  potassium  iodid  from  which  an  equivalent  of  iodin  is  set 
free.  The  amount  of  the  latter  is  then  found  by  titration  with  sodium 
thiosulphate.  (See  page  218.) 

(b)  The  chlorin  may  be  allowed  to  react  with  a  known  weight  of 
ferrous  salt,  and  the  amount  of  iron  salt  which  at  the  completion  of 
the  reaction  is  unchanged  is  found  by  titration  with  standard  perman- 
ganate.    Thus  the  amount  of  manganese  oxid,  of  known  composition 
or  of  available  oxygen,  is  calculated  from  the  quantity  of  ferrous  salt 
oxidized. 

(c)  The  chlorin  may  be  passed  into  a  solution  of  sodium  carbonate 
and  this  solution  titrated  with  standard  arsenous  acid  solution,  or  the 

N 
chlorin  may  be  distilled  directly  into  a  measured  volume  of  —  arsenous 

N  I0 

acid,  and  the  latter  then  retitrated  with  —  iodin  solution.     (See  page 

397 


398  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

If  sulphuric  acid  is  used  instead  of  hydrochloric  acid,  the  available 
oxygen  of  the  manganese  oxid  will  be  evolved  instead  of  its  equivalent 
of  chlorin.  See  equations: 


MnO2  +  H2SO4=MnSO4-f-H2O  +  O. 

This  reaction  may  be  made  use  of  for  determining  the  available 
oxygen  of  manganese  ores  in  two  ways: 

First.  By  causing  the  evolved  oxygen  to  oxidize  a  ferrous  salt, 
and  from  the  quantity  of  ferrous  salt  so  oxidized,  the  amount  of  avail- 
able oxygen  is  obtained.  The  reactions  between  the  higher  oxids  of 
manganese  and  ferrous  salts  are  illustrated  by  the  following  equations: 


Mn2O3+2FeSO4+3H2SO4=2MnSO4+Fe2(SO4)3+3H2O; 


Second.  The  oxids  of  manganese  (with  the  exception  of  MnO), 
when  digested  with  oxalic  acid  in  the  presence  of  sulphuric  acid,  con- 
vert the  oxalic  acid  into  carbon  dioxid  and  water.  If  a  known  weight 
of  oxalic  acid  (in  excess)  be  taken,  the  amount  of  the  latter  remaining 
unchanged  after  the  reaction  is  found  by  titration  with  permanganate, 
and  the  difference  between  this  amount  and  that  originally  added  is 
the  measure  of  the  available  oxygen  in  the  manganese  ore.  The 
reactions  are: 


Mn2O3+H2C2O4+2H2SO4=2MnSO4+2CO2  +  3H2O; 
MnO2+H2C2O4+  H2SO4=MnSO4+2CO2+2H2O; 
K2MnO4+H2C2O4+2H2SO4=MnSO4+K2SO4+4CO2  +  4H2O. 

Example  of  First  Method.  By  Oxidation  of  Ferrous  Salt,  i  gin. 
of  soft  iron  wire  is  weighed  out  and  dissolved  in  about  40  cc.  of  dilute 
sulphuric  acid  (1:4)  in  the  apparatus,  shown  in  Fig.  55,  or  that  shown 
in  Fig.  56.  When  the  iron  is  completely  dissolved  0.8  gm.  of  finely 
powdered  manganese  ore  (previously  dried  at  100°  C.)  is  introduced 
into  the  flask  and  the  cork  with  its  tube  replaced.  The  mixture  is 
then  gently  warmed  until  the  ore  is  completely  dissolved.  The  solu- 


MANGANESE  399 

tion  is  then  cooled,  diluted  with  recently  boiled  water,  and  the  excess 
of  ferrous  sulphate  found  by  titration  with  permanganate.  The  excess 
of  unoxidized  iron  thus  found  deducted  from  the  weight  of  iron  taken 
gives  the  quantity  of  iron  which  reacted  with  and  was  oxidized  by  the 
manganese  dioxid,  and  from  this  the  quantity  of  the  latter,  or  of  avail- 

N 
able  oxygen,  can  easily  be  calculated.     Thus  14.2  cc.  of  —  perman- 

ganate (i  cc.  =  0.00555  gm-  Fe)  are  consumed. 

0.00555X14.2  =  0.07881  gm.  =  excess  of  iron. 

i  gm.  of  iron  (99.6  per  cent  of  Fe)  =  0.996  gm.,  the  quantity  of  iron 
originally  taken. 

Therefore  0.996—0.07881  =  0.91719  gm.  of  iron  oxidized  by  the 
0.8  gm.  of  manganese  oxid. 

Then  55.5:0.91719:  :43.i8:^;  #=0.712  +  gm.  of  MnO2  or  89  per 
cent. 

Example  of  Second  Method.  By  the  Oxidation  of  Oxalic  Acid 
(Mohr).  0.5  gm.  of  the  finely  pulverized  and  dried  manganese  ore  is 
weighed  into  a  flask  and  10  cc.  of  normal  oxalic  acid  solution  added. 
25  cc.  of  dilute  sulphuric  acid  (1:4)  are  then  added  and  the  mixture 
gently  heated  until  solution  is  complete.  The  solution  is  then  cooled 
and  made  up  to  100  cc.  with  cold  water,  and  the  excess  of  oxalic  acid 
found  by  titration  with  decinormal  potassium  permanganate. 

Assuming  that  6.1  cc.  of  the  permanganate  were  employed  in 
titrating  25  cc.  of  the  above  solution,  then  the  whole  will  require 
24.4  cc.  of  the  permanganate  solution.  The  10  cc.  of  normal  oxalic 
acid  originally  used  are  equivalent  to  100  cc.  of  decinormal  oxalic 
acid  and  the  same  amount  of  decinormal  permanganate.  Therefore  by 
deducting  24.4  cc.  from  100  cc.  we  obtain  the  quantity  of  decinormal 
acid  which  was  oxidized,  i.e.,  75.6  cc. 


Each  cc.  of  decinormal  oxalic  acid=o.oo43i8  gm.  of 

=  0.000794  "     of  available  O. 


Thus  75.6X0.004318  gm.  =  o.32644o8  gm.  of  MnO2; 

75.6X0.000794  gm.  =  0.0600264  gm.  of  O  (available). 

The  Distillation  Method,  described  on  page  214,  is  undoubtedly 
the  quickest  and  most  accurate  method  for  determining  the  quantity  of 
available  oxygen  present  in  the  ore,  which  to  manufacturers  of  bleach- 
ing powder,  is  a  matter  of  considerable  moment,  inasmuch  as  this 
method  directly  expresses  the  quantity  of  chlorin  which  is  evolved  by 


400  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

treating  the  ore  with  hydrochloric  acid.  It  furthermore  enables  them 
to  estimate  the  quantity  of  hydrochloric  acid  which  any  particular 
sample  of  ore  may  require  for  its  complete  decomposition.  (See 
Scherer  and  Rumpf,  Chem.  News,  xx,  302,  Am.  Reprint,  vi,  1870, 
page  82).  . 

With  regard  to  the  method  of  Mohr,  involving  the  oxidation  of 
oxalic  acid,  it  is  generally  conceded  that  this  method  has  advantages 
over  the  iron  method.  There  is  no  fear  of  false  results  occurring 
from  the  presence  of  air,  and  it  requires  only  one  weighing  for  each 
test. 

Furthermore,  the  results  are  very  uniform,  according  to  B.  H. 
Paul  (Chem.  News,  xxi,  16,  Am.  Reprint,  vi,  145  (1870)).  The 
method  has  also  the  advantage  of  giving  results  which  fairly  represent 
the  amount  of  available  oxygen  in  manganese  ores;  for  any  iron  that 
may.  be  present  as  metal  or  protoxid  will  consume  an  equivalent  quan- 
tity of  permanganate  solution,  and  thus  apparently  reduce  the  quantity 
of  oxalic  acid  decomposed  by  the  dioxid  to  an  extent  proportionate  to 
the  quantity  of  iron  existing  in  the  ore.  Thus,  for  instance,  if  the 
quantity  of  oxalic  acid  decomposed  by  100  grains  of  manganese  ore 
free  from  iron  or  protoxid  of  iron  were  109.53  grains,  the  ore  would 
contain  76.5  per  cent  dioxid,  and  the  whole  of  that  would  be  available. 
But,  if  the  100  grains  of  ore  also  contain  5.6  grains  of  metallic  iron, 
or  an  equivalent  of  protoxid,  the  permanganate  required  for  peroxidizing 
that  iron  would  represent  6.3  grains  of  oxalic  acid,  and  the  quantity 
of  oxalic  acid  decomposed  by  the  dioxid  would  appear  so  much  less 
than  it  really  was,  or  103.23  grains  instead  of  109.53  grains.  Accord- 
ingly, the  amount  of  dioxid  would  be  represented  as  72.1  per  cent 
instead  of  76.5  per  cent,  and  the  latter  would  in  fact  be  the  amount  of 
dioxid  available  for  generating  chlorin. 

Manganese  Dioxid  may  also  be  estimated  by  the  following 
method: 

0.2  gm.  of  the  substance  is  placed  in  a  25o-cc.  flask,  an  excess  of 
potassium  iodid  solution  added,  and  then  strongly  acidulated  with 
hydrochloric  acid.  The  flask  is  stoppered,  and  allowed  to  stand  until 
the  manganese  dioxid  is  completely  dissolved.  The  solution  is  then 
diluted  with  water  to  the  25o-cc.  mark,  and  25  cc.  of  it  taken  out  and 
titrated  with  centinormal  sodium  thiosulphate,  using  starch  solution 
as  the  indicator.  The  method  depends  upon  the  fact  that  MnO2,  when 
treated  with  HC1,  gives  off  two  atoms  of  chlorin  for  each  molecule  of 
MnO2,  and  in  the  presence  of  an  iodid  the  chlorin  liberates  an  equiva- 
lent of  iodin.  Hence  two  atoms  of  iodin  represent  one  molecule  of 
The  reaction  is  thus  expressed: 


MANGANESE  401 

N 
Thus  eachcc.  of  --  sodium  thiosulphate  represents  0.0012590  gm. 

of  iodin,  which  is  equivalent  to  0.004318  gm.  of  manganese  dioxid. 

Direct   Titration   with   Permanganate  (Guyard).*      A  dilute, 
neutral,  or  faintly  acid  solution  of  manganese  salt  is  heated  to  80°  C., 

N 
and  —  potassium  permanganate  solution  added,  so  long  as  a  brownish 

10 

red  precipitate  of  hydrated  MnC>2  forms  and  until  the  occurrence 
of  the  characteristic  rose  color  of  permanganate.  In  neutral  solu- 
tion the  reaction  is  exact,  but  a  large  excess  of  hydrochloric  or 
sulphuric  acid  causes  irregularity.  Iron  and  chromium  must  also  be 
absent.  The  manganese  compound  may  be  dissolved  in  nitro-muriatic 
acid,  boiling  if  necessary,  and  the  excess  of  acid  neutralized.  The 
reaction  is  written  thus: 


N 

i  cc.  of  —  permanganate  =0.001638  gm.  of  Mn. 
10 

Example.  One  or  two  grams  of  the  substance  to  be  examined  is 
dissolved  in  nitro-hydrochloric  acid  and  the  mixture  boiled  for  some 
time  in  order  to  convert  all  of  the  manganese  into  chlorid.  The 
solution  is  nearly  neutralized  with  sodium  hydroxid,  diluted  with  boiling 
water  to  i  or  2  liters,  and  maintained  at  a  temperature  of  80°  C., 
whilst  titrating  with  standard  potassium  permanganate. 

Volhard's  Permanganate  Method.  This  depends  upon  the  fact 
that  when  a  faintly  acid,  hot  solution  of  a  manganous  salt  is  treated 
with  potassium  permanganate,  a  precipitation  of  hydrated  manganese 
dioxid  results,  the  permanganate  solution  being  decolorized  so  long 
as  any  manganous  salt  is  present.  The  reaction  is  as  in  the  foregoing 
method,  and  takes  place  according  to  Volhard  only  in  the  presence 
of  other  non-oxidizable,  highly  basic  metals,  as  for  instance  zinc 
oxid  or  a  zinc  salt. 

The  end-reaction  is  most  distinctly  observed  when  working  in  a 
solution  slightly  acidified  with  sulphuric  acid,  together  with  a  few 
drops  of  nitric  acid.  Organic  matter  must  be  absent,  as  must  also 
quantities  of  chlorid  over  0.5  gm.  per  liter.  Large  quantities  of  iron, 
except  in  the  ferric  state,  must  be  likewise  absent.  Excessive  quan- 
tities of  iron,  if  present,  are  best  removed  by  the  addition  of  zinc  oxid, 
whereby  the  former  is  precipitated  as  hydroxid,  the  manganese  remain- 
ing in  solution. 

The  permanganate  solution  used  in  this  titration  is  standardized 

*  Chem.  News,  1863,  292. 


402  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

against  a  solution  of  manganic  sulphate.  The  zinc  oxid  or  zinc  sul- 
phate used  should  not  have  a  reducing  action  upon  the  permanganate. 
The  method  of  carrying  out  this  process  is  determined  by  the  presence 
or  absence  of  iron. 

//  iron  is  absent  (or  present  in  very  small  amounts  only),  the  man- 
ganese solution  in  the  form  of  manganese  sulphate  is  introduced  into 
a  flask,  together  with  10  cc.  of  a  zinc  sulphate  solution  (100  gms. 
crystallized  zinc  sulphate  to  a  liter),  and  the  mixture  diluted  so  that  it 
will  contain  not  more  than  0.25  gm.  of  manganese  in  100  cc.  This, 
if  neutral,  is  acidified  by  the  addition  of  a  few  drops  of  concentrated 
nitric  acid  and  heated  to  boiling.  The  heat  is  then  withdrawn  and 
the  solution  titrated  at  first  rapidly,  then  slowly  and  carefully,  with 
the  permanganate  until  the  characteristic  pinkish  tint  is  obtained. 
The  brownish  precipitate  of  manganese  dioxid  will  to  some  extent 
interfere  with  an  accurate  observation  of  the  end-point,  but  if  the 
flask  in  which  the  titration  is  performed  is  rotated  throughout  the 
process,  the  precipitate  settles  as  a  rule  quickly  enough  to  enable 
the  operator  to  distinguish  clearly  the  end  color. 

//  larger  quantities  of  iron  are  present,  as,  for  instance,  in  spiegel- 
eisen  ferro -manganese,  and  other  ores.  A  quantity  of  the  substance 
(=to  about  0.3  gm.  of  Mn)  is  dissolved  in  nitric  acid,  the  solution 
evaporated  to  dryness  in  a  porcelain  dish,  the  residue  heated  until 
the  nitrate  is  completely  decomposed,  thereby  destroying  any  organic 
matter  present.  The  residual  oxids  are  then  digested  with  hydro- 
chloric acid,  adding  a  little  strong  sulphuric  acid  and  evaporating  to 
dryness,  The  resulting  sulphate  is  then  slightly  acidified  writh  nitric 
acid,  dissolved  in  warm  water,  then  washed  into  a  liter  flask,  and 
nearly  neutralized  with  sodium  carbonate  or  hydroxid.  Sufficient 
pure  zinc  oxid,  made  into  a  cream,  is  now  added  to  precipitate  all  of 
the  iron.  The  liquid  above  the  precipitate  should  be  milky  white. 
The  flask  is  then  filled  to  the  mark  with  water,  shaken,  and  200  cc. 
filtered  off  into  a  boiling  flask,  acidified  with  two  drops  of  nitric  acid 
(sp.gr.  1.2),  boiled,  and  titrated  with  the  permanganate  whilst  still 
hot. 

The  permanganate  method  is  rapid  and  simple,  nevertheless  it  is 
very  generally  ignored,  because  of  the  fact  that  under  certain  condi- 
ditions  inconsistent  results  are  obtained.  The  sources  of  error  are: 
(a)  Incomplete  destruction  of  the  organic  matter  present ;  (b)  excessive 
addition  of  zinc  oxid  and  in  hot  solution;  (c)  standardizing  the  per- 
manganate with  iron  instead  of  with  manganese. 

By  Reduction  of  Potassium  Ferricyanid  (Lenssen).*  This 
method  is  based  upon  the  fact  that  when  a  manganous  salt  in  solu- 

*  Journ.  f.  prakt.  Chem.,  LXXX,  408. 


MANGANESE  403 

tion,  mixed  with  one  equivalent  of  a  ferric  salt  for  every  equivalent 
of  MnO,  is  acted  upon  at  a  boiling  temperature  by  excess  of  an  alka- 
line solution  of  potassium  ferricyanid;  all  of  the  manganese  is  pre- 
cipitated as  MnC>2  and  a  corresponding  quantity  of  potassium  ferro- 
cyanid  is  formed,  as  per  the  equation: 


The  ferrocyanid  formed  is  then  measured  by  means  of  standard 
potassium  permanganate,  as  explained  on  page  278,  and  the  amount 
of  manganese  calculated  from  this. 

According  to  the  above  equation,  one  atomic  weight  of  manganese 
gives  rise  to  two  molecular  weights  of  potassium  ferrocyanid.  The 
manganese  must  be  present  in  the  form  of  a  manganous  salt,  and  all 
other  reducing  agents  must  of  course  be  absent.  If  no  ferric  salt 
is  present  in  the  solution  the  precipitate  will  consist  of  MnO2  and 
varying  proportions  of  MnO,  and  the  results  will  hence  be  inaccurate. 

The  Process.  The  manganous  salt  is  dissolved  in  water  or  acid, 
and  enough  ferric  chlorid  solution  added  to  make  certain  of  having 
at  least  one  molecule  of  Fe2Cle  to  one  atom  of  Mn.  This  is  added 
gradually  to  a  boiling  strongly  alkaline  solution  of  potassium  ferri- 
cyanid contained  in  a  flask.  After  boiling  the  mixture  for  a  few 
minutes  the  brownish-black  precipitate  becomes  granular  and  less 
bulky.  It  is  then  allowed  to  cool  completely,  and  is  poured  into  a 
half-liter  flask,  diluted  to  the  mark  with  water,  shaken,  and  the  pre- 
cipitate allowed  to  settle  (or  it  may  be  filtered  before  diluting  and 
the  precipitate  washed  with  water,  and  the  filtrate  after  acidulation 
with  sulphuric  or  hydrochloric  acid  titrated  with  permanganate). 
An  aliquot  portion  of  the  clear  solution  is  removed  by  means  of  a 
pipette,  acidified  with  sulphuric  acid,  and  the  amount  of  ferrocyanid 
contained  in  it  determined  by  titration  with  standard  permanganate. 
A  small  error  is  introduced  here  from  neglecting  the  volume  of  the 
precipitate.  It  is  therefore  a  better  plan  to  filter,  wash  the  precipitate, 
and  titrate  an  aliquot  portion  of  the  mixed  filtrate  and  washings.  The 
filtration  should,  however,  be  done  only  when  the  solution  is  com- 
pletely cooled,  for  if  the  liquid  is  filtered  hot,  the  filter  will  have  a  re- 
ducing action,  and  too  high  results  will  be  obtained.  Fresenius  suggests 
certain  precautions  which  must  be  observed: 

(a)  The  solution  of  potassium  ferricyanid  and  potassium  hydroxid 
must  not  be  boiled  too  long,  as  this  causes  a  small  amount  of  ferri- 
cyanid to  be  reduced  to  ferrocyanid. 

(b}  The  potassium  hydroxid  used  must  be  quite  free  from  organic 
matter,  and  if  there  be  any  doubt  upon  this  point,  it  should  be  fused 
in  a  silver  dish  before  being  used. 


404  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

(c)  The  precipitate  of  MnC>2  must  be  thoroughly  washed. 

(d)  The  ferric  salt  should  not  be  in  large  excess,  otherwise  inac- 
curacies result. 

N 
i  cc.  of  —  permanganate =0.002 73    gm.  of  Mn; 

=  0.003524    ;<     "  MnO; 
=  0.004318   "     "    MnO2. 

The  Bismuthate  Method  (Blair).*  This  method,  as  originally 
proposed  by  Schneider,  depends  upon  oxidizing  a  manganous  salt  to 
permanganic  acid  by  means  of  bismuth  tetroxid  in  the  presence  of 
an  excess  of  nitric  acid.  The  permanganic  acid  formed  is  very  stable 
in  nitric  acid  of  sp.gr/  1.135,  when  the  solution  is  cold.  But  in  hot 
solutions  the  excess  of  bismuth  tetroxid  is  rapidly  decomposed  and 
nitric  then  acts  upon  and  decomposes  the  permanganic  acid. 

In  the  cold,  however,  the  excess  of  bismuth  salt  may  be  filtered 
off,  and  to  the  clear  liquid  an  excess  of  ferrous  salt  added  and  the 
amount  of  the  latter  necessary  to  oxidize  the  permanganic  acid,  deter- 
mined by  titration  with  potassium  permanganate. 

The  method  is  accurate,  except  in  the  presence  of  even  traces  of 
hydrochloric  acid.  Reddrop  and  Ramage  first  suggested  the  use 
of  sodium  bismuthate  instead  of  bismuth  tetroxid,  because  the  latter 
is  difficult  to  obtain  free  from  chlorids.  Sodium  bismuthate  may  be 
prepared  as  follows:  Heat  twenty  parts  of  caustic  soda  nearly  to  redness 
in  an  iron  or  nickel  crucible  and  add,  in  small  quantities  at  a  time, 
ten  parts  of  pure,  dried  basic  bismuth  nitrate.  Then  add  two  parts 
of  sodium  peroxid,  and  pour  the  brownish-yellow  fused  mass  on  an 
iron  plate  to  cool;  then  break  up  in  a  mortar,  extract  with  water, 
and  collect  on  an  asbestos  filter.  The  residue,  after  thorough  wash- 
ing by  decantation,  is  dried  in  a  water-oven,  then  broken  up,  and  passed 
through  a  fine  sieve. 

The  Method  for  Steels.  Dissolve  i  gm.  of  drillings  in  50  cc.  of  nitric 
acid  (sp.gr.  1.135)  in  an  Erlenmeyer  flask  of  2oo-cc.  capacity.  Cool, 
and  add  about  0.5  gm.  of  bismuthate.  Heat  for  a  few  minutes,  or 
until  the  pink  color  has  disappeared,  with  or  without  the  precipitation 
of  manganese  dioxid.  Add  sulphurous  acid,  ferrous  sulphate  solution 
or  sodium  thiosulphate  in  sufficient  amount  to  clear  the  solution  and 
heat  until  all  nitrous  oxid  has  been  driven  off.  Cool  to  about  15°  C, 
add  an  excess  of  bismuthate  and  agitate  for  a  few  minutes;  add  50  cc. 
of  water  containing  30  cc.  nitric  acid  to  the  liter,  and  filter  through 
an  asbestos  felt  on  a  platinum  cone  into  a  3oo-cc.  Erlenmeyer  flask, 
and  wash  with  50  cc.  to  100  cc.  of  the  same  acid.  The  arrangement 


*  J.  A.  C.  S.,  XXVI,  793. 


MANGANESE 


405 


shown  in  Fig.  78  has  proved  very  satisfactory.  Run  into  the  flask 
from  the  pipette,  shown  in  Fig.  79,  a  measured  volume  of  ferrous 
sulphate  solution  and  titrate  to  a  faint  pink  color  with  permanganate. 
The  number  of  cc.  of  the  permanganate  solution  obtained,  subtracted 
from  the  number  corresponding  to  the  volume  of  ferrous  sulphate 
used,  will  give  the  volume  of  permanganate  equivalent  to  the  manganese 
in  the  sample,  which,  multiplied  by  the  value  of  the  permanganate  in 
manganese,  gives  the  amount  of  manganese  in  the  steel. 

Pig  Iron.  Dissolve  i  gm.  in  25  cc.  of  nitric  acid  (sp.gr.  1.135)  m 
a  small  beaker,  and  as  soon  as  the 
action  has  ceased  filter  into  a  2oo-cc. 
Erlenmeyer  flask,  wash  with  30  cc. 
of  the  same  acid,  and  proceed  as 
for  steel. 

Iron  Ores  Containing  Less  than 
2  Per  Cent  Manganese.  Treat 
i  gm.  in  a  platinum  dish  with  4 
cc.  of  strong  sulphuric  acid,  10  cc. 


FIG.  78. 


FIG.  79. 


of  water  and  10  to  20  cc.  of  hydrofluoric  acid.  Evaporate  until  the 
sulphuric  acid  fumes  freely.  Cool,  and  dissolve  in  25  cc.  of  1.135 
nitric  acid.  If  no  appreciable  residue  remains,  transfer  to  a  2oo-cc. 
flask,  using  25  cc.  of  1.135  nitric  acid  to  rinse  the  dish  and  proceed 
as  usual.  If  there  is  an  appreciable  residue,  filter  on  a  small  filter 
into  a  beaker,  wash  with  water,  burn  the  filter  and  residue,  and  fuse 
with  a  small  amount  of  potassium  bisulphate;  dissolve  in  water  with 
the  addition  of  a  little  nitric  acid,  add  to  main  filtrate,  evaporate  nearly  to 
dryness,  take  up  in  1.135  nitric  acid  and  transfer  to  the  flask  as  before. 


406  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Manganese  Ores  and  Iron  Ores  High  in  Manganese.  Treat 
i  gm.  as  in  the  case  of  iron  ores,  using  a  little  sulphurous  acid  if  neces- 
sary. Transfer  the  solution  to  a  500-0:.  flask,  dilute  to  the  mark, 
mix  thoroughly,  and  measure  into  a  flask  from  a  pipette  such  a  volume 
of  the  solution  as  will  give  from  i  to  2  per  cent  of  manganese  and 
enough  strong  nitric  acid  (sp.gr.  1.4)  to  yield  a  mixture  of  1.135  acid  in 
a  volume  of  50  to  60  cc.  For  example,  in  a  50  per  cent  ore  use  10  cc. 
of  the  solution  and  add  30  cc.  of  water  anol  10  cc.  of  nitric  acid  (1.4). 
In  this  case  the  manganese  must  be  calculated  on  -5^  of  a  gram,  or 
20  mg.  of  ore.  When  the  ore  contains  a  much  smaller  amount  of 
manganese,  say  5  or  10  per  cent,  it  is  better  to  make  up  the  solution 
to  say  100  cc.  instead  of  500  cc. 

Reagents.  Nitric  Acid  (sp.gr.  1.135).  A  mixture  of  three  parts 
of  water  and  one  part  of  strong  nitric  acid  will  answer. 

Nitric  Acid  (3  per  cent).     30  cc.  of  strong  nitric  acid  to  the  liter. 

Permanganate  Solution  and  Ferrous  Sulphate  Solution,  i  gm. 
of  potassium  permanganate  to  the  liter  gives  a  solution  of  convenient 
strength,  and  12.4  gms.  of  ferrous  ammonium  sulphate  and  50  cc.  of 
sulphuric  acid  made  up  to  the  liter,  gives  a  solution  which  is  almost 
equal  to  the  permanganate  solution.  The  strength  of  the  ferrous 
sulphate  solution  changes  quite  rapidly,  while  the  permanganate 
remains  unaltered  for  months. 

By  using  a  constant  volume  of  the  ferrous  sulphate  solution  and 
testing  it  against  the  permanganate  solution  every  day  the  calculation 
of  the  result  is  very  simple.  It  is  necessary  that  the  conditions  should 
be  the  same  in  testing  the  ferrous  sulphate  solution  as  in  titrating  a 
solution  of  manganese.  The  following  method  of  procedure  may  be 
adopted:  Measure  into  a  2oo-cc.  flask  50  cc.  of  nitric  acid  (1.135); 
cool,  and  add  a  small  amount  of  bismuthate,  dilute  with  50  cc.  of 
3  per  cent  nitric  acid;  filter  into  a  300-0:.  flask,  and  wash  with  50  cc. 
of  3  per  cent  nitric  acid.  If  the  felt  is  well  coated  with  bismuthate,  it 
is  unnecessary  to  add  any  to  the  nitric  acid  in  the  flask,  as  filtration 
through  the  mass  of  bismuthate  on  the  felt  will  answer  the  purpose. 
Run  in  from  the  pipette  (Fig.  79)  25  cc.  of  ferrous  sulphate  solution 
and  titrate  with  the  permanganate  to  a  faint  pink.  This  gives  the 
value  in  permanganate  of  the  ferrous  sulphate  solution.  With  this 
method  of  procedure  any  number  of  determinations  may  be  made 
with  a  variation  of  less  than  o.i  cc. 

The  permanganate  solution  may  be  standardized  with  iron  in  the 
usual  way  and  calculating  its  value  in  manganese.  The  proportion 
is  55.5:54.6.  Or  it  may  be  standardized  by  titrating  a  steel  con- 
taining a  known  amount  of  manganese  and  getting  the  value  of  the 
solution  by  dividing  the  percentage  of  manganese  by  the  number  of 
cc.  of  the  permanganate  used.  Or,  thirdly,  by  making  a  solution  of 


MANGANESE  407 

pure  manganese  sulphate  and  evaporating  a  weighed  amount  of  it 
to  dryness,  heating  it  to  redness  and  weighing  as  manganese  sul- 
phate, which,  multiplied  by  0.36424,  gives  the  amount  of  manganese. 
5  gms.  of  "C.  P."  manganese  sulphate  dissolved  in  500  cc.  of  water 
and  filtered  will  give  a  solution  containing  about  0.0035  gm-  of 
manganese  to  the  gram  of  solution.  Weigh  i  to  3  gms.  of  the  solu- 
tion in  a  crucible,  transfer  to  a  2oo-cc.  flask,  using  50  cc.  of  nitric 
acid  (1.135);  co°l>  add  0.5  to  i  gm.  of  bismuthate,  and  allow  it  to 
stand  for  three  or  four  minutes,  shaking  at  intervals.  Add  50  cc. 
of  3  per  cent  nitric  acid,  filter  through  the  asbestos  filter  and  wash 
with  50  or  60  cc.  of  the  same  acid.  Run  25  cc.  of  the  ferrous  sul- 
phate solution  into  the  flask  from  the  pipette  and  titrate  with  the 
permanganate  solution  to  a  faint  pink. 

Subtract  the  number  of  cc.  of  the  permanganate  solution  obtained 
from  the  value  of  the  25  cc.  of  ferrous  sulphate  solution  in  perman- 
ganate, and  the  result  is  the  number  of  cc.  of  permanganate  corre- 
sponding to  the  manganese  in  the  manganese  sulphate  solution  used. 
Divide  the  weight  of  the  manganese  in  the  manganese  sulphate  used 
by  the  number  of  cc.  of  permanganate,  and  the  result  is  the  value  of 
0.006354  gm.,  or  the  sample  contains  0.635  Per  cent  °f  manganese. 

Example,  i  gm.  manganese  sulphate  solution  contains  0.003562  gm. 
manganese;  2.0372  gms.  manganese  sulphate  solution  equals  0.0072565 
gm.  manganese;  25  cc.  ferrous  sulphate  solution  equals  24.5  cc.  per- 
manganate solution;  2.0372  gms.  manganese  sulphate,  after  oxida- 
tion and  addition  of  25  cc.  of  ferrous  sulphate  solution,  require  3.6  cc. 
of  permanganate;  24.5  cc.— 3.6  cc.=  20.9  cc.;  0.0072565  divided  by 
20.90  =  .000347 2,  or  i  gm.  permanganate  equals  0.0003472  gm.  man- 
ganese. If  then  i  gm.  of  steel,  after  oxidation  and  addition  of  25  cc. 
of  ferrous  sulphate  solution,  requires  6.2  cc.  of  permanganate  to  give 
the  pink  color,  then  24.5— 6.2  cc.  =  18.3X0.0003472  =0.006354  gm. 
or  the  sample  contains  0.635  per  cent  manganese. 

When  the  proper  precautions  are  observed,  this  method  gives 
very  accurate  results,  especially  for  material  containing  up  to  two 
per  cent  of  manganese.  The  reaction  is  very  delicate. 


CHAPTER  XXXVIII 

MERCURY 

By  Precipitation  as  Mercurous  Chlorid.  The  metal  must  be 
present  in  the  form  of  mercurous  salt  only.  A  weighed  quantity  of 

N 

the  solution  is  treated  with  a  measured  excess  of  —  sodium  chlorid, 

10 

which  precipitates  all  the  mercury  as  mercurous  chlorid.     The  mix- 
ture is  filtered,  the  precipitate  well  washed,  and  the  mixed  filtrates 

N 

titrated  with  —  silver  nitrate  solution,  using  potassium  chromate  as 
10 

indicator.     The  solution  must  be  neutral. 

The  quantity  of  silver  solution  used  is  deducted  from  the  quan- 
tity of  sodium  chlorid  solution  added,  and  the  difference  is  tlje  quan- 
tity of  the  latter  which  reacted  with  the  mercurous  salt. 

N 

i  cc.  of  —  sodium  chlorid  =  0.026007  gm.  HgNOs, 
10 

=  0.01985     "    Hg. 
Estimation  of  Mercurous  Chlorid  by  lodin  and  Thiosulphate 

(Hempel).     The   mercurous   chlorid  is  treated  with  potassium  iodid 

N 

and  —  iodin  solution  until  it  is  completelv  dissolved.     The  reaction 
10 

is  as  follows: 


Starch  solution  is  then  added,  which  gives  a  blue  color  with  the 

N 
excess  of  iodin.     The  mixture  is  then  titrated  with  —  thiosulphate 

N  .     I0 
until  the  blue  color  disappears,  and  lastly  with  —  iodin  until  the  blue 

color  returns.  Subtract  the  cc.  of  thiosulphate  from  the  total  quan- 
tity of  iodin  added,  and  the  remaining  cc.  will  be  the  measure  of  the 
mercury. 

N 
Each  cc.  —  iodin  =  0.01  985    gm.  Hg; 

=  0.023368  "     Hg2Cl2 

408 


MERCURY  409 

In  the  analysis,  to  i  gm.  of  calomel  take  about  2.5  gms.  of  potas- 

N  . 

sium  iodid  and  100  cc.  of  —  lodin. 
10 

Mercurous  Salts  other  than  the  chlorid  may  be  converted  into 
the  chlorid  by  precipitation  with  sodium  chlorid,  the  precipitate  well 
washed,  and  treated  as  directed  for  mercurous  chlorid. 

Mercuric  Salts  may  be  converted  into  mercurous  chlorid  by  add- 
ing sodium  chlorid,  an  excess  of  ferrous  sulphate,  and  sodium  hy- 
droxid  to  alkaline  reaction.  The  mixture  is  allowed  to  stand  for  a 
short  time,  shaking  frequently.  Then  hydrochloric  acid  is  added 
until  the  solution  becomes  clear,  and  the  mercurous  chlorid  white, 
and  free  from  iron.  The  precipitate  is  then  separated,  washed,  and 
treated  as  directed  under  Mercurous  Chlorid.  The  reaction  is  as 
follows  : 


Estimation    of    Mercuric    Salts    by    Precipitation    as    Iodid 

(Personnel).*  This  method  is  described  under  the  estimation  of 
iodids  (page  134). 

The  mercuric  solution  must  be  added  to  the  potassium  iodid 
solution;  a  reversal  of  the  process  is  not  reliable. 

The  mercuric  salt  must  be  in  the  form  of  a  neutral  solution  of 
mercuric  chlorid,  and  it  should  be  considerably  diluted,  say  to  300 
or  500  cc.,  and  as  a  preliminary  trial  20  cc.  of  decinormal  potassium 
iodid  solution  should  be  taken,  and  titrated  with  the  mercuric  solu- 
tion. Two  or  more  titrations  should  be  made,  the  first  will  give  the 
approximate  figure. 

This  method  depends  upon  the  fact  that  when  a  neutral  solution 
of  mercuric  chlorid  is  added  to  a  solution  of  potassium  iodid,  a  color- 
less solution  is  formed,  until  the  proportion  of  the  mercuric  chlorid 
is  to  that  of  potassium  iodid  as  i  to  4.  As  soon  as  this  proportion  is 
exceeded,  and  more  mercuric  chlorid  added,  the  solution  is  colored 
red  through  precipitation  of  mercuric  iodid,  which  marks  the  end- 
reaction. 

The  reagents  required  are: 

Decinormal  Potassium  Iodid,  made  by  dissolving  32.952  gms.  of 
the  pure  salt  and  diluting  to  a  liter; 

Decinormal  Mercuric  Chlorid,  made  by  dissolving  13.443  gms.  of 
pure  mercuric  chlorid,  together  with  about  30  gms.  of  sodium  chlorid 
(to  assist  solution)  in  water  to  make  one  liter. 

*  Compt.  rend.,  LVI,  63. 


410  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  mercuric  chlorid  to  be  estimated,  which  must  be  neutral,  is 
dissolved  in  water  and  added  to  an  excess  of  the  decinormal  potassium 
iodid.  The  excess  of  potassium  iodid  is  then  formed  by  titrating 
from  a  burette,  with  decinormal  mercuric  chlorid,  until  the  red  color 
makes  its  appearance. 

The  reaction  is  expressed  by  the  folio  wing  equation: 

4KI+HgCl2=2KCl+HgI2 .  KI. 

N 
Each  cc.  of  -—  potassium  iodid  represents  0.009975  gm.  of  Hg; 

0.013443  "     "  HgCl2. 

Estimation  of  Mercuric  Salts  by  Oxidation  of  Ferrous  Salts 
(Mohr).  The  mercury  in  the  form  of  mercuric  chlorid  is  dissolved  in 
water  in  a  flask,  a  weighed  amount  of  ferrous  ammonium  sulphate 
dissolved  in  water  added,  and  then  sodium  hydroxid,  until  the 
ferrous  oxid  is  precipitated  and  the  solution  is  decidedly  alkaline. 
The  mixture  is  then  allowed  to  stand  for  a  few  minutes,  shaking  fre- 
quently. The  mercuric  chlorid  is  thus  reduced  to  mercurous  chlorid, 
and  the  ferrous  salt  to  ferric  hydroxid.  The  mixture  is  muddy  and 
dark  in  color  because  of  the  presence  of  ferric  and  ferrous  hydroxid 
and  mercurous  chlorid. 

The  reaction  may  be  expressed  by  the  following  equation: 

HgCl2  +  FeCl2 = HgCl  +  FeCl3. 

Sufficient  hydrochloric  or  sulphuric  acid  is  now  added  to  com- 
pletely dissolve  the  iron  hydroxids,  leaving  the  mercurous  chlorid 
perfectly  white.  The  solution  is  then  diluted  to  the  mark,  shaken, 
filtered  through  a  dry  filter,  and  a  portion  measured  out  and  titrated 
for  excess  of  ferrous  salt,  using  either  dichromate  or  permanganate. 
If  permanganate  is  to  be  used,  it  is  advisable  to  use  sulphuric  instead 
of  hydrochloric  acid  for  dissolving  the  precipitated  iron  hydroxids. 

Example,  i  gm.  of  mercuric  chlorid  in  solution  was  treated  with 
3  gms.  of  the  double  iron  salt,  and  the  solution  then  made  freely  alka- 
line with  sodium  hydroxid.  The  precipitated  iron  hydroxids  are  dis- 
solved, through  the  addition  of  sulphuric  acid,  and  the  solution  diluted 

N 
to  300  cc.,  filtered,  and  100  cc.  titrated  with  —  permanganate.     13.4  cc. 

were  required.  Therefore  the  entire  quantity  would  require  three 
times  13.4  cc.  =  40.2  cc.,  which,  deducted  from  77  cc.  (the  quantity 
required  for  3  gms.  of  double  iron  salt),  left  36.8  cc.  which  =  1.43152  gms. 
of  undecomposed  iron  salt,  which,  multiplied  by  the  factor  0.6905  gm. 


MERCURY  411 

=  0.9884+  gm.  of  pure  mercuric  chlorid;  or  the  36.8  cc.  may  be  multi- 

N 

plied  by  the  —  factor  for  mercuric  chlorid. 
10 

Mercurous  Salts  may  be  converted  into  mercuric  chlorid  by  pre- 
cipitating them  as  mercurous  chlorid  by  means  of  sodium  chlorid, 
washing  the  precipitate,  adding  sodium  hydroxid,  and  passing  chlorin 
through  the  solution.  The  solution  is  then  acidified  with  hydrochloric 
acid,  and  evaporated  on  a  water-bath  to  expel  the  excess  of 
chlorin. 

Mercuric  Salts  may  be  converted  into  mercuric  chlorid  by  evapo- 
rating with  hydrochloric  acid,  being  careful,  however,  not  to  boil  the 
solution  or  some  of  the  chlorid  will  be  lost. 

By  Potassium  Cyanid  (Hannay).*  This  method  depends  upon 
the  fact  that  ammonia  produces  in  mercurial  solutions  a  precipitate 
or  an  opalescence  (according  to  the  degree  of  concentration),  which 
is  removed  by  a  definite  amount  of  potassium  cyanid.  To  the  mer- 
cury in  the  form  of  mercuric  chlorid,  ammonia  is  added  until  a  con- 
siderable amount  of  a  white  precipitate  has  formed  (an  excess  of 
ammonia  does  no  harm).  A  solution  of  potassium  cyanid  is  then 
added  until  the  precipitate  redissolves  and  leaves  the  liquid  perfectly 
clear.  The  strength  of  the  standard  potassium  cyanid  is  best  deter- 
mined by  titration  in  the  same  manner  against  a  weighed  amount  of 
pure  mercuric  chlorid.  Since  it  decomposes  slowly  by  keeping,  the 
strength  must  be  redetermined  before  each  series  of  analyses.  The 
temperature  at  which  the  titration  is  conducted  should  not  be  below 
8°  nor  above  20°  C. 

This  method  is  not  interfered  with  by  the  presence  of  organic 
matter,  and  in  its  modifications  by  Tuson  and  Neison  (J.  Chem.  S., 
1877,  679),  Chapman  Jones  (same  journal,  LXI,  364),  and  Sutton 
("Manual  of  Volumetric  Analysis"),  it  is  useful  for  a  great  variety 
of  salts. 

See  Estimation  of  Cyanogen,  page  275. 

By  Direct  Titration  with  Sodium  Thiosulphate  (Scherer).f 
This  method  depends  upon  the  fact  that  when  sodium  thiosulphate 
is  added  to  a  mercurial  solution,  a  precipitate  is  produced  so  long 
as  any  mercury  remains  in  solution.  The  standard  sodium  thio- 
sulphate is  added  until  the  last  drop  fails  to  produce  a  precipitate. 
The  precipitate  settles  rapidly  and  the  proper  end-point  is  easily 
found. 

The  thiosulphate  used  is  of  twentieth-normal  strength.  It  is  made 
by  dissolving  12.323  gms.  of  the  pure  crystallized  salt  in  sufficient 


*  J.  Chem.  S.,  1873,  565.  f  Lehrbuch  der  chemie,  I,  513. 


412  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

water  to  make  one  liter.     The  reaction  which  takes  place  with  this 
solution  and  mercurous  nitrate  is 


The  Estimation  of  Mercurous  Nitrate.  The  solution,  which 
must  be  free  from  mercuric  salt,  is  diluted,  heated,  and  the  standard 
thiosulphate  run  in  from  a  burette  until  no  further  precipitate  is  pro- 
duced. The  sulphid  settles  rapidly  and  the  end-point  is  easily  found. 

Each  cc.  of  the  thiosulphate  =0.0  1985    gm.  Hg; 

=  0.026007  "    Hg2(NO3)2; 
=  0.020644  "     Hg2O. 

Estimation  of  Mercuric  Nitrate.  Sodium  thiosulphate  pre- 
cipitates the  mercury  from  mercuric  nitrate,  according  to  the  follow- 
ing reaction  : 

3Hg(NO3)2   2Na2S2O3+2H2O=2HgS  .  Hg(NO3)2+2Na2SO4+4HNO3. 

The  mercurial  solution  is  highly  diluted,  put  into  a  stoppered 
flask,  a  little  nitric  acid  added,  and  then  the  thiosulphate  solution 
delivered  from  a  burette,  the  flask  being  vigorously  shaken  meanwhile 
until  the  last  drop  produces  no  further  yellow  precipitate.  In  order 
to  better  distinguish  the  end-reaction  Scherer  recommends  that  the 
solution  be  diluted  to  a  definite  volume,  when  the  greater  part  of  the 
metal  is  precipitated,  and  a  measured  volume  of  the  clear  fluid  taken 
out  with  a  pipette,  and  the  titration  finished  upon  this  portion.  A 
second  like  quantity  may  then  be  taken  out  and  titrated  in  the  .same 
way  to  check  the  analysis. 

Each  cc.  of  the  thiosulphate  =  0.0  1488  gm.  Hg, 

=  0.1607   "     HgO. 

Estimation  of  Mercuric  Chlorid.  The  reaction  in  this  case  is 
as  follows: 

3HgCl2+2Na2S2O3+2H2O=2HgS  .  HgCl2+2Na2SO4+4HCI. 

The  solution  is  acidified  with  hydrochloric  acid,  considerably 
diluted,  and  heated  nearly  to  boiling;  and  then  the  thiosulphate 
carefully  added  as  long  as  a  white  precipitate  is  formed.  Care  must 
be  taken  not  to  add  an  excess,  otherwise  a  dirty  looking  gray,  or  even 
black,  color  will  be  produced.  The  end-reaction  in  this  analysis  is 


MERCURY  413 

not  nearly  so  easily  seen  as  in  the  foregoing,  and  filtration  by  means 
of  a  Beale's  filter  is  necessary  in  order  to  distinguish  the  exact  end- 
point. 

Each  cc.  of  the  thiosulphate=  0.01488  gm.  Hg, 

=  0.10607  "     HgO- 

This  method  gives  good  results  with  pure  mercurous  nitrate.  But 
it  is  practically  inaccurate,  if  any  mercuric  nitrate  is  present,  and  this 
is  usually  the  case. 

Mercuric  Chlorid  may  also  be  estimated  by  reduction  with  stan- 
nous  chlorid,  as  described  on  page  233. 

Estimation  of  Mercuric  Chlorid  in  Colored  Tablets  or  Solu- 
tions.* Aniline  colors,  now  frequently  used  in  solutions  and  tablets 
of  mercuric  chlorid  for  antiseptic  use,  interfere  with  the  volumetric 
estimation  of  the  mercury  contained  in  them.  This  may  be  overcome 
by  precipitating  the  mercury  with  metallic  iron,  oxidizing  the  ferrous 
chlorid  formed  with  potassium  permanganate,  which  destroys  the 
dye,  and  then  estimating  the  ferric  chlorid  with  potassium  iodid  and 
decinormal  sodium  thiosulphate.  From  20  cc.  of  a  four  per  cent  solu- 
tion of  mercuric  chlorid  all  the  mercury  is  precipitated  when  a  few 
grams  of  reduced  iron  are  added  and  the  mixture  agitated  frequently 
during  one  hour.  The  reactions  are: 


(a)  HgCl2  +  Fe=FeCl2 

(b)  ioFeCl2+2KMn04+i6HCl=2KCl+2MnCl2+5Fe2Cl6+8H2O; 
(0  Fe2Cl6-f2KI=FeCl2+l2+2KCl; 

(d) 


The  Process,  i  gm.  of  the  mercuric  chlorid  tablets  is  dissolved 
in  50  cc.  of  water.  25  cc.  of  this  solution  containing  0.5  gm.  of  sub- 
stance is  agitated  frequently  during  one  hour  with  a  few  grams  of 
reduced  iron.  20  cc.  of  the  solution  are  then  filtered  into  a  flask,  5  cc. 
of  dilute  sulphuric  acid  and  5  cc.  of  a  10  per  cent  manganese  sulphate 
solution  are  added.  The  addition  of  the  latter  prevents  the  liberation 
of  chlorin,  and  thus  obviates  the  necessity  for  heating  to  drive  it  off. 
A  sufficient  quantity  of  a  one  per  cent  solution  of  potassium  perman- 
ganate is  now  added  to  produce  a  permanent  red-colored  solution, 
and  then  a  few  drops  of  alcohol  or  tartaric  acid  solution  are  added 
to  decompose  the  excess  of  potassium  permanganate.  This  accom- 
plished, i  to  2  gms.  of  potassium  iodid  are  added,  the  solution  digested 
for  half  an  hour,  and  then  titrated  with  sodium  thiosulphate  in  the 
presence  of  starch. 

*  E.  Rupp,  Arch.  d.  Pharm.,  238,  98. 


414  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

Further  work  on  this  subject  by  E.  Rupp  (Arch.  d.  Pharm.,  1905, 
300)  resulted  in  the  following  method : 

Several  cubic  centimeters  of  a  35  per  cent  formaldehyde  solution 
are  made  alkaline  by  the  addition  of  some  weak  sodium  hydroxid 
solution,  and  the  mercurial  solution  added  whilst  agitating  the  flask. 
This  mixture  is  warmed  for  ten  or  fifteen  minutes  on  a  water-bath, 
and  then  after  cooling,  considerably  acidified  with  acetic  acid.  A  suffi- 

N 

cient  excess  of  -  -  iodin  is  then  added,  the   flask  securely  stoppered 
10 

and  shaken  for  five  minutes;    and  when  the  precipitated  mercuric 

N 

potassium  iodid  is  dissolved,  the  solution  is  titrated  with  —  thiosul- 

10 

phate.  using  starch  as  indicator. 

N 

Each  cc.  of  —  iodin  solution  =  0.01002  gm.  Hg, 
10 

=  0.01355   "     HgCl2. 

The  Process.  Five  sublimate  tablets  are  dissolved  in  water  to 
make  500  cc.  of  solution.  Of  this  20  cc.  are  taken,  3  cc.  of  formal- 
dehyde solution  are  added,  followed  by  10  cc.  of  caustic  soda  solution 
and  20  cc.  of  water.  This  mixture  is  warmed,  as  described  above, 
and  allowed  to  cool,  when  30  cc.  of  30  per  cent  acetic  acid  are  added, 

N 
followed  by  25  cc.  of  —   iodin  solution,  and  the  mixture  titrated  with 

N  ,  I0 

—  thiosulphate.     The   quantity  of  the  latter  used,   deducted  from 

10  N 

25  cc.,  gives  the  quantity  of  —  iodin  which  represents  the  mercuric 

chlorid. 

The  Personne  Method,  described  on  page  409,  may  be  employed 
with  good  results  for  the  estimation  of  mercuric  chlorid  in  sublimate 
tablets.  The  presence  of  anilin  colors,  or  sodium  chlorid,  does  not 
in  the  least  interfere. 

Titration  by  Means  of  Potassium  Bichromate.  This  method, 
which  is  recommended  by  F.  M.  Litterscheid,*  is  based  upon  the 
observation  that  when  a  solution  of  mercuric  chlorid  is  treated  with 
an  excess  of  solution  of  potassium  dichromate  in  the  cold,  and  then 
with  ammonia  to  distinctly  neutral  reaction,  the  mercury  is  completely 
precipitated,  forming  a  lemon-yellow  precipitate,  separating  rapidly 
after  vigorous  shaking  and  soon  assuming  a  granular  character.  After 
twenty  minutes  standing,  the  filtrate  gives  no  reaction  for  mercury 
with  either  sulphureted  hydrogen  or  stannous  chlorid.  The  resulting 


*  Arch.  d.  Pharm.,  1903,  306-313. 


MERCURY   IN   ITS  ORGANIC  COMPOUNDS  415 

compound,  dimer  cur -ammonium  chromate,  is  practically  insoluble  in 
water. 

The  dichromate  solution  used  is  the  same  as  that  used  for  stand- 

N 
ardizing  —  sodium  thiosulphate  (4.8713  gms.  in  1000  cc).     See  page 

179.     The  method  is  carried  out  as  follows: 

Mercuric  chlorid  solution  (i  140)  is  mixed  in  a  loo-cc.  flask  with  an 

N 

excess  of  —  potassium  dichromate.     Ten  per  cent  ammonia-water  is 
10 

then  added,  drop  by  drop,  with  rotation  of  the  flask  until  the  liquid 
is  distinctly  alkaline.  The  mixture  is  then  shaken  frequently  during 
ten  minutes  and  diluted  to  100  cc. ;  again  shaken  and  allowed  to  stand 
for  six  hours.  An  aliquot  part  is  then  filtered  off  (about  one  half, 
rejecting  the  first  portion  of  the  filtrate)  and  in  this  determining  the 
excess  of  dichromate.  This  is  done  after  acidulation  with  sulphuric  acid 
(i :  5),  and  the  addition  of  an  excess  of  potassium  iodid,  by  titrating  with 

N 

—  sodium  thiosulphate.   The  quantity  of  the  latter  used,  deducted  from 

10 

N 

the  quantity  of  —  dichromate  solution  added,  gives  the  quantity  of 
10 

the  latter  which  reacted  with  and  hence  represents  the  mercuric  chlorid. 
The  potassium  iodid  used  must  -of  course  be  free  from  iodate. 

Each  cc.  of  the  dichromate  solution =0.2 76  gm.  Hg. 

Estimation  of  Mercury  in  its  Organic  Compounds.  E.  Rupp 
and  Th.  Noll  *  recommend  the  following  procedure  for  hydrargyrum 
salicylicum:  0.3  gm.  of  the  salicylate  is  heated  with  4  gms.  of  potas- 
sium sulphate  and  5  cc.  of  concentrated  sulphuric  acid  in  a  i5o-cc. 
flask,  resting  on  wire  gauze  and  provided  with  an  oblique  reflux  tube 
about  50  cm.  long.  As  soon  as  the  mixture  is  clear  and  colorless, 
the  reflux  tube  is  rinsed  with  5  to  10  cc.  of  sulphuric  acid,  and  o.i  to 
0.2  gm.  of  potassium  permanganate  added.  The  heating  is  renewed 
until  the  solution  is  colorless.  The  whole  is  now  diluted  to  100  cc. 
and  titrated  with  decinormal  sulphocyanate,  using  iron  alum  as  indi- 
cator, until  a  reddish-brown  color  appears. 

Each  cc.  of  the  decinormal  solution  =  0.0 10015  gm-  °f  Hg. 

Analysis  of  Mercuric  Cyanid.j  One  gram  is  dissolved  in  water 
to  make  100  cc.  of  solution.  In  this  a  determination  of  mercury  and 
of  cyanogen  may  be  made  as  follows: 

Determination  of  Mercury.  20  cc.  of  the  solution  is  introduced 
into  a  glass -stoppered  bottle,  1-2  gms.  of  potassium  iodid  and  3-5  cc. 


*  Arch.  d.  Pharm.,  243,  No.  I  (Feb.  27,  1905),  1-5. 
t  E.  Rupp  and  F.  Lehmann,  Pharm.  Ztg.,  52,  1020. 


416  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

of  a  15  per  cent  potassium  hydroxid  solution.  To  this  is  added  a 
mixture  of  2-3  cc.  of  formaldehyde  solution  (35  per  cent)  and  20  cc. 
of  water,  and  the  solution  shaken  for  two  minutes,  or  until  complete 
reduction  ensues.  Sufficient  acetic  acid  is  then  added  to  acidify  the 

N 

solution,  and  this  is  followed  by  25  cc.  of  —  iodin  and  shaken  until 

10 

the  mercury  is  entirely  dissolved.  Then  there  are  added  10  cc.  of 
diluted  sulphuric  acid,  and  the  excess  of  iodin  is  found  by  titration  with 

N 

—  sodium  thiosulpnate. 

10  N 

Each  cc.  of  —  iodin  =  0.01  2509  gm.  Hg(CN)2. 
10 

Determination    of  Cyanid   Radicle.     Place  into  a  glass  -stoppered 
bottle  10  cc.  of  the  mercuric  cyanid  solution  ;   add  a  little  water  and 

N 

5-10  cc.  of  potassium  hydroxid  solution  (15  per  cent),  and  25  cc.  of  — 

10 

iodin,  mix,  warm  thirty  minutes  on  a  water-bath,  then  dilute  with 
water  to  make  100  cc.  Acidify  with  about  25  cc.  of  diluted  hydro- 

N 

chloric  acid,  and  after  two  minutes,  titrate  the  excess  of  iodin  with  - 

10 

sodium  thiosulphate. 

N  . 

Each  cc.  of  —  iodin  =  0.0062  545  gm. 
10 

The  following  equations  illustrate  the  reactions: 


(i) 

(2)  Hg(CN)24-2KIO  +  2KI=K2HgI4+2KCNO; 

(3)  KIO  +  KI  +  2HCl=2l  +  2KCl+H2O  .  Hg(CN)2. 


Assay  of  Ammoniated  Mercury.*  Triturate  0.3  gm.  of  the 
substance,  and  rinse  into  a  stoppered  bottle  with  50  cc.  of  water  and 
add  3  gms.  of  potassium  iodid.  Shake  until  solution  is  effected; 
add  two  drops  of  a  0.2  per  cent  alcoholic  solution  of  methyl  orange, 

N 

and  titrate  with  —  hydrochloric  acid.     The  reaction  is  as  follows: 
10 


N 

Each  cc.  of  —  hydrochloric  acid=  0.01248  gm.  of  HgNH2Cl. 
10 


*  Rupp  and  Lehmann,  Pharm.  Ztg.,  52,  1014. 


CHAPTER  XXXIX 

SILVER 

Estimation  by  Precipitation  as  Chlorid.  Silver  in  solution 
(slightly  acidified  with  nitric  acid)  when  treated  with  sodium  chlorid 
is  completely  precipitated  as  silver  chlorid.  See  page  130. 

The  end-reaction  may  be  determined  in  three  ways: 

First.  By  shaking  the  solution  violently  after  each  addition  of 
the  sodium  chlorid  solution,  and  allowing  the  silver  chlorid  to  sub- 
side before  adding  more  of  the  reagent.  The  end-point  is  then  reached 
when  the  further  addition  of  sodium  chlorid  fails  to  produce  a  pre- 
cipitate in  the  clear  supernatant  liquid. 

Second.  By  filtering  a  small  portion  by  means  of  a  Beale's  filter 
and  testing  the  clear  liquid. 

Third.  By  the  use  of  neutral  potassium  chromate  as  indicator. 
In  this  case  it  is  always  better  to  add  the  standard  solution  of  sodium 
chlorid  (usually  decinormal)  in  slight  excess;  then  add  a  few  drops 
of  the  indicator  and  titrate  the  excess  of  sodium  chlorid  solution  by 
means  of  decinormal  silver  nitrate.  The  end-point  is  known  by  the 
solution  becoming  red,  through  formation  of  silver  chromate.  Accu- 
racy cannot  so  easily  be  obtained  if  the  chromate  is  added  to  a  silver 
solution,  and  then  sodium  chlorid  until  the  red  color  changes  to  white: 

First.  Because  the  precipitated  silver  chromate,  if  allowed  to 
stand  any  length  of  time,  is  not  easily  decomposed  by  the  sodium 
chlorid  solution  and  the  operator  is  apt  to  overstep  the  proper  point. 

Second.  Because  the  change  from  red  to  white  is  less  easy  to 
observe  than  that  from  white  to  red. 

These  methods  and  the  preparation  of  the  standard  solutions  are 
fully  described  in  the  chapter  on  Precipitation  Analyses. 

The  Sulphocyanate  Method  is  described  on  page  131. 

The  Starch  lodid  Method  (Pisani).  This  method  is  especially 
applicable  to  the  analysis  of  ores  containing  lead  and  copper,  but 
not  mercury,  tin,  iron,  manganese,  antimony,  arsenic,  or  gold. 

The  method  is  based  upon  the  fact  that  when  starch  iodid  is  added 
to  a  neutral  solution  of  a  silver  salt,  the  silver  is  precipitated  as  iodid, 
and  the  blue  color  of  the  starch  iodid  is  destroyed  as  long  as  any  silver 
remains  in  solution.  When,  however,  the  silver  is  entirely  precipi- 
tated, the  starch  iodid  colors  the  solution  blue. 

4i7 


418  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

Preparation  of  the  Starch  lodid  Solution.  Weigh  out  2  gms.  of 
iodin  into  a  mortar,  add  15  gms.  of  starch,  and  rub  it  up  well  with 
6  or  8  drops  of  water.  This  mass  is  transferred  to  a  well-stoppered 
flask  and  heated  on  a  water-bath  for  about  one  hour  or  until  it  has 
assumed  a  dark  bluish-gray  color.  Sufficient  water  is  then  added 
to  dissolve  it.  The  strength  of  this  solution  is  then  ascertained  by 
means  of  a  solution  of  silver.  About  o.oi  gm.  of  silver  is  weighed 
out,  dissolved  in  nitric  acid,  evaporated  to  dryness,  dissolved  again  in 
about  80  cc.  of  water;  some  precipitated  calcium  carbonate  added, 
and  the  starch  iodid  solution  run  in  from  a  burette  until  the  solution 
takes  a  greenish-blue  tint.  The  o.oi  gm.  of  silver  should  require 
about  50  cc.  of  the  starch  iodid  solution. 

The  amount  of  silver  taken  for  an  analysis  should  not  contain  more 
than  o.oi  to  0.02  gm.  of  silver,  and  in  the  presence  of  copper  must  be 
greatly  diluted,  in  order  to  weaken  the  color  of  the  copper,  say  from 
60  to  100  cc.  If  more  than  0.02  gm.  of  silver  be  present,  the  larger 

N 

portion  may  be  precipitated  by  means  of  —  sodium  chlorid,  the  pre- 
cipitate filtered  off,  and  the  remaining  silver  determined  as  above. 
The  amount  so  found  is  then  added  to  that  which  has  been  precipitated 
by  the  sodium  chlorid.  If  lead  is  present  in  the  solution  it  may  be 
removed  by  the  addition  of  sulphuric  acid,  filtering,  and  then  neu- 
tralizing the  excess  of  acid  by  means  of  calcium  carbonate,  and  filter- 
ing again  before  titrating.  Field  (Chem.  News,  n,  17)  uses  a  solution 
of  iodin  in  potassium  iodid  with  starch. 


CHAPTER  XL 

STRONTIUM 

Strontium  Oxid  and  Hydroxid  may  be  estimated  by  dissolving 
in  water  and  titrating  with  decinormal  hydrochloric  acid  in  the  presence 
of  an  indicator.  A  better  way  is  by  residual  titration,  i.e.,  adding  a 

N  N 

measured  excess  of  —  hydrochloric  acid,  and  retitrating  with  —  NaOH 
10  10 

in  the  presence  of  phenolphthalein,  after  boiling  the  acid  solution. 

N 

i  cc.  —  hydrochloric  acid =0.004347  gm.  Sr; 
10 

=0.006035  "     Sr(OH)2.< 

Strontium  Carbonate  may  be  estimated  by  residual  titration  as 
above  described. 

N 

i  cc.  —  hydrochloric  acid=o.oo7322  gm.  SrCOs. 
10 

Strontium  Chlorid  may  be  estimated  by  precipitating  the  metal 
as  sulphate  by  means  of  potassium  sulphate,  and  then  titrating  the  fil- 
trate containing  potassium  chlorid,  with  decinormal  silver  nitrate 
solution,  and  chromate  as  indicator. 

SrCl2  +  K2S04=  SrS04+  2KC1; 

KC1 + AgN03 = AgCl + KN03. 

N 

i  cc.  —  silver  nitrate  solution  =  0.007865  gm.  SrCl2. 
10 

The  same  method  may  be  employed  for  strontium  iodid  and  bromid. 
These  haloid  salts  may  also  be  titrated  direct  with  silver  nitrate,  as 
described  on  page  115. 

Strontium  Nitrate  may  be  estimated  by  adding  to  its  solution  an 
excess  of  sodium  carbonate,  thoroughly  washing  the  resulting  pre- 
cipitate of  strontium  carbonate  with  hot  water,  and  then  estimating 
the  carbonate  as  described  above. 

All  soluble  strontium  salts  may  be  estimated  by  precipitation  as 
oxalate,  in  the  absence  of  calcium,  barium,  and  other  metals  precipi- 
table  by  oxalic  acid. 

419 


420  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

A  strong  solution  of  oxalic  acid  is  added  in  excess,  then  an  equal 
volume  of  alcohol.  Let  stand  over  night,  filter  through  sand,  dry 
the  precipitate  thoroughly  to  expel  the  alcohol,  wash  it  into  a  beaker, 
add  dilute  sulphuric  acid  to  decompose  the  oxalate,  and  titrate  with 

N 

—  permanganate. 

10  N 

i  cc.  —  permanganate =0.004347  gm.  Sr. 
10 


CHAPTER  XLI 

TIN 

Titration  with  lodin  in  Alkaline  Solution  (Lenssen).*  In 
this  method  tin  is  estimated  in  the  form  of  stannous  chlorid.  To  a 
solution  of  this  salt  acidulated  with  hydrochloric  acid,  or  a  solution 
of  metallic  tin  in  hydrohloric  acid,  a  tolerable  quantity  of  Rochelle 
salt  is  added,  and  then  a  strong  solution  of  sodium  bicarbonate,  until 
the  solution  is  alkaline.  If  the  solution  becomes  cloudy  upon  addition 
of  the  sodium  bicarbonate,  more  Rochelle  salt  must  be  added.  Starch 

•  N 

solution  is  then  added  and  —  •  iodin  solution  from  a  burette  until  the 

10 

blue  color  appears.     See  equation  below. 

Stannous  Chlorid  of  Commerce  is  estimated  according  to 
Dietze  (Ph.  Ztg.,  1897,  191)  as  follows: 

One  gram  is  dissolved  in  water,  a  few  drops  of  hydrochloric  acid 
added,  and  the  solution  diluted  to  100  cc.  25  cc.  of  this  solution  are 
mixed  with  i  gm.  of  tartaric  acid,  2  or  3  gms.  of  sodium  bicarbonate, 

N 

and  a  few  drops  of  starch  solution,  and  the  mixture  titrated  with  — 

10 

iodin  solution  until  a  blue  color  appears.     21.9  cc.  to  22.1  cc.  of  the 
latter  should  be  employed. 

N 
Each  cc.  of  —  iodin  =  0.01  1  21  1  gm.  of  SnCl2  +  2H2O. 

The  cc.  used  multiplied  by  4  and  then  by  the  factor  gives  the 
quantity  in  the  i  gm.  taken.  This  multiplied  by  100  gives  the  per 
cent.  The  reaction  is  as  follows  : 


If  a  salt  of  tin,  other  than  stannous  chlorid,  is  to  be  estimated, 
it  may  easily  be  brought  into  this  condition  by  precipitating  the 
metal  from  its  solution  by  means  of  pure  metallic  zinc,  washing  the 
finely  divided  tin,  and  dissolving  in  strong  hydrochloric  acid  in  con- 
tact with  pieces  of  metallic  platinum  in  an  atmosphere  of  hydrogen 
or  carbon  dioxid. 

*  Jour.  f.  prak.  Chem.,  LXXVIII,  200. 

421 


422  A    MANUAL  OP   VOLUMETRIC  ANALYSIS 

Estimation  in  Acid  Solution.  As  pointed  out  by  S.  W.  Young 
(J.  A.  C.  S.,  xix,  809),  the  estimation  of  stannous  chlorid  may  be 
accurately  made  in  an  acid  solution  by  means  of  iodin. 

The  method  of  operation  is  very  simple,  and  consists  in  bringing 
the  stannous  salt  into  solution,  preferably  with  dilute  hydrochloric 
acid,  adding  starch  paste,  and  then  titrating  with  standard  iodin  to  a 
blue  color.  Neither  oxidizing  nor  reducing  substances  must  be 
present. 

Great  care  must  be  exercised  to  prevent  oxidation  of  the  stannous 
salt  solution  by  undue  exposure  to  air.  It  should  be  prepared  rapidly 
and  immediately  titrated. 

The  potassium  iodid  must  be  free  from  iodate. 

Since  thiosulphate  cannot  be  satisfactorily  used  in  acid  solution 
to  titrate  against  iodin,  a  dilute  stannous  chlorid  solution  may  be 
employed  for  this  purpose.  This  solution  may  be  preserved  under 
coal-oil,  and  even  then  should  be  checked  against  the  iodin  very  fre- 
quently because  it  changes  its  strength  very  rapidly. 

The  results  obtained  by  the  author  with  this  method  are  a  trifle 
high,  which  is  attributed  by  him  to  the  standardization  of  the  iodin 
by  thiosulphate  in  neutral  solution,  whereas  the  assay  is  done  in  an  acid 
solution. 

Young  obtained  better  results  by  standardizing  in  the  following 
way: 

A  fresh  solution  of  stannous  chlorid  was  prepared  of  such  strength 
as  to  be  approximately  equivalent  to  the  iodin  solution.  Portions  of 
C.  P.  potassium  dichromate  were  then  weighed  out  (about  0.2  gm.). 
These  were  titrated  with  the  stannous  chlorid  solution  until  the  dichro- 
mate was  completely  reduced,  and  then  titrated  back  with  iodin  solu- 
tion until  the  starch-iodid  blue  was  produced.  The  reaction  is  as 
follows  : 


the  relationship  between  the  iodin  and  the  stannous  chlorid  being 
thus  accurately  determined. 

See  also  Young  and  Adams,  J.  A.  C.  S.,  xix,  515;  "Action  of 
Iodin  on  Solutions  of  Stannous  Chlorid;"  and  Wilson  H.  Low, 
"Determination  of  Antimony  and  Tin  in  Babbitt,  Type  Metal,  or 
Other  Alloys,"  J.  A.  C.  S.,  xxix,  66. 

Indirect  Titration  by  Ferric  Chlorid  and  Permanganate 
(Lowenthal;*  also  Stromeyer  f  ).  This  method  is  based  upon  the 

*  Journ.  f.  prakt.  Chera.,  LXXVI,  484. 

f  Annal.  d.  Chem.  u.  Pharm  ,  CXVII,  261. 


TIN  423 

fact  that  when  stannous  chlorid  is  brought  in  contact  with  ferric 
chlorid  the  latter  is  reduced  to  the  ferrous  state,  whilst  the  stannous 
chlorid  is  oxidized  to  the  stannic  condition.  The  quantity  of  ferrous 
salt  then  found  by  means  of  potassium  permanganate  is  the  measure 
of  the  stannous  salt  present  originally.  This  method  is  more  fully 
described  on  page  177.  Cupric  chlorid  may  be  used  instead  of  ferric 
chlorid. 

Stannic  Salts,  also  tin  compounds  containing  iron,  are  dissolved 
in  water,  hydrochloric  acid  added,  and  some  pieces  of  pure  metallic 
zinc  introduced,  and  left  in  for  ten  or  twelve  hours.  The  tin  which 
is  so  precipitated  is  (after  being  thoroughly  washed)  dissolved  in 
hydrochloric  acid  and  titrated  as  above  described,  or  it  may  at  once 
be  mixed  with  the  ferric  chlorid  solution,  a  little  hydrochloric  acid 
added,  and  when  solution  is  complete  titrated  with  permanganate. 

Stannic  salts  may  also  be  reduced  to  stannous  sulphid  by  treat- 
ment with  H2S.  The  precipitated  sulphid  is  then  well  washed  mixed 
with  ferric  chlorid,  gently  warmed,  the  sulphur  separated  by  filtration, 
and  the  filtrate  titrated  with  permanganate  as  above.  The  reaction  is: 

SnS2  +  2Fe2Cl6=SnCl4+4FeCl2  +  S2. 

By  Means  of  Potassium  Dichr ornate  (Reynolds).*  This  is  a 
modification  of  Streng's  original  method.  The  oxidation  of  stannous 
chlorid  by  air  is  prevented  by  carbon  dioxid,  and  a  new  and  delicate 
indicator  is  used.  The  indicator  is  prepared  by  heating  azobenzene 
with  concentrated  sulphuric  acid  till  the  beginning  of  a  violent  reaction; 
the  whole  is  then  poured  into  a  large  volume  of  water,  forming  a  deep 
red  solution.  Stannous  chlorid  will  decolorize  this  solution,  while 
potassium  dichromate  will  restore  the  color.  It  is  not  affected  by 
ferrous  salts  nor  by  sodium  thiosulphate. 

The  Process.  Place  20  cc.  of  concentrated  hydrochloric  acid  in 
a  200-cc.  Erlenmeyer  flask,  fitted  with  a  cork  through  which  passes 
a  tube  reaching  to  within  1.5  cm.  of  the  bottom  and  a  short  tube  about 
6  mm.  wide  at  the  top  and  2  at  the  bottom,  cut  slantwise. 

The  tube  has  a  side  hole  just  below  the  cork  to  allow  the  gas  to  escape 
when  adding  the  standard  dichromate. 

Fill  the  flask  with  CO2  from  a  cylinder,  the  gas  passing  first  through 
titanous  sulphate  and  then  through  sulphuric  acid.  Heat  the  hydro- 
chloric acid  nearly  to  boiling  and  allow  to  cool  in  a  current  of  CO2. 

Weigh  out  from  0.6  to  i  gm.  of  the  metal  to  be  analyzed,  in  a 
platinum  gauze  basket,  and  lower  into  the  acid.  When  dissolved, 

*  Chem.  News,  97,  13  (Jan.  10). 


424  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

heat  nearly  to  boiling  to  drive  out  hydrogen,  cool,  and  run  in  the 
standard  dichromate  through  the  short  tube  to  within  one  cc.  of  the 
correct  amount.  Then  add  enough  indicator  to  insure  masking  the 
green  of  the  chromic  salt  when  the  color  of  the  indicator  is  restored. 
Remove  the  basket  and  finish  the  titration  by  delivering  the  remainder 
of  the  dichromate  directly  into  the  solution. 

The  method  gives  good  results  in  the  case  of  pure  tin,  ferro-tin, 
and  britannia  metal,  but  is  not  reliable  for  pewter,  phosphor-tin,  or 
manganese-tin. 


CHAPTER  XLII 
ZINC 

Zinc  Oxid  and  Carbonate.  Benedikt  and  Cantor  (Zeit.  angew. 
Chem.,  1888,  236,  237)  show  that  the  above  compounds  can  be 
accurately  titrated  by  adding  a  measured  excess  of  normal  acid  solu- 
tion and  titrating  back  with  normal  alkali,  using  methyl-orange  as 
indicator. 

ZnO+H2SO4=ZnSO4-|-H2O; 

2)80.78      2)98 

40.39          49=  1000  cc.  —  V.  S. 

i 


2)124-45      2)98 

62.225         49=  1000  cc.  —  V.  S. 

i 

N 
i  cc.  —  £[2804=0.03245    gm.  Zn; 

=  0.04039     "     ZnO; 
^=0.062225  "     ZnCO3. 

Other  salts  of  zinc  may  be  treated  by  the  same  method.  The  zinc 
salt  is  dissolved  in  water  (50  cc.  =  about  o.i  gm.  of  ZnO),  phenoph- 
thalein  is  added  and  the  solution  titrated  with  standard  sodium 
hydroxid  to  an  intense  red  color,  a  few  cc.  more  of  the  standard 
alkali  then  added,  and  after  boiling  the  mixture  titrating  the  excess 
of  sodium  hydroxid  with  standard  acid. 

Estimation  as  Oxalate  (Leison).  The  zinc  salt  in  solution  pref- 
erably as  sulphate  and  neutral  in  reaction,  is  treated  with  an  excess 
of  a  strong  solution  of  oxalic  acid,  and  then  a  volume  of  strong  alcohol 
equal  in  bulk  to  the  zinc  solution,  is  added.  This  is  allowed  to  stand 
twelve  hours,  and  then  the  precipitated  zinc  oxalate  separated  by 
filtration  through  a  plug  of  asbestos,  and  thoroughly  washed  with 
alcohol,  and  dried  in  an  air-bath.  The  asbestos  plug  and  precipitate 
are  placed  in  hot  dilute  sulphuric  acid,  by  which  the  oxalate  is  dis- 

425 


426  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

N 

solved,  and  the  mixture  titrated  with  —   potassium  permanganate 

10 

-solution. 

N 

Each  cc.  of  —  permanganate =0.003  245    g111-  of  Zn; 
10 

=  0.004039     "     "  ZnO; 
=  0.0080125  "     "  ZnSO4. 

Zinc  Dust.  Zinc  dust  is  generally  a  mixture  of  metallic  zinc, 
zinc  oxid,  and  often  some  zinc  carbonate.  It  is  largely  used  as  a  reduc- 
ing agent,  and  its  value  in  this  respect  is  proportionate  to  the  metallic 
zinc  it  contains.  Hence  it  is  important  to  be  able  to  estimate  the 
quantity  of  free  metal  in  a  sample.  This  may  be  done  as  follows: 

A  weighed  portion  of  the  zinc  dust,  free  from  lumps,  is  introduced 
into  a  flask  provided  with  a  ground -glass  stopper,  and  a  measured 
excess  of  a  centinormal  solution  of  iodin  added  and  the  mixture  digested 
for  some  time.  The  metallic  zinc  is  acted  upon  by  the  iodin,  and 
the  zinc  iodid  is  formed;  the  oxid  is  not  affected. 

When  the  reaction  is  completed,  the  excess  of  iodin  solution  is 
determined  by  retitration  with  centinormal  sodium  thiosulphate  solu- 
tion, and  the  quantity  of  the  latter  employed  is  deducted  from  the 
quantity  of  iodin  solution  added. 

N 
Each  cc.  of  —  iodin  V.  $.  =  0.0003245  gm.  of  metallic  zinc. 

Indirect  Method.  Precipitation  as  Sulphid  and  Conversion 
of  the  Latter  into  Chlorid,  with  Subsequent  Titration  of  the 
Chlorid  by  Volhard's  Method  (Mann).  The  reagents  required  are: 

Silver  Chlorid.  Well  washed,  preserved  under  water,  and  pro- 
tected from  light. 

Decinormal  Silver  Nitrate. 

Decinormal  Ammonium  Sulphocyanate. 

Ferric  Alum  Indicator. 

Pure  Nitric  Acid. 

The  Process.  0.5  to  i  gm.  of  the  zinc  ore  is  dissolved  in  nitric 
acid.  H2S  is  passed  through  the  solution  to  remove  heavy  metals. 
Iron  and  aluminum  are  separated  by  double  precipitation  with 
ammonia.  The  mixed  nitrates  are  acidified  with  acetic  acid  and  the 
zinc  precipitated  as  ZnS  by  means  of  H2S.  The  liquid  is  then  boiled 
in  order  to  drive  off  excess  of  H2S,  which  is  known  to  be  removed 
completely  when  a  drop  of  the  filtered  liquid  fails  to  stain  lead  paper. 

The  precipitate  is  then  allowed  to  settle,  the  supernatant  liquid 
decanted,  and  the  precipitate  transferred  to  a  filter  with  a  little  hot 
water  without  further  washing.  The  filter  and  contents  are  then 
placed  into  a  beaker;  50  cc.  of  hot  water  added,  and  a  sufficient  quan- 


ZINC  427 

tity  of  moist  silver  chlorid  added  to  decompose  the  zinc  sulphid  and 
be  present  in  slight  excess.  The  mixture  is  now  boiled  until  it  begins 
to  settle  clear;  several  drops  of  dilute  sulphuric  acid  are  added,  and 
the  whole  of  the  zinc  sulphid  will  soon  be  converted  into  zinc  chlorid. 
The  insoluble  matter,  consisting  of  silver  chlorid  and  free  sulphur,  is 
removed  by  nitration  and  washed.  The  chlorid  in  the  mixed  nitrate 
and  washings  is  then  estimated  by  the  following  method: 

To  200  or  300  cc.  of  the  cooled  nitrate  are  added  5  cc.  of  the  ferric 
alum  indicator  and  enough  pure  nitric  acid  to  remove  the  yellow  color 
of  the  iron.  A  measured  quantity  (excess)  of  the  standard  silver 
nitrate  is  then  added,  and  finally  the  titration  with  standard  sulpho- 
cyanate  begun.  The  volume  of  the  latter  required  to  produce  the 
red  end-color  is  deducted  from  the  quantity  of  the  silver  nitrate  added, 
and  the  difference  is  the  quantity  of  silver  nitrate  which  represents 
the  zinc  chlorid  present.  The  reactions  are: 

ZnS  +  2  AgCl = ZnCl2 + Ag2S, 
ZnCl2  +  2AgN03=  2AgCl+Zn(N03)2. 

This  method  gives  exceedingly  good  results. 

Precipitation  with  Standard  Sodium  Sulphid.  This  process, 
which  gives  very  good  results,  is  the  reverse  of  the  method  for  the 
estimation  of  sulphids  described  on  page  327.  The  materials  required 
are: 

Standard  Sodium  Sulphid.  This  is  prepared  by  passing  H2S  into 
a  solution  of  sodium  hydroxid  until  saturated,  adding  sodium  hydroxid 
until  the  solution  no  longer  smells  of  H2S,  and  then  diluting  it  until 
it  nearly  corresponds  to  the  standard  zinc  solution. 

Standard  Zinc  Solution.  Made  by  dissolving  43.976  gms.  of  pure 
zinc  sulphate  (ZnSO4-f7H2O)  in  water  to  make  1000  cc.  Such  a 
solution  contains  o.oi  gm.  of  zinc  per  cc. 

Alkaline  Lead  Indicator.  Made  by  heating  together  lead  acetate, 
tartaric  acid,  and  sodium  hydroxid  solution  in  excess,  until  a  clear 
solution  is  produced.  Sodium  nitro-prussid,  nickelous,  and  cobaltous 
chlorids  are  among  the  substitutes  suggested  as  indicator. 

Determination  of  the  Relative  Strength  of  the  Standard 
Solutions.  50  cc.  of  the  zinc  solution  (=0.5  gm.  Zn)  are  placed  into 
a  beaker  and  sufficient  of  a  solution  containing  three  parts  of  ammonia 
and  one  part  of  ammonium  carbonate  are  added  until  the  precipitate 
which,  at  first,  forms  is  redissolved.  The  sodium  sulphid  solution  is 
then  added  from  a  burette,  stirring  after  each  addition.  From  time 
to  time  a  drop  of  the  alkaline  lead  solution  is  taken  out  from  a  bottle 
containing  it,  by  means  of  a  glass  rod,  and  placed  upon  a  piece  of 
white  filtering  paper,  and  near  it  upon  the  same  piece  of  paper,  a  drop 


428  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

of  the  turbid  zinc  solution.  The  liquids  spread  over  the  paper  and 
run  into  each  other,  and  as  soon  as  an  excess  of  sodium  sulphid  solu- 
tion is  present,  show  a  dark  line  where  the  drops  meet.  It  is  not 
always  easy  to  hit  the  exact  end -point  on  the  first  trial,  but  a  second 
or  a  third  trial  will  give  accurate  results.  In  the  analysis  itself  the 
zinc  solution  must  be  made  alkaline  with  ammonia,  as  above  described. 
Two  burettes  are  to  be  used,  one  containing  the  standard  sodium 
sulphid  solution  and  the  other  the  standard  zinc  solution.  The  sul- 
phid is  added  until  the  lead  indicator  shows  it  to  be  in  excess.  If 
this  point  is  overstepped  in  the  first  trial,  as  may  easily  happen,  a  few 
cc.  of  the  standard  zinc  solution  may  be  added,  and  the  titration  again 
proceeded  with. 

If  zinc  ores  are  to  be  examined,  an  ammoniacal  zinc  solution  is  pre- 
pared for  analysis  as  follows : 

One  or  two  grams  of  the  ore,  according  to  the  quantity  of  zinc 
present,  are  finely  pulverized,  introduced  into  a  wide-mouthed  flask, 
and  treated  with  hydrochloric  acid,  to  which  a  little  nitric  acid  is 
added.  This  mixture  is  warmed  in  order  to  facilitate  solution,  and 
then  the  excess  of  acid  removed  by  heat.  Lead,  if  present,  is  removed 
by  the  addition  of  a  few  drops  of  H2SO4  previous  to  complete  dryness. 
The  residue  is  then  extracted  with  water  and  filtered.  Should  heavy 
metals  be  present,  they  are  to  be  removed  by  H^S;  the  solution  will 
still  contain  iron  and  possibly  also  manganese.  Boiling  with  a  little 
nitric  acid  is  necessary,  if  the  iron  is  not  fully  oxidized.  If  only  traces 
of  manganese  are  present,  a  few  drops  of  bromized  HC1  should  be 
added.  When  cold  the  solution  is  supersaturated  with  ammonia  to 
precipitate  the  iron,  which  is  separated  by  filtration  and  washed  with 
ammoniacal  warm  water.  If  the  quantity  of  iron  is  large,  the  pre- 
cipitate will  hold  much  zinc  (about  one-fifth  of  its  weight).  It  should 
then  be  dissolved  in  HC1  and  reprecipitated  as  basic  acetate.  The 
filtrate  from  this  second  precipitate  is  then  added  to  the  original  zinc 
filtrate,  and  the  whole  made  up  to  one  liter. 

Precipitation  as  Sulphid  and  then  Treatment  with  Ferric  Salt 
and  Titration  of  the  Resulting  Ferrous  Salt  with  Permanganate 
(Schwartz).  This  method  is  based  upon  the  fact  that  when  zinc 
sulphid  is  mixed  with  ferric  sulphate  and  sulphuric  acid,  the  following 
reaction  takes  place: 

ZnS+Fe2(SO4)3=2FeSO4+ZnSO4+S. 

The  ferrous  sulphate  so  produced  is  estimated  with  permanganate, 
and  the  proportional  quantity  of  zinc  present  ascertained. 

Ferric  chlorid  and  hydrochloric  acid  may  be  used  instead  of  the 


ZINC 


429 


sulphate  and  sulphuric  acid,  and  dichromate  may  be  employed  instead 
of  permanganate. 

The  Process.  The  zinc  solution  to  be  examined,  in  a  tall  cylinder, 
is  made  alkaline  by  the  addition  of  sodium  or  ammonium  hydroxid 
(or  in  the  assay  of  zinc  ores  the  ammoniacal  zinc  solution,  described 
in  the  foregoing  method,  is  taken).  A  slight  excess  of  sodium  or 
ammonium  sulphid  is  then  added,  the  cylinder  filled  with  warm  dis- 
tilled water,  covered  closely,  and  set  aside  in  a  warm  place  for  several 
hours.  A  tube  drawn  out  at  one  end  and  containing  a  loosely  fitting 
plug  of  asbestos,  is  then  arranged,  as  in  Fig.  80.  The  bent  tube  is 
connected  with  a  suction-pump.  After  the  liquid 
above  the  precipitate  in  the  cylinder  has  become 
perfectly  clear,  and  the  precipitate  settled,  decant 
the  clear  fluid  through  the  filter.  As  soon  as 
the  clear  fluid  has  run  through,  close  the  tube 
with  a  tight  cork,  fill  the  cylinder  with  warm, 
recently-boiled,  water  containing  a  little  ammo- 
nium hydrate,  cork  tightly,  and  allow  the  flask  to 
stand  until  the  precipitate  again  subsides;  then 
pour  the  clear  fluid  upon  the  filter,  and  repeat 
this  three  or  four  times ;  finally,  wash  most  of  the 
precipitate  upon  the  asbestos  filter,  and  wash 
rapidly  with  warm  water  containing  ammonia 
until  a  drop  of  the  filtrate  produces  no  black  pre- 
cipitate in  an  alkaline  lead  solution.  Then 
remove  the  tube  containing  the  asbestos  and  zinc 
sulphid  from  the  Woulfe's  bottle,  and,  by  means 
of  a  stiff  wire  inserted  at  the  small  end,  force 
the  asbestos  and  zinc  sulphid  into  the  cylinder 
in  which  the  precipitation  was  made,  wash  any  zinc  sulphid  still  adhering 
to  the  tube  into  the  cylinder,  and  add  a  quantity  of  an  acid  solution  of 
ferric  sulphate  more  than  sufficient  to  decompose  the  zinc  sulphid. 
Stopper  the  cylinder  securely,  and  allow  to  stand  in  a  warm  place, 
shaking  occasionally.  The  reaction  which  takes  place  may  be  repre- 
sented thus: 

Fe2(SO4)3+ZnS=2FeSO4+ZnSO4+S. 

After  the  reaction  is  completed,  the  solution  should  possess  a  yellow 
color  due  to  undecomposed  ferric  salt.  The  stopper  is  now  removed, 
no  odor  of  H2S  should  be  perceptible,  and  the  solution  should  be  diluted 

N 
and  titrated  with   —  permanganate.     If  the  solution  is  very  dilute, 


FIG.  80. 


10 


the  finely  divided  precipitate  of  sulphur  does  not  interfere. 


t 


430  A    MANUAL  OF    VOLUMETRIC   ANALYSIS 

N 

Each  cc.  of  —  permanganate =0.003 245    gm.  Zn; 
10 

=  0.004039     "     ZnO; 
=  0.0080125   "     ZnSO4. 

Precipitation  as  Sulphid  with  Iron  Indicator  (Schaffner).* 

Reagents  Required.  Sodium  Sulphid  Solution  and  Zinc  Solution 
are  prepared  as  directed  on  page  427.  i  cc.  of  the  former  should 
precipitate  o.oi  gm.  of  zinc,  and  i  cc.  of  the  latter  should  contain  just 
o.oi  gm.  of  zinc. 

Ferric  Hydroxid  Indicator.  3  gms.  of  iron  wire  are  dissolved  in 
HC1  with  the  aid  of  heat;  a  little  HNO3  is  added,  and  the  solution 
boiled  so  as  to  convert  the  ferrous  into  ferric  chlorid,  and  the  solution 
then  diluted  to  100  cc. 

Just  before  using,  add  one  or  two  drops  (always  taking  the  same 
number  of  drops)  to  i  cc.  of  undiluted  ammonia -water.  Each  drop 
produces  a  ring  of  ferric  hydroxid,  which  requires  but  a  few  moments 
to  impart  the  desired  opacity  to  the  liquid.  In  about  one  minute, 
the  suspended  ferric  hydroxid  is  ready  for  use.f 

The  Process.  Prepare  the  ammoniacal  zinc  solution  as  described 
on  page  428.  To  500  cc.  of  this  solution  add  ferric  hydroxid  indi- 
cator suspended  in  ammonia  (see  above),  and  then  run  in  the  standard 
sodium  sulphid  solution  until  the  greater  part  of  the  ferric  hydroxid, 
collected  on  the  sides  and  bottom  of  the  beaker,  acquires  a  brown  or  a 
black  tint  (the  same  tint  should  be  used  in  all  the  titrations).  It  is 
advisable  to  keep  the  solution  for  titration  at  from  40°  to  60°  C.  A 
check  titration  carried  out  under  exactly  equal  conditions,  with  a 
known  quantity  of  zinc,  gives  comparative  data  for  calculation. 

Estimation  as  Ferrocyanid.  The  use  of  potassium  ferrocyanid 
for  the  volumetric  estimation  of -zinc  was  first  suggested  by  Galletti.J 
The  precipitation  is  effected  in  acetic  acid  solution  at  40°  C.,  and  the 
milky  appearance  which  the  liquid  assumes  when  the  potassium  ferro- 
cyanid is  in  excess  serves  as  an  indication  of  the  end-reaction. 

The  Standard  Potassium  Ferrocyanid  solution  contains  43.104  gms. 
per  liter,  i  cc.  of  this  =  o.oi  gm.  of  zinc,  but  its  actual  working  power 
must  be  found  by  titration  against  a  standard  zinc  solution,  made  by 
dissolving  10  gms.  pure  metallic  zinc  in  hydrochloric  acid,  and  making 
up  to  i  liter.  The  ferrocyanid  solution  should  be  freshly  prepared 

*  Jour.  f.  prakt.  Chem.,  LXXIII,  410. 

f  Ferric  chlorid  solution  may  be  used  instead  and  dropped  directly  into  the 
ammoniacal  zinc  solution. 

J  Zeitschr.  f.  analyt.  Chem.,  IV,  213,  and  the  method  later  modified  by  him 
in  VIII,  135  and  XIV,  190. 


ZINC  431 

frequently,  or  should  be  adjusted  when  needed.  The  adjustment 
should  be  made  in  precisely  the  same  way  as  the  actual  analysis  of 
the  ores,  which  is  as  follows: 

25  cc.  of  the  zinc  solution  are  treated  in  a  beaker  with  15  cc.  of 
ammonia-water  (sp.gr.  0.900).  Acetic  acid  is  then  very  cautiously 
added  to  acidify  the  solution.  50  cc.  of  acid  ammonium  acetate 
solution  (made  by  adding  together  20  cc.  of  ammonia -water,  sp.gr. 
0.900,  15  cc.  of  acetic  acid,  and  65  cc.  of  distilled  water)  are  then 
added  to  liquid,  and  the  whole  diluted  to  250  cc.,  and  warmed  to  about 
50°  C.  The  ferrocyanid  solution  is  then  delivered  from  a  burette, 
until  the  wThole  of  the  zinc  is  precipitated.  The  first  change  of  color, 
from  white  to  ash  gray,  is  taken  as  the  end -point.  A  better  end- 
reaction  is,  however,  attainable  if  a  drop  of  the  solution  is  brought 
in  contact  on  a  white  porcelain  slab  with  a  drop  of  uranium  acetate 
solution.  The  ferrocyanid  solution  should  correspond,  volume  for 
volume,  with  the  standard  zinc  solution. 

This  method  is  available  in  the  absence  of  copper,  manganese, 
nickel,  and  cobalt,  but  moderate  quantities  of  iron  and  lead  may  be 
present. 

Precipitation  in  Hydrochloric  Acid  Solution  (Fahlberg).*  This 
method  is  somewhat  simpler  than  the  foregoing.  The  same  ferro- 
cyanid solution  is  used.  The  zinc  solution  is  prepared  by  dissolving 
10  gms/  of  pure  zinc  in  hydrochloric  acid,  adding  50  gins,  of  ammo- 
nium chlorid,  and  diluting  to  one  liter.  The  ammonium  chlorid 
causes  the  precipitate  to  assume  a  more  fine  flocculent  character, 
which  is  quite  an  advantage. 

The  working  value  of  the  ferrocyanid  solution  is  ascertained  as 
follows : 

Fill  two  burettes,  one  with  the  ferrocyanid  solution  and  the  other 
with  the  standard  zinc  solution.  Then  place  in  a  beaker  50  cc. 
of  the  zinc  solution,  add  10  to  15  cc.  of  hydrochloric  acid  (sp.gr.  1.12), 
and  450  cc.  of  water,  and  while  gently  stirring,  run  in  the  ferrocyanid 
solution  in  quantities  of  i  cc.  at  a  time  until  a  drop  of  the  liquid, 
brought  in  contact  with  a  drop  of  uranium  nitrate  solution  on  a  white 
slab,  gives  a  permanent  brownish-red  color. 

Now  carefully  run  in  the  zinc  solution  until  the  reaction  disappears, 
then  again  add  the  ferrocyanid  solution,  drop  by  drop,  until  -the  end- 
reaction  is  again  attained.  The  ferrocyanid  solution  is  then  diluted, 
so  that  equal  volumes  of  it  and  the  zinc  solution  react.  Ferric  chlorid 
may  also  be  used  as  indicator. 

Miller  (J.  A.  C.  S.,  xvm,  1102)  suggests  the  use  of  a  strong  solution 


*  Zeitschr.  f.  analyt.  Chem.,  XIII,  379. 


432  A    MANUAL  OF   VOLUMETRIC   ANALYSIS 


of  hydrochloroplatinic  acid  (H^PtCle),  acidified  with  HC1,  as  an  indi- 
cator for  the  titration  of  zinc  by  K4Fe(CN)e  when  performed  in  a  hot 
solution.  The  end-reaction  is  a  bright  emerald  green,  which  develops 
in  a  few  seconds  but  does  not  work  in  a  cold  solution.  This  indicator 
is  used  in  the  same  way  as  uranium  solution,  and  is  less  affected  by 
varying  amounts  of  HC1. 

The  nature  of  the  precipitate  produced  in  zinc  solutions  upon  the 
addition  of  potassium  ferrocyanid  has  been  the  subject  of  consider- 
able discussion.  The  prevailing  idea  that  a  normal  ferrocyanid  is 
formed  and  that  the  reaction  occurs  as  per  the  equation, 

K4FeCy6+  2ZnCl2=  Zn2FeCy6+  4KC1, 

is  criticised  by  Miller,*  who  asserts  that  the  precipitate  has  the  com- 
position, Zn3K2[Fe(CN)e]2-  In  proof  of  which  a  solution  of  potas- 
sium ferrocyanid  (i  cc.  of  which  =0.010  gm.  of  Zn)  would  contain 
32.32  gms.  of  K4Fe(CN)6+3H2O  to  the  liter,  if  the  reaction  proceeds, 
as  per  the  above  equation;  whereas  it  has  been  proven  by  experiment, 
that  i  cc.  of  a  solution  containing  43.104  gms.  of  K4Fe(CN)6+3H2O  to 
the  liter,  equals  o.oio  gm.  of  zinc.  This  would  indicate  that  two  molec- 
ular weights  of  potassium  ferrocyanid  are  equivalent  to  three  atoms 
of  zinc,  and  hence  the  composition  of  the  precipitate  formed  is  probably 
as  asserted  by  Miller,  Zn3K2[Fe(CN)e]2>  and  the  equation  may  be 
written  ; 

2K4FeCy6+3Zna2=Zn3K2[Fe(CN)6]2+6KCl. 

In  a  series  of  researches  Wyronboff  comes  to  the  conclusion  that 
the  precipitate  produced  by  the  action  of  potassium  ferrocyanid  upon 
a  solution  of  a  zinc  salt  has  the  composition,  3Zn2Fe(CN)e  •  K4Fe(CN)e 
+  I2H2O,  which  to  an  extent  agrees  with  the  findings  of  Miller. 

The  fact  is  that  the  composition  of  the  precipitate  differs  with  vary- 
ing conditions  in  an  analysis,  such  as  quantity  and  temperature  of 
the  solution  and  quantity  of  acid  present.  The  indicator  used  also 
has  an  influence. 

According  to  Stone  and  Van  Ingen  (J,  A.  C.  S.,  xix,  545),  in  a 
cold  solution  containing  one  seventh  of  a  cc.  of  HC1  in  100,  uranium 
showed  the  end-reaction  when  the  quantities  of  zinc  and  ferrocyanid 
were  sufficient  to  form  Zn2Fe(CN)6-  In  a  hot  solution  containing  10 
cc.  of  HC1,  uranium  gave  the  reaction  with  Zn3K2[Fe(CN)6]2-  In  both 
of  these  cases  a  drop  of  the  indicator  was  mixed  with  a  drop  of  the 
solution  on  a  porcelain  plate.  In  a  cold  solution  containing  one 
seventh  of  ace.  of  the  acid,  uranium  gave  a  reaction  with  Zn4K4[Fe(CN)6]3 

*  J.  A.  C.  S.,  XVIII,  uoi. 


ZINC  433 

when  drops  of  it  and  the  solution  were  placed  side  by  side  on  a  filter- 
paper,  so  that  the  uranium  did  not  touch  the  precipitate,  but  only 
the  clear  solution  that  filtered  from  it.  Under  the  same  conditions 
copper  and  ferric  chlorid,  as  indicators,  gave  the  same  end-point. 

A  strong  solution  of  cobalt  placed  on  a  porcelain  plate  and  a  drop 
of  the  solution  by  it,  so  that  the  two  touch  but  do  not  mix,  showed  the 
reaction  for  Zn4K4[Fe(CN)6]3  when  the  solution  was  mixed  with  very 
dilute  cobalt  the  reaction  was  for  Zn3K2[Fe(CN)6]3.  In  both  cases 
one-seventh  cc.  of  hydrochloric  acid  was  present  and  the  solution  was 
cold.  Platinum  indicator,  when  used  in  a  hot  solution,  gives  the 
reaction  with  Zn3K2[Fe(CN)6]2-  For  fuller  discussion  of  this  subject 
see  Miller  and  Mathews;  also  Stone  and  Van  Ingen  (J.  A.  C.  S.,  xix, 
542  and  547).  Similar  variations  occur  in  the  composition  of  the 
precipitate,  produced  with  manganese  and  potassium  ferrocyanid. 

Determination  of  Zinc  in  the  Presence  of  Manganese,  using 
Cobalt  Nitrate  as  Indicator.*  The  methods  commonly  used  for  the 
separation  of  manganese,  before  titrating  with  ferrocyanid,  take  too 
long  and  are,  moreover,  unsatisfactory.  Stone  therefore  suggests  the 
following  procedure,  in  which  the  two  metals  (after  separation  of 
other  metals  of  the  iron  group)  are  titrated  together;  the  manganese 
determined  in  a  separate  portion  by  titration  with  permanganate, 
and  the  zinc  found  by  difference.  The  usual  indicators  for  ferro- 
cyanid, namely,  uranium  salts,  ferric  chlorid,  and  copper  sulphate, 
are  not  suited  here,  because  they  react  with  the  precipitated  man- 
ganese ferrocyanid.  Cobalt  nitrate,  however,  gives  a  very  delicate 
reaction,  and  is  not  affected  by  as  much  as  one  part  of  hydrochloric 
acid  in  fourteen  of  water,  nor  does  it  react  on  the  precipitates. 

The  indicator  should  be  quite  dilute  and  should  be  used  on  a 
porcelain  slab,  a  drop  of  it  is  brought  in  contact  with  a  drop  of  the 
solution  to  be  tested.  The  drops  should  touch,  but  not  mix.  A  faint 
green  line  appearing  immediately,  marks  the  end-point.  The  reagents 
required  are: 

Standard  Potassium  Ferrocyanid,  30  gms.  to  i  liter.  This  is 
standardized  by  titrating  against  solutions  containing  known  amounts 
of  zinc  or  manganese  in  slightly  acidulated  solutions,  using  cobalt 
nitrate  solution  as  an  outside  indicator.  The  solution  should  be  about 
the  volume  used  in  an  actual  analysis.  The  amount  of  ferrocyanid 
solution  required  to  give'  a  reaction  with  cobalt  in  this  volume  of 
acidulated  solution  must  be  noted  and  deducted  for  each  titration. 
It  is  about  0.7  cc.  for  a  volume  of  140  cc. 

Standard  Potassium  Permanganate,  1.99  gms.  per  liter,  i  cc.= 
o.ooi  gm.  Mn. 

*  G.  C.  Stone,  J.  A.  C.  S.,  XVII,  473. 


434  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  Process,  The  ore  is  dissolved  in  HC1,  using  KC1O3  as  an 
oxidizer,  taking  care  to  have  sufficient  acid  to  keep  the  manganese 
in  solution. 

Lead  alone  need  not  be  separated;  copper  can  be  precipitated  by 
lead,  or  lead  and  copper  can  both  be  precipitated  by  aluminum; 
cadmium  may  be  precipitated  by  H^S  and  the  filtrate  oxidized. 
Iron  and  alumina  are  best  separated  by  barium  carbonate,  which  must 
be  free  from  alkali  carbonates  and  hydroxids,  barium  hydroxid, 
and  ammonium  salts.  A  sufficiently  pure  salt  for  this  purpose  can  be 
obtained  by  suspending  the  commercial  pure  barium  carbonate  (free 
from  ammonium  salts)  in  water  and  warming  for  several  hours  with 
two  or  three  per  cent  of  its  weight  of  barium  chlorid. 

The  thoroughly  oxidized  solution  of  the  ore  is  washed  into  a  5oo-cc. 
flask,  cooled,  and  barium  carbonate  suspended  in  water  added  until 
the  precipitate  curdles.  Then  pour  into  a  beaker,  mix  thoroughly, 
let  it  settle,  decant  the  clear  liquid  through  a  dry  filter,  dilute  to  500  cc., 
and  take  portions  of  50,  100,  or  200  cc.  for  each  titration.  The  titra- 
tion  should  be  started  at  once,  or  some  zinc  will  precipitate. 

One  portion,  which  should  contain  between  o.oi  and  0.04  gm.  of 
manganese,  is  diluted  to  about  200  cc.,  heated  nearly  to  boiling  in  a 
porcelain  dish,  and  titrated  rapidly  with  permanganate,  stirring 
vigorously. 

A  second  portion,  made  slightly  acid  with  HC1,  is  now  titrated  for 
the  zinc  and  manganese  together  in  the  cold. 

A  large  excess  of  HC1  must  be  avoided  because  of  its  solvent  action 
upon  manganese  ferrocyanid,  5  cc.  of  HC1  to  100  cc.  of  solution  con- 
taining about  0.03  gm.  of  Mn  is  about  right. 

If  the  manganese  is  present  in  appreciable  quantity,  the  color  of 
the  precipitate  will  darken  as  the  ferrocyanid  is  run  in,  and  quite 
suddenly  change  to  light  greenish-yellow  shortly  before  the  end  is 
reached.  It  is  not  necessary  to  test  with  the  cobalt  solution,  until 
i  or  2  cc.  of  the  ferrocyanid  have  been  added  after  the  lightening  of 
the  precipitate. 

Example,  i  cc.  of  the  ferrocyanid  solution  used  equaled  0.00606 
gm.  of  zinc,  or  0.00384  gm.  of  manganese;  i  cc.  of  the  permanganate 
solution  equaled  o.ooi  gm.  of  manganese.  2.5  gms.  of  the  ore  were 
treated  as  above  described. 

50  cc.  of  the  solution  was  diluted,  heated,  and  titrated  with  per- 
manganate, requiring  18.45  cc.  =  to  7.38  per  cent  of  manganese.  100  cc. 
titrated  with  ferrocyanid  required  27.85  cc. 

The  previous  titration  had  shown  that  there  was  0.0369  gm.  of 
manganese  present,  which  would  require  9.61  cc.  of  ferrocyanid; 
deducting  this  from  27.85  left  18.24  cc.  for  the  zinc=to  0.11053  gm* 
or  22.11  per  cent. 


ZINC  435 

By  Precipitation  as  Arsenate,  and  Titration  of  the  Arsenate 
with  Standard  Sodium  Thiosulphate  (Meade).*  This  method 
consists  in  precipitating  zinc  by  means  of  sodium  arsenate,  and  then 
after  dissolving  the  precipitated  zinc  ammonium  arsenate  in  dilute 
hydrochloric  acid,  adding  potassium  iodid  and  titrating  the  liberated 
iodin  by  means  of  standard  thiosulphate.  The  reactions  may  be 
expressed  as  follows  : 

+  Na2HAs04-f-NH40H=ZnNH4As04+2NaCl+H2O, 


N 
Each  cc.  of  —  thiosulphate  solution  represents  0.003245  gm.  of  Zn. 

The  Process.  A  definite  weight  of,  say  i  gm.,  of  zinc  oxid  is  dis- 
solved in  dilute  hydrochloric  acid;  an  excess  of  ammonia  is  added, 
and  then  50  cc.  of  a  10  per  cent  solution  of  sodium  arsenate.  This 
mixture  is  then  diluted  to  about  750  cc.,  warmed,  and  nitric  acid  added 
until  a  slight  turbidity  appears;  then  acetic  acid  is  added  instead, 
i  cc.  at  a  time,  until  the  solution  reacts  acid  to  test  paper. 

The  precipitate  has  now  changed  its  character  from  curdy  and 
flocculent  to  heavy  and  granular;  this  change  being  facilitated  by 
heating  and  stirring.  The  precipitate  after  settling  is  filtered  off 
and  washed.  The  filter  is  then  punctured  and  the  precipitate  washed 
through  into  a  beaker;  50  to  60  cc.  of  dilute  hydrochloric  acid  are 
added.  The  paper  and  the  beaker  in  which  the  precipitate  was 
formed  are  washed  with  dilute  acid  until  the  solution  and  washings 
measure  85  to  100  cc.  3  gms.  of  potassium  iodid  are  then  added, 
and  after  a  few  minutes  standing,  the  titration  with  thiosulphate  is 
begun.  The  use  of  starch  as  indicator  is  unnecessary,  as  the  dis- 
charge of  the  iodin  color  gives  a  sufficiently  delicate  end-reaction. 

The  separation  of  zinc  from  calcium  and  magnesium  may  be 
made  by  taking  advantage  of  the  fact  that  calcium  and  magnesium 
are  precipitated  from  alkaline  solutions  by  sodium  arsenate,  while 
zinc  separates  from  an  acid  solution. 

The  solution  is  made  strongly  ammoniacal,  sodium  arsenate  added, 
and  the  calcium  and  magnesium  arsenates  filtered  off.  The  solution 
is  then  made  acid  with  nitric  and  acetic  acids  and  filtering  off  the 
precipitated  zinc  arsenate,  and  determining  as  above  described. 

Precipitation  as  Phosphate  (Walker).  j-  This  method  is  an 
adaptation  of  the  principles  of  Stolba's  {  method  for  magnesium. 
It  gives  better  results  with  zinc  than  with  magnesium. 

*J.A.C.S..  XXII,  353. 

f  J.  A.  C.  S.,  XXIII,  468. 

|  Chem.  Centrabl.,  1866.  727;   see  also  pages  311  and  394,  this  book. 


436  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

The  Process.  "To  the  zinc  solution,  which  should  contain  ammo- 
nium chlorid,  a  large  excess  of  ammonia  is  added;  then  a  large  excess 
of  sodium  phosphate.  The  solution  remains  clear,  but  if  the  excess 
of  ammonia  is  cautiously  neutralized,  a  white  cloud  is  formed,  as 
each  drop  of  the  acid  falls  into  the  strong  ammoniacal  liquid.  On 
stirring,  this  cloud  dissolves  until  nearly  all  the  ammonia  is  neutralized, 
when  the  whole  solution  becomes  milky. 

"  It  should  now  be  heated  to  about  75°  C.  and  stirred  constantly, 
at  the  same  time  continuing  the  addition  of  dilute  acid,  drop  by  drop. 
In  a  very  few  minutes  the  precipitate  becomes  crystalline,  and  with 
care  the  liquid  may  be  almost  perfectly  neutralized.  It  is  a  good 
plan  to  add  a  small  piece  of  litmus-paper  to  the  liquid;  this  should 
not  turn  red  but  should  remain  blue  or  violet,  while  the  hot  liquid 
should  have  no  odor  or  only  a  very  faint  odor  of  ammonia.  When 
the  precipitation  is  made  as  above,  the  zinc  ammonium  phosphate  is 
easily  filtered,  which  may  be  safely  done  after  five  minutes  standing. 
The  precipitate  should  be  washed  with  cold  water  until  the  washings 
show  only  a  faint  trace  of  chlorids;  then  the  paper  with  the  precipitate 
is  returned  to  the  beaker  in  which  the  precipitation  was  made,  an 
excess  of  standard  acid  added,  a  few  drops  of  methyl  orange,  and 
the  exact  point  of  neutrality  determined  with  standard  alkali.  Accord- 
ing to  the  equation 

ZnNH4P04+H2S04=ZnS04+NH4H2P04, 

we  see  that  i  cc.  of  the  normal  acid  corresponds  to  0.03245  gm.  Zn." 

Since  the  zinc  ammonium  phosphate  is  not  precipitated  in  the 
presence  of  a  large  excess  of  ammonia,  the  process  may  be  used  in  the 
presence  of  magnesium,  which  is  precipitated  in  the  strongly  alkaline 
liquid,  and  the  nitrate  neutralized  to  precipitate  the  zinc. 

REFERENCES 

"Assay  of  Zinc  Ores."    A.  H.  Low.    J.  A.  C.  S.,  xxii,  198. 

"Assay  of  Zinc  Ores."     W   G.  Waring.     J.  A.  C.  S.,  xxvi,  4. 

"Comparison  of  Methods  of  Zinc  Analyses."  Nissenson  and  Kettem- 
beil.  Chem.  Ztg.,  xxix,  951. 

"A  Study  of  the  Ferrocyanid  Method."  W.  H.  Seaman.  J.  A.  C.  S., 
xxix,  205. 

"Report  of  Committee  on  Zinc  Ore  Analyses."     J.  A.  C.  S.,  xxix,  262. 

"Tables  for  Use  with  the  Ferrocyanid  Method  for  Zinc  Analyses." 
Chem.  Eng.,  I,  190. 

"Volumetric  Method  for  Zinc."  W.  H.  Keen.  J.  A.  C.  S.,  xxx,  225 
Note  on  same  method,  Ibid.  904. 

"Assay  of  Zinc."     E.  W.  Buskett.     Mines  and  Minerals,  xxviif,  183. 

"Estimation  of  Zinc  by  Schaffner's  Method."  V.  Hassreidter.  Z. 
angew.  Chem.,  xxi  (2),  66. 


PART    III 

CHAPTER  XLIII 
SANITARY  ANALYSIS  OF  WATER 

IN  collecting  samples  of  water  great  care  must  be  exercised  in 
order  to  secure  a  fair  representation  of  the  water  and  to  avoid  the 
introduction  of  foreign  matters. 

The  samples  should  be  collected  in  clean,  stoppered  bottles  having 
a  capacity  of  about  one  gallon. 

It  is  well  to  completely  fill  the  bottle  with  water,  then  empty  it, 
and  again  fill  with  the  water  to  be  analyzed. 

In  taking  samples  from  lakes,  reservoirs,  or  slow  streams  the  bottle 
should  be  submerged,  so  as  to  avoid  collecting  any  water  that  has 
been  in  direct  contact  with  the  air. 

In  collecting  from  pump-wells  a  few  gallons  should  be  pumped 
out  before  taking  the  sample,  in  order  to  remove  that  which  has  been 
standing  in  the  pump. 

If  the  public  water-supply  is  to  be  analyzed,  take  the  water  from 
a  hydrant  communicating  directly  with  the  street  main,  and  not  from 
a  cistern. 

At  the  time  of  collecting,  a  record  should  be  made  of  those  sur- 
roundings and  conditions  which  might  influence  the  character  of  the 
water,  such  as  proximity  of  cesspools,  sewers,  stables,  and  factories. 

It  should  also  be  noted  whether  the  sample  is  from  a  deep  or 
shallow  well,  a  river,  spring,  or  artesian  well. 

The  nature  of  the  soil  and  the  different  strata  of  the  locality  must 
also  be  taken  into  account. 

The  sample  should  be  kept  in  the  dark  and  analyzed  with  as  little 
delay  as  possible. 

Color.  This  may  be  taken  by  looking  down  through  a  column 
of  water  in  a  colorless  glass  tube  about  two  feet  long,  standing  upon  a 
piece  of  white  paper. 

A  comparison  is  made  with  a  second  tube  containing  distilled 
water. 

437 


438  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Another  way  of  determining  the  color  is  by  the  use  of  a  colorless 
glass  tube  two  feet  long  and  two  inches  in  diameter,  closed  at  each 
end,  with  disks  of  colorless  glass  cemented  on,  but  having  a  small 
opening  at  one  end  for  filling  and  emptying  the  tube. 

To  use  this  tube,  it  is  half  filled  with  the  water  to  be  examined 
and  placed  in  a  horizontal  position.  A  piece  of  white  paper  is  held 
at  one  end  of  the  tube,  and  then  by  looking  through  from  the  other 
end  the  color  of  the  liquid  is  observed,  and  a  comparison  of  tint 
made  between  the  lower  half  of  the  tube  containing  the  water  and 
the  upper  half  containing  air. 

Odor.  Three  or  four  ounces  of  water  are  placed  in  a  small 
flask  fitted  with  a  cork  through  which  is  passed  a  thermometer;  the 
flask  is  placed  in  a  water-bath  and  heated  to  100°  F.  The  flask  is 
then  shaken,  the  cork  withdrawn,  and  the  odor  immediately  observed. 

In  this  way,  satisfactory  and  uniform  tests  are  obtained,  and  a 
practised  nose  can  frequently  detect  the  pollution. 

Reaction.  This  may  be  determined  by  the  use  of  a  neutral 
solution  of  litmus.  If  an  acid  reaction  is  obtained,  the  water  should 
be  boiled  in  order  to  determine  if  it  is  due  to  carbonic  acid;  if  the 
red  color  disappears  upon  boiling,  the  acid  reaction  is  due  to  car- 
bonic acid. 

Phenolphthalein  or  lacmoid  may  also  be  used  for  this  purpose. 

Suspended  Matter.  A  liter  of  the  turbid  water  is  passed  through 
a  dried  and  weighed  filter.  The  filter  is  then  again  dried  and  weighed, 
and  the  increase  in  weight  represents  the  suspended  matter  in  one 
liter  of  the  water. 

TOTAL   SOLIDS 

A  platinum  dish  having  a  capacity  of  about  120  cc.  is  heated  to 
redness,  cooled  under  a  desiccator,  and  weighed.  100  cc.  of  the 
water  is  then  introduced  and  evaporated  on  a  water-bath  (the  dish 
being  placed  on  a  sheet  of  filter-paper  to  prevent  any  deposition  from 
the  water  on  the  bottom)  until  the  residue  appears  dry.  The  dish 
and  residue  are  then  placed  in  an  air  oven  (kept  at  a  temperature  of 
about  105°  C.)  for  thirty  minutes,  cooled  in  a  desiccator,  and  weighed. 
In  waters  of  exceptional  purity  it  may  be  advisable  to  use  a  larger 
quantity,  such  as  250  cc. 

The  increase  in  weight  of  the  dish  represents  approximately  the 
total  solids  contained  in  the  water  taken. 

It  has  been  found  that  the  figure  for  total  solids  obtained  thus 
does  not  truly  represent  the  sum  of  the  organic  and  mineral  matters 
in  all  cases. 

Experiments   have    been    made   with   urea   dissolved   in   varying 


SANITARY  ANALYSIS  OF  WATER  439 

quantities  of  water.  Where  the  solution  contained  i  gm.  of  urea 
the  residue  after  evaporation  varied  from  0.98  to  0.007  gm. 

Besides  the  possible  loss  of  organic  matter  during  the  evapora- 
tion, some  of  the  mineral  constituents  may  retain  with  great  obstinacy 
krge  quantities  of  water  in  the  form  of  water  of  crystallization,  which 
would  cause  an  error  in  the  opposite  direction. 

Thus  the  determination  of  total  solids  is  only  an  approximation. 

ORGANIC    AND   VOLATILE   MATTER — LOSS    ON   IGNITION 

Though  the  mineral  matter  in  a  water  must  to  some  extent  be 
taken  into  account  in  judging  of  a  water,  the  organic  matter  is  of 
far  greater  importance.  The  really  injurious  matters  are  more  prob- 
ably the  organic. 

It  is  therefore  important  to  determine  as  near  as  possible  their 
quantity  and  nature. 

It  was  naturally  supposed  that  by  igniting  the  residue  obtained 
from  evaporation  of  the  water,  the  organic  matter  would  be  burned 
out,  and  that  the  loss  of  weight  would  then  represent  the  organic 
matter. 

Ignition,  however,  decomposes  other  salts  which  may  be  con- 
tained in  water,  and  may  even  volatilize  some  wholly;  therefore  the 
loss  on  ignition  cannot  be  truly  called  the  organic  matter.  Hence 
the  expressions  "Organic  and  Volatile  Matter,"  and  "Loss  on 
Ignition."  The  determination  is  made  as  follows: 

Heat  the  dish  and  residue  from  the  "total  solids"  estimation 
to  redness,  cool  in  a  desiccator,  and  weigh.  The  difference  in  the* 
weight  is  reported  as  "Organic  and  Volatile  Matter,"  or  "Loss  on 
Ignition."  The  degree  of  blackening  which  takes  place,  gives  some 
idea  of  the  probable  amount  of  organic  matter  present. 

CHLORIN 

may  be  estimated  by  the  use  of  decinormal  or  centinormal  silver 
nitrate  solution;  but  analysts  generally  use  a  solution  of  such  strength 
that  i  cc.  will  represent  o.ooi  gm.  of  chlorin. 

Standard  Silver  Nitrate  Solution.  Dissolve  4.8022  gms.  of  pure 
recrystallized  silver  nitrate  in  sufficient  water  to  make  1000  cc. 

Potassium  Chromate  Solution.  Five  grams  of  neutral  potassium 
chromate  are  dissolved  in  100  cc.  of  water  and  a  weak  solution  of 
silver  nitrate  added,  drop  by  drop,  until  a  slight  permanent  red  pre- 
cipitate is  produced,  which  is  allowed  to  settle  in  the  bottle,  or  sepa- 
rated by  nitration. 

The  Process.  Measure  out  100  cc.  of  the  water  to  be  analyzed  into 
a  beaker  or  white  basin;  add  a  few  drops  of  the  potassium  chromate 


440  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

solution;  then  run  in  slowly  from  a  burette  the  silver  nitrate  solution 
until  a  slight  red  tint  appears.  Note  the  number  of  cc.  of  silver 
solution  used.  Each  cc.  represents  o.ooi  gm.  (i  milligram)  of  chlorin. 
If  the  chlorin  is  present  in  small  quantity,  about  250  cc.  of  the  water 
should  be  evaporated  to  about  one  fifth  before  titrating  with  the 
silver  nitrate  solution.  If  the  basin  which  is  being  employed  in  the 
titration  is  placed  next  to  a  similar  basin  containing  the  same  amount 
of  potassium  chromate  and  a  volume  of  distilled  water  equal  to  the 
sample,  the  reading  of  the  end-reaction  is  very  much  simplified. 

Example.  100  cc.  of  water  taken,  4  cc.  of  silver  solution  con- 
sumed; thus  showing  that  the  100  cc.  of  water  contained  0.004  gm. 
of  chlorin. 

For  method  of  stating  results  see  Calculation  of  Results. 

The  water  must  be  perfectly  neutral  before  titration.  If  acid, 
it  must  be  shaken  with  a  little  pure  precipitated  calcium  carbonate. 

AMMONIA 

When  organic  matter  decomposes  spontaneously,  it  first  forms 
ammonia,  then  nitrites,  and  finally  nitrates.  Thus  the  presence  of 
ammonia  in  water  is  generally  conceded  to  indicate  decomposing 
organic  matter,  and  hence  its  determination  is  an  important  part  of 
the  sanitary  examination  of  water. 

The  ammonia  is  generally  spoken  of  as  free  ammonia  and  albu- 
minoid ammonia,  or,  more  properly,  as  ammonium  salts  and  ammonia 
from  organic  nitrogen. 

The  sanitary  examination  of  a  water  should  always  include  a 
quantitative  determination  of  nitrogen  in  both  compounds. 

The  process  requires  several  solutions  and  considerable  care  in 
manipulation.  The  solutions  required  are: 

1.  Nessler's  Solution,   made  by  dissolving  35  gms.   of  potassium 
iodid  in  100  cc.  of  water  and  17  gms.  of  mercuric  chlorid  in  300  cc. 
of  water.     The  liquids  may  be  heated  to  aid  solution,  but  if  so,  must 
be  again  cooled.     When  solution  is  complete,  add  the  latter  to  the 
former  until  a  permanent  precipitate  is  produced;   then  dilute  with  a 
20   per   cent    solution    of   sodium   hydroxid    to    1000  cc.     Now    add 
mercuric    chlorid   solution    again    until    a    permanent    precipitate    is 
formed.     Let   the   mixture  stand  until  settled,   then   decant   off  the 
clear  solution  for  use.     The  bulk  of  the  solution  should  be  kept  in  a 
well-stoppered   bottle,   and  a  small   quantity  transferred  from   time 
to  time  to  a  small  bottle,  from  which  it  should  be  used.     This  solution 
improves   on  keeping,  and  reacts  with  extremely  minute  quantities 
of  ammonia. 

2.  Sodium  Carbonate  Solution.     A  20  per  cent  solution  of  pure 
freshly-ignited  sodium  carbonate  in  water  free  from  ammonia. 


SANITARY   ANALYSIS  OF   WATER  441 

3.  Standard   Ammonium    Chlorid   Solution.     Dissolve   0.3138  gm. 
of  pure  ammonium  chlorid  in  water  to  100  cc.     For  use  dilute  i  cc. 
of  this  solution  with  99  cc.  of  distilled  water  free  from  ammonia. 
Each  cc.  of  this  solution  contains  o.ooooi  gm.  of  ammonia. 

4.  Alkaline  Potassium  Permanganate  Solution.     Dissolve  200  gms. 
of  pure  potassium  hydroxid  and  8  gms.  of  pure  potassium  perman- 
ganate in  sufficient  ammonia-free  water  to  make  1000  cc. 

5.  Ammonia-free  Water.     If  the  distilled  water  of  the  laboratory 
gives  a  reaction  with  Nessler's  solution,  it  should  be  treated  with 
sodium  carbonate,  about  i  gm.  to  the  liter,  and  boiled  until  one  fourth 
has  been  evaporated. 

A  good  clear  hydrant  water  when  treated  with  sodium  carbonate 
and  distilled  yields  ammonia-free  water.  The  first  portion  which  comes 
over  has  of  course  some  ammonia  in  it,  and  small  portions  of  the 
distillate  should  be  tested  with  Nessler's  reagent  until  no  more  reaction 
is  obtained;  the  remainder,  except  the  very  last  portion,  should  be 
collected. 

Ammonia-free  water  may  also  be  obtained  by  distilling  water 
acidulated  with  sulphuric  acid.  In  the  first  two  processes  the  ammonia 
is  converted  into  a  volatile  salt  and  is  easily  dissipated,  or  appears 
in  the  first  distillate;  in  the  last  process  it  is  converted  into  a  non- 
volatile salt,  which  does  not  distil  over. 

Apparatus  Required.  A  round  bottom  flask  having  a  capacity 
of  about  1000  cc.,  fitted  with  a  one-holed  rubber  stopper  (which  has 
been  boiled  in  a  solution  of  potassium  hydroxid,  until  free  from 
ammonia  and  then  boiled  in  distilled  water),  through  which  passes 
a  tube  projecting  for  a  considerable  distance  into  the  tube  of  a  Liebig 
condenser.  The  tube  of  the  flask  and  the  Liebig  condenser  are 
connected  by  a  rubber  connection,  thus  making  the  apparatus  air- 
tight at  this  point.  To  obtain  the  most  satisfactory  results,  the  tube 
from  the  flask  should  be  of  block  tin  and  should  lead  to  an  upright 
condenser  (which  is  preferably  of  copper).  Instead  of  a  flask  a 
retort  may  be  used,  as  shown  in  Fig.  81. 

The  heat  is  applied  by  means  of  a  low-temperature  burner,  the 
iron  ring  of  which  is  removed,  so  that  the  retort  rests  directly  upon 
the  gauze. 

Cylinders  for  Comparative-color  Tests.  These  cylinders  are  made 
of  pure  colorless  glass,  about  one  inch  in  diameter,  having  a  capacity 
of  about  100  cc.  and  graduated  at  50  cc.  These  should  either  have 
a  milk-glass  foot,  or  should  stand  upon  white  paper.  Six  or  more 
of  these  are  required. 

The  Process.  The  flask  and  condenser  are  thoroughly  rinsed 
with  ammonia -free  water.  Then  about  200  cc.  of  ammonia-free 
water  are  introduced  and  about  10  cc.  of  sodium  carbonate  solution 


442 


A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


added  to  make  the  water  alkaline.  The  whole  is  then  boiled  by  placing 
the  flask  on  a  piece  of  gauze  over  a  Bunsen  burner,  until  50  cc.  of  dis- 
tillate are  obtained.  This  distillate  is  transferred  to  one  of  the  color- 
comparison  cylinders,  and  2  cc.  pf  Nessler's  reagent  added.  If  a 
yellow  color  is  produced,  which  develops  more  fully  in  three  or  five 
minutes,  ammonia  is  present,  either  from  imperfect  cleansing  of  the 
apparatus  of  from  the  sodium  carbonate.  In  this  event  the  distilla- 
tion must  be  carried  on  until  the  distillate  no  longer  reacts  with 
Nessler's  reagent.  The  apparatus  is  then  known  to  be  ammonia- 


FIG   81. 

free.  500  cc.  of  the  water  under  analysis  are  then  introduced,  made 
alkaline  with  10  cc.  of  sodium  carbonate  solution,  and  distilled. 
Each  50  cc.  of  distillate  is  then  Nesslerized  as  follows,  until  the 
distillates  cease  to  give  a  reaction,  thus  indicating  that  all  the  free 
ammonia  has  been  expelled. 

Prepare  several  standard  ammonia  tubes  by  adding  to  tube  No.  i, 
£  cc.  ammonium  chlorid  solution;  to  tube  No.  2,  i  cc.  of  ammonium 
chlorid  solution;  to  tube  No.  3,  ij  cc.;  to  tube  No.  4,  2  cc. ;  and  to 
tube  No.  5,  -2\  cc.  The  tubes  then  contain  respectively  0.000005  gm-> 
o.ooooi  gm.,  0.000015  gm->  0.00002  gm.,  and  0.000025  gm-  °f  ammonia, 


SANITARY  ANALYSIS  OF  WATER  443 

and  are  filled  to  the  5o-cc.  mark  with  ammonia-free  water.  2  cc.  of 
Nessler's  reagent  is  then  added  to  each  tube.  The  amount  of  ammonia 
is  then  determined  in  each  50  cc.  of  distillate  by  adding  2  cc.  of  Nessler's 
reagent  and  matching  the  color  so  produced  with  the  standard  ammonia 
tubes,  until  one  is  found  which  exactly  matches  the  color  of  the  dis- 
tillate. Knowing  the  strength  of  the  standard  it  is  easy  to  calculate 
the  amount  of  ammonia  in  the  distillate.  The  amount  in  each 
distillate  is  noted;  the  total  being  the  amount  of  ammonia  in  the 
500  cc.  of  water  under  analysis.  Example: 

First      50  cc.  of  distillate  tube,  No.  5 0.000025  gm-  ammonia 

Second      "     "         "          "      "     3 0.000015   " 

Third        "     "        "          "      "     i 0.000005   " 

Fourth      "     "         "       no  effect. 


, .       0.000045   ' 

The  residue  in  the  retort  serves  for  the  determination  of  the 
nitrogen  of  the  organic  matter  (albuminoid  ammonia),  which  is  con- 
verted by  the  alkaline  permanganate  into  ammonia. 

50  cc.  of  the  alkaline  permanganate  are  placed  in  a  porcelain 
dish  of  about  150  cc.  capacity,  the  dish  nearly  filled  with  distilled 
water,  and  then  the  liquid  boiled  down  to  50  cc. 

This  is  added  to  the  residue  in  the  retort,  the  distillation  resumed, 
and  the  ammonia  estimated  in  each  50  cc.  as  described.  Another  way 
is  to  continue  the  distillation  after  the  addition  of  the  alkaline  perman- 
ganate until  the  distillate  no  longer  gives  a  reaction  for  ammonia. 
This  insures  absence  of  ammonia  which  might  be  accidently  intro- 
duced with  the  reagents  or  otherwise.  The  apparatus  being  ammonia- 
free,  now  introduce  500  cc.  of  the  water  under  analysis  and  distil; 
Nesslerize  each  50  cc.,  as  described  before.  Deduct  the  amount  of 
free  ammonia  before  found  from  the  total  found  in  this  determination 
and  the  difference  will  be  the  "albuminoid  ammonia." 

It  is  the  practice  of  some  analysts  to  mix  the  distillates  of  each 
of  the  above  operations,  and  thus  make  determinations  merely  of 
the  total  quantity  of  ammonia  and  albuminoid  ammonia.  By  so 
doing  valuable  information  may  be  lost,  since  it  has  been  pointed  out 
that  the  ammonia  may  be  differently  distributed  in  the  distillates, 
according  to  the  state,  decomposing  or  otherwise,  in  which  the  ammonia 
exists  in  the  water.  If  the  ammonia  distils  over  very  rapidly,  it  indicates 
that  the  organic  matter  is  in  a  putrescent  or  decomposing  condition. 

If,  on  the  other  hand,  it  distils  gradually,  it  indicates  the  presence 
of  organic  matter  in  a  comparatively  stable  or  fresh  condition.  It 
is  best,  therefore,  to  keep  the  record  of  each  distillate,  so  that  the 
rapidity  with  which  the  ammonia  is  set  free,  as  well  as  the  actual 
amount,  may  be  known. 


444  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

The  greatest  care  should  be  exercised  in  order  to  avoid  the  intro- 
duction of  ammonia  in  any  way  during  the  course  of  the  analysis, 
since  small  quantities  of  ammonia  compounds  and  nitrogeneous 
matters  are  everywhere  present.  All  measuring  vessels,  cylinders, 
etc.,  should  be  thoroughly  rinsed  with  ammonia-free  water  before 
using. 

Determinations  of  ammonia  to  be  accurate  should  be  performed 
in  a  room  removed  from  the  main  laboratory,  where  ammoniacal 
fumes  from  reagents  will  by  no  means  contaminate  the  atmosphere. 

NITROGEN    AS    NITRATES 

Solutions  Required.  Acid  Phenyl  Sulphate.  370  gms.  of  strong 
sulphuric  acid  are  added  to  30  gms.  of  pure  phenol  and  placed  in  a 
flask,  which  is  submerged  in  boiling  water  for  six  hours.  This 
reagent  should  be  preserved  in  a  glass-stoppered  bottle. 

Standard  Potassium  Nitrate.  0.722  gm.  of  pure  potassium  nitrate, 
previously  heated  to  a  temperature  just  sufficient  to  fuse  it,  is  dis- 
solved in  water,  and  the  solution  made  up  to  1000  cc.  i  cc.  of  this 
solution  will  contain  o.oooi  gm.  of  nitrogen. 

The  Process.  A  measured  volume  of  water  is  evaporated  just  to 
dryness  in  a  platinum  or  porcelain  dish,  having  previously  added 
-fa  cc.  of  a  saturated  solution  of  sodium  carbonate.  2  cc.  of  the  acid 
phenyl  sulphate  are  added  and  thoroughly  mixed  with  the  residue  by 
means  of  a  glass  rod,  and  the  dish  gently  warmed.  The  liquid  is  then 
diluted  with  about  25  cc.  of  water,  a  slight  excess  of  ammonium 
hydroxid  (about  5  cc.)  added,  and  the  solution  made  up  to  100  cc. 

The  color  produced  is  compared  with  that  of  prepared  tubes  of 
standard  nitrate  solution  as  follows:  10  cc.  of  standard  potassium 
nitrate  solution  are  now  similarly  evaporated  in  a  platinum  or  porce- 
lain basin  on  a  water-bath,  just  to  dryness,  and  the  residue  moistened 
with  2  cc.  of  the  acid  phenyl  sulphate,  and  mixed  thoroughly  with  a 
glass  rod.  A  small  amount  of  water  is  added  and  the  mixture  made 
up  to  1000  cc.  with  distilled  water.  Each  cc.  of  this  solution  == 
o.oooooi  gm.  of  nitrogen  from  nitrates.  Definite  but  varying  amounts 
of  this  solution  are  then  measured  into  Nessler  tubes  of  100  cc.  capacity. 
An  excess  of  ammonia  water  (about  5  cc.)  is  added  to  each  tube  and 
then  distilled  water  to  the  mark.  The  matching  of  the  colors  is  then 
done  in  a  similar  manner  to  that  described  for  ammonia. 

If  100  cc.  of  water  under  analysis  are  taken  the  amount  of  nitrate 
found  will  be  that  contained  in  100  cc.  The  reactions  are: 


Acid  phenyl  sulphate.  Trinitrophenal  (picric  acid). 

C6H2(OH)(N02)3+NH40H=C6H20(NH4)(N02)3+H20, 

Ammonium  picrate. 


SANITARY   ANALYSIS  OF  WATER  445 

The  nitric  acid  used  in  the  above  equation  is  derived  from  the 
potassium  nitrate  by  the  action  of  sulphuric  acid. 

The  ammonium  picrate  imparts  a  yellow  color  to  the  solution, 
the  intensity  of  which  is  proportional  to  the  amount  present. 

Care  should  be  taken  that  the  same  quantity  of  acid  phenyl  sul- 
phate is  used  for  the  water  and  for  the  comparison  liquid,  otherwise 
different  tints  instead  of  depths  of  tints  are  produced. 

With  river  or  spring  waters  25  to  100  cc.  should  be  evaporated  for 
the  test,  but  with  subsoil  and  other  waters  which  probably  contain 
much  nitrates  10  cc.  will  be  sufficient. 


NITROGEN   AS    NITRITES 

Solutions  Required,  i.  Naphthylammonium  Chlorid  (Naph- 
thylamin  Hydrochlorate).  Saturated  solution  in  water  free  from 
nitrites.  It  should  be  colorless  (0.5  gm.  dissolved  in  100  cc.  of  boiling 
water).  This  solution  should  be  kept  in  a  glass-stoppered  bottle  with 
a  little  animal  charcoal,  which  will  keep  the  solution  colorless. 

2.  Sulphanilic  Acid  (Para-amido-benzene — Sulphonic  Acid}.  A  sa- 
turated solution  in  water  free  from  nitrites  (i  gm.  in  100  cc.  of  hot  water). 

Hydrochloric  Acid.  25  cc.  of  concentrated  pure  hydrochloric 
acid  mixed  with  75  cc.  of  water  free  from  nitrites. 

Standard  Sodium  Nitrite.  0.220  gm.  pure  silver  nitrite  is  dis- 
solved in  pure  water,  and  a  dilute  solution  of  pure  sodium  chlorid 
added  until  a  precipitate  ceases  to  form.  It  is  then  diluted  with  pure 
water  to  1000  cc.  and  allowed  to  stand  until  clear.  For  use  5  cc. 
of  this  solution  are  diluted  to  1000  cc.  It  must  be  kept  dark,  i  cc. 
of  the  dilute  solution  is  equivalent  to  o.ooooooi  gm.  of  nitrogen. 

A  standard  solution  of  silver  nitrite  is  used  by  some  chemists, 
but  the  above  is  said  to  give  better  results. 

The  Process.  IOQ  cc.  of  the  water  are  placed  in  one  of  the  color- 
comparison  cylinders,  the  measuring  vessels  and  cylinder  having 
previously  been  rinsed  with  the  water  to  be  tested.  By  means  of  a 
pipette  introduce  into  the  water  i  cc.  each  of  the  solutions  of  sulphanilic 
acid,  dilute  hydrochloric  acid,  and  naphthylammonium  chlorid 
in  the  order  named.  It  is  convenient  to  have  three  pipettes — one 
for  each  of  these  solutions,  and  to  use  them  for  no  other  purpose. 
In  all  cases  the  pipettes  should  be  rinsed  with  ammonia-free  water 
before  using  them.  Into  a  series  of  100  cc.  Nessler  tubes,  introduce 
varying  but  definite  amounts  of  the  standard  nitrite  solution  and 
make  up  to  100  cc.  with  pure  water;  then  add  the  same  reagents  as 
were  added  to  the  water  in  the  other  cylinder. 

A  pink  color  is  produced  in  the  presence  of  nitrites,  which  re- 
quires in  dilute  solutions  half  an  hour  for  complete  development. 


446  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

At  the  end  of  that  time  the  sample  under  analysis  is  matched,  and 
the  calculation  made  as  explained  under  nitrates. 

The  reactions  are  explained  by  the  following  equations: 


C2H4NH2HSO3  +  HNO2  = 

Sulphanilic  acid.  Para-diazo-benzene-sulphonic  acid. 

C6H4N2S03  +  Ci0H7NH3Cl=  CioH6(NH2)NNC6H4HS03+HCl. 

Naphthammonium          Azo-alpha-amido-naphthalene- 
chlorid.  parazo-benzene-sulphonic  acid. 

The  last-named  body  gives  the  color  to  the  liquid. 


OXYGEN-CONSUMING  POWER 

Potassium  permanganate  readily  yields  up  its  oxygen,  especially 
in  the  presence  of  a  strong  mineral  acid,  as  sulphuric.  It  oxidizes 
many  salts  and  organic  matter. 

This  property  lead  to  the  idea  that  this  salt  may  be  used  for  burn- 
ing up  (chemically  speaking)  the  organic  matter  in  water,  and  that 
the  quantity  of  permanganate  used  could  be  relied  upon  as  a  means 
of  measuring  the  organic  matter  in  water. 

This  method  does  not  distinguish  between  animal  and  vegetable 
matter,  nor  does  the  quantity  of  permanganate  consumed  represent 
only  the  organic  matter. 

The  organic  matters  in  water  are  very  variable  in  character  and 
condition,  and  their  oxidability  is  subject  to  much  difference. 

Nevertheless,  as  a  high  oxygen-consuming  power  certainly  indi- 
cates pollution  by  organic  matter,  the  process  is  of  considerable  value. 

The  following  is  a  convenient  method  for  approximating  the 
oxygen-consuming  power  of  a  water: 

Solutions  Required.  Potassium  Permanganate.  0.3952  gm.  of 
pure  potassium  permanganate  is  dissolved  in  distilled  water,  and 
the  solution  made  up  to  1000  cc.  i  cc.  of  this  solution  will  yield 
under  favorable  circumstances  o.oooi  gm.  of  oxygen. 

Diluted  Sulphuric  Acid.  50  cc.  of  pure  sulphuric  acid  are  mixed 
with  100  cc.  of  water,  and  then  just  sufficient  of  the  permanganate 
solution  added  to  give  the  mixture  a  faint  pink  color,  which  remains 
after  standing  in  a  warm  place  four  hours. 

The  Process.  Five  stoppered  bottles  having  a  capacity  of  500  cc. 
are  thoroughly  cleansed  with  strong  sulphuric  acid  and  then  carefully 
rinsed  with  pure  water,  and  250  cc.  of  the  water  to  be  tested  put  into 
each  one.  10  cc.  of  the  dilute  sulphuric  acid  is  then  added  to  each, 
together  with  regularly  increasing  quantities  of  the  standard  perman- 
ganate, say  2,  4,  6,  8,  and  10  cc.  respectively. 


SANITARY   ANALYSIS   OF   WATER  447 

At  the  end  of  an  hour  they  should  be  examined,  to  see  which,  if 
any,  are  decolorized.  At  the  end  of  the  fourth  hour  they  should  again 
be  examined,  and  again  at  the  expiration  of  twenty-four  hours. 

If  all  of  the  bottles  are  decolorized  at  or  before  the  fourth  hour 
an  additional  10  cc.  of  the  permanganate  solution  should  be  added 
to  each  bottle. 

With  ordinary  waters  the  first  and  probably  the  second  bottle 
will  be  decolorized,  while  a  little  color  will  remain  in  the  third,  and 
the  color  in  the  fourth  and  fifth  will  be  but  little  diminished.  In 
this  way  an  approximate  figure  for  the  oxygen-consuming  power  of 
the  water  may  be  obtained,  which  in  most  cases  is  all  that  is  necessary. 
If  a  closer  figure  is  desired,  the  experiment  may  be  repeated,  using 
quantities  of  permanganate  intermediate  between  those  marking  the 
limits  of  the  reaction. 

Thus  if  the  second  bottle  is  decolorized  and  a  faint  color  still 
remains  in  the  third,  repeat  the  experiment  with  5  cc.  of  the  per- 
manganate. 

This  method  of  procedure  has  an  advantage  over  some  of  the 
other  processes,  because  the  rate  of  oxidation  can  easily  be  seen. 
This  is  considered  by  some  to  be  of  more  importance  than  the  actual 
amount  of  oxygen  consumed. 

It  must  be  remembered  that  nitrites,  ferrous  salts,  sulphids,  etc., 
consume  oxygen  as  well  as  organic  matter.  It  is  therefore  important 
to  boil  water  containing  hydrogen  sulphid  in  order  to  drive  the  latter 
off.  Nitrites  may  be  removed  by  treating  the  water  with  sulphuric 
acid,  and  boiling.  The  nitrite  is  thus  converted  into  nitrous  acid, 
which  is  driven  off  by  the  heat.  Or  the  oxygen  required  to  convert 
the  nitrites  present  into  nitrates  may  be  deducted  from  the  total 
amount  of  oxygen  consumed.  Fourteen  parts  of  nitrogen  as  nitrite 
require  sixteen  parts  of  oxygen  for  oxidation  into  nitrate. 

PHOSPHATES 

Solutions  Required.  Ammonium  Molybdate.  Made  by  dis- 
solving 10  gms.  of  molybdic  anhydrid  in  a  mixture  of  15  cc.  of  con- 
centrated ammonia  (sp.gr.  0.900)  and  25  cc.  of  water.  This  solu- 
tion is  poured  slowly,  and  with  constant  stirring,  into  a  mixture  of 
65  cc.  of  concentrated  nitric  acid  (sp.gr.  1.4)  and  65  cc.  of  water,  and 
allowed  to  stand  until  clear.  It  should  be  kept  dark. 

The  Process.  One  liter  of  the  water  is  evaporated  to  about  50  cc.; 
a  few  drops  of  a  dilute  solution  of  ferric  chlorid  are  added,  followed 
by  a  slight  excess  of  ammonia.  Ferric  hydroxid  is  thus  precipitated, 
which  carries  down  with  it  all  the  phosphate.  This  precipitate  is 
separated  by  filtration,(  dissolved  on  the  filter  in  the  smallest  possible 


448  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

quantity  of  hot  dilute  nitric  acid,  and  a  little  water  passed  through 
the  filter.  The  filtrate  and  washings  should  not  exceed  5  cc.,  and 
should  if  more,  be  evaporated  to  this  bulk. 

The  solution  is  now  heated  nearly  to  boiling  and  2  cc.  of  the 
ammonium  molybdate  solution  added.  If  after  half  an  hour  an 
appreciable  precipitate  is  formed,  it  is  collected  on  a  small  weighed 
filter  and  its  weight  found  after  thorough  drying.  This  weight,  multi- 
plied by  0.05,  gives  the  amount  of  PO4.  If  the  quantity  is  too  small 
to  be  collected  and  weighed  in  this  manner,  it  is  usually  reported  as 
"traces,"  "heavy  traces,"  or  "very  heavy  traces." 

HARDNESS 

The  hardness  of  water,,  that  is,  its  soap-destroying  power,  is  due 
principally  to  the  presence  of  calcium  salts;  but  salts  of  magnesium, 
iron,  and  other  metals  may  also  contribute  to  this  effect. 

Two  kinds  of  hardness  are  recognized: 

1.  "Temporary,"  which  is  due  to  the  presence  in  water  of  the 
bicarbonates    of  calcium,    magnesium,    etc.      By  boiling,   these    salts 
are   decomposed,   the   carbonic   acid  gas   being  driven   off,   and   the 
neutral  carbonate   formed,   which   is   precipitated.     Thus   the   water 
loses  its  hardness  upon  boiling. 

CaH2(C03)2=CaC03+H20  +  C02. 

2.  "Permanent  "  hardness  is  due  to  the  presence  in  water  of  salts 
of  the  above-mentioned  metals  which  are  not  removed  by  boiling, 
such  as  the  sulphates. 

Hardness  is  estimated  by  means  of  a  standard  soap  solution. 

Many  samples  of  water  possess  both  temporary  and  permanent 
hardness,  and  it  is  sometimes  desirable  to  estimate  them  sepa- 
rately. 

The  total  hardness  is  estimated  in  one  sample,  and  the  hardness 
in  another  sample  is  determined  after  boiling  and  filtering  off  the 
precipitated  calcium  carbonate. 

The  hardness  found  after  boiling  is  the  permanent  hardness,  and 
is  the  most  objectionable  form.  The  difference  between  the  total 
and  permanent  hardness  is  the  temporary  hardness.  To  express 
the  hardness  in  some  tangible  form,  the  usual  custom  in  this  country 
and  in  England  is  to  give  results  in  the  corresponding  amounts  of 
calcium  carbonate,  i.e.,  practically  to  determine  the  amount  of 
soap  destroyed  by  a  measured  quantity  of  water,  and  then  to  state 
the  results  as  the  amount  of  calcium  carbonate  which  would  destroy 
that  quantity  of  soap. 


SANITARY   ANALYSIS  OF  WATER  449 

The  reaction  which  takes  place  when  soap  is  added  to  a  hard 
water,  is  illutsrated  in  the  following  equations: 


Acid  calcium          Sodium  stearate        Calcium  stearate. 
carbonate.  (Soap). 


or, 


CaSO4  + 

Calcium  sulphate. 


The  calcium  stearate,  which  is  an  insoluble  calcium  soap,  is  pre- 
cipitated in  both  cases  as  a  white  curd-like  mass. 

The  method  for  estimating  hardness  in  water  by  the  use  of  soap 
solution  is  known  as  Clark's  method. 

Solutions  Required.  Standard  Soap  Solution.  Dissolve  10  gms. 
of  shavings  of  air-dried  Castile  soap  in  a  liter  of  dilute  alcohol. 
Filter  the  solution  if  it  is  not  clear,  and  keep  it  in  a  tightly-stoppered 
bottle. 

Standard  Calcium  Chlorid  Solution.  Dissolve  i  gm.  of  pure 
calcium  carbonate  in  the  smallest  excess  of  hydrochloric  acid,  then 
carefully  neutralize  with  ammonia  water,  and  add  sufficient  water  to 
make  up  to  one  liter. 

One  cc.  of  this  solution  will  contain  the  equivalent  of  o.ooi  gm. 
of  calcium  carbonate.  This  solution  is  used  for  determining  the 
strength  of  the  soap  solution,  which  is  done  as  follows : 

Measure  10  cc.  of  this  solution  into  a  glass -stoppered  bottle  capable 
of  holding  about  250  cc.;  add  90  cc.  of  distilled  water,  and  run  in  the 
soap  solution,  drop  by  drop,  from  a  burette  until  a  lather  is  formed, 
which  remains  for  five  minutes,  and  extends  over  the  entire  surface 
of  the  liquid  when  the  bottle  is  placed  in  a  horizontal  position.  Note 
the  number  of  cc.  of  soap  solution  used. 

We  now  repeat  the  experiment  with  100  cc.  of  distilled  water. 
The  amount  of  the  soap  solution  required  to  produce  a  permanent 
lather  with  the  distilled  water  must  be  deducted  from  the  amount 
used  in  the  first  test.  Usually  it  will  be  about  one  half  or  one  cc. 

The  10  cc.  of  the  calcium  chlorid  solution  contained  the  equiv- 
alent of  o.oio  gm.  of  CaCOs.  Suppose  in  the  above-mentioned  test 
8.5  cc.  of  the  soap  solution  were  used  to  produce  a  permanent  lather, 
and  0.5  cc.  were  used  by  the  distilled  water.  Then  8  cc.  were  used 
to  precipitate  o.oio  gm.  of  CaCOs.  Thus  each  cc.  of  this  soap 
solution  will  represent  J  of  o.oio  gm.  =  0.0012 5  of  calcium  carbonate. 

The  soap  solution  may  either  be  used  as  it  is,  or  it  may  be  diluted 
with  dilute  alcohol  so  that  about  10.5  or  n  cc.  of  it  will  be  required 
to  produce  a  permanent  lather  with  10  cc.  of  the  standard  calcium 


450  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

chlorid  solution.  If  so  diluted  each  cc.  will  represent  o.ooi  gm.  of 
CaC03. 

This  is  a  convenient  strength,  because  if  100  cc.  of  water  are 
operated  upon,  each  cc.  of  the  soap  solution  used  will  represent  i  part 
of  CaCOs  in  100,000  parts  of  water. 

Measure  100  cc.  of  the  water  under  analysis  into  the  well-stoppered 
bottle.  Add  the  soap  solution  gradually  from  a  burette  in  fractions 
of  a  cc.  at  a  time,  shaking  well  after  each  addition  until  a  soft  lather 
is  obtained,  which,  if  the  bottle  is  placed  at  rest  on  its  side,  remains 
continuous  over  the  whole  surface  for  five  minutes. 

The  soap  should  not  be  added  in  large  quantities  at  a  time,  even 
if  the  volume  required  is  approximately  known. 

If  magnesium  salts  are  present,  a  kind  of  scum  (simulating  a 
lather)  will  be  seen  before  the  reaction  is  completed.  The  character 
of  this  scum  must  be  carefully  watched,  and  the  soap  solution  added 
very  carefully,  with  an  increased  amount  of  shaking  after  each  addi- 
tion. The  point  when  the  false  lather  due  to  the  magnesium  salt 
ceases  and  the  true  persistent  lather  is  produced  is  comparatively 
easy  to  distinguish. 

If  more  than  23  cc.  of  the  soap  solution  are  consumed  by  the 
100  cc.  of  water,  a  smaller  quantity  of  water  should  be  taken  (say 
50  or  25  cc.)  and  made  up  to  100  cc.  with  distilled  water,  recently 
boiled.  In  such  case  the  quantity  of  soap  solution  used  must  be 
multiplied  by  2  or  4. 

If  the  first-mentioned  soap  solution  is  used  each  cc.  represents 
0.00125  gm.  If  the  second  solution  is  used  each  cc.  represents  o.ooi  gm. 
of  CaCOs,  and  if  100  cc.  of  water  are  acted  upon  each  cc.  represents 
i  part  of  CaCOs  in  100,000. 

If  70  cc.  of  water  are  acted  upon,  instead  of  100  cc.,  each  cc.  of 
soap  solution  used  represents  i  gm.  per  70,000  cc.,  which  corresponds 
to  i  gr.  per  imperial  gallon  (70,000  grs.)  or  i  degree  of  hardness. 

These  estimations  are,  however,  only  approximate,  for  the  lather 
does  not  form  until  the  reaction  between  the  soap  and  the  calcium 
in  the  water  is  completed,  and  then  the  quantity  of  soap  solution 
required  to  produce  the  lather  depends  upon  its  strength. 

Dr.  Clark,  the  originator  of  this  method,  has  shown  that  1000  grains 
of  distilled  water  (free  from  hardness)  require  1.4  measures  of  soap 
solution,  each  measure  being  the  volume  of  10  grains  of  distilled 
water  at  16°  C. 

For  Permanent  Hardness.  To  determine  the  hardness  after 
boiling,  measure  100  cc.  of  the  water  under  analysis  into  a  flask,  and 
with  a  file  make  a  mark  on  the  flask  to  indicate  this  volume;  then 
add  100  cc.  of  distilled  water,  and  boil  until  the  amount  is  reduced 
to  the  loo-cc.  mark;  filter  rapidly,  and  test  in  the  same  manner  as 


SANITARY   ANALYSIS  OF  WATER  451 

described  above.  One  half  or  one  cc.  is  deducted  from  the  soap 
solution  used  for  the  calculation. 

Among  German  chemists  it  is  customary  to  designate  the  soap- 
destroying  power  equivalent  to  one  part  of  CaCQ  in  100,000,  as  one 
degree  of  hardness. 

Among  French  chemists  each  degree  of  hardness  represents  one 
part  of  CaCOs  in  100,000. 

INTERPRETATION  OF  RESULTS 

Statement  of  Analysis.  The  composition  of  water  is  generally  ex- 
pressed in  terms  of  a  unit  of  weight  in  a  definite  volume  of  liquid, 
but  no  fixed  standard  is  used.  The  proportions  are,  however,  most 
generally  expressed  in  parts  per  million,  this  being  the  most  satis- 
factory and  convenient,  since  there  are  1,000,000  milligrams  in  a 
liter,  it  is  readily  seen  that  the  number  of  milligrams  of  a  constituent 
present  in  a  liter  is  at  once  the  parts  per  million. 

Example.  If  in  the  determination  of  chlorin  it  is  found  that  100  cc. 
of  water  contain  0.004  gm.  (4  milligrams),  then  one  liter  will  contain 
0.040  gm.  (40  milligrams)  or  40  parts  per  million. 

Many  chemists  prefer  to  express  the  results  in  parts  per  hundred 
thousand.  Sometimes,  generally  by  English  chemists,  the  figures 
are  given  in  grains  per  imperial  gallon  of  70,000  grains ;  less  frequently, 
in  grains  per  U.  S.  gallon  of  58,328  grains. 

To  calculate  the  grains  in  one  U.  S.  gallon,  multiply  the  milli- 
grams in  a  liter  by  the  factor  0.058328.  If  it  is  desired  to  report  the 
parts  per  100,000,  the  figure  is  obtained  from  the  number  of  milli- 
grams in  100  cc.  For  example,  100  cc.  of  water  contain  4  milligrams 
(0.004  gm.),  then  there  are  4  parts  per  100,000. 

Passing  Judgment.  In  order  to  pass  judgment  upon  the  analytical 
results  from  a  sample  of  water,  the  analyst  must  know  to  which  class 
of  water  it  belongs — whether  river  water,  well  water,  or  artesian- 
well  water.  He  must  know  something  of  the  soil  and  geological  charac- 
ter of  the  locality  from  which  the  water  is  obtained,  as  well  as  other 
conditions  of  the  locality  which  might  affect  the  quality  of  the  water, 
such  as  proximity  of  stables,  cesspools,  sewers,  factories,  etc. 

Color.  Water  of  the  highest  purity  is  clear,  colorless,  odorless, 
and  nearly  tasteless.  But  the  color  of  water  is  no  indication  of  its 
quality.  A  turbid  or  colored  water  is  not  necessarily  a  dangerous 
one,  neither  is  a  clear,  colorless  water  always  a  safe  water. 

Odor.  For  comfort,  if  for  nothing  else,  potable  water  should  be 
free  from  odor.  Water  sometimes  has  an  unpleasant  odor  and  taste, 
yet  it  may  be  used  with  perfect  safety  for  domestic  purposes.  At 
other  times  the  odor  may  give  rise  to  suspicions  which  a  subsequent 


452  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

examination  may  confirm.     Thus  by  the  odor  alone  the  safety  of  the 
water  cannot  be  told. 

Total  Solids.  This  is  intended  to  represent  the  total  solid  matters 
dissolved  in  the  water;  but  since  much  of  the  organic  matter  as  well 
as  some  of  the  inorganic  matter  is  volatilized  by  evaporation,  the 
total  solids  obtained  by  this  method  are  only  the  total  non-volatile 
solids.  The  indication  is  thus  lower  than  it  should  be.  On  the  other 
hand,  certain  salts,  especially  calcium  sulphate,  retain  water  of  crystal- 
lization, thus  producing  an  effect  in  the  opposite  direction. 

The  total  solids  so  obtained  contain  both  organic  and  inorganic 
matters,  either  of  which  may  be  injurious  or  not.  Mineral  waters 
contain  large  quantities  of  inorganic  salts.  Much  smaller  quantities 
of  total  solids  in  other  waters  might  indicate  pollution. 

Large  quantities  of  mineral  solids,  especially  of  marked  physio- 
logical action,  are  known  to  render  water  non-potable;  but  no  absolute 
maximum  or  minimum  can  be  assigned  as  the  limit  of  safety.  An 
arbitrary  limit  has,  however,  been  fixed  by  sanitary  authorities  of 
60  parts  per  100,000;  and  if  the  solid  residue  does  not  exceed  57  parts 
per  100,000,  there  is  no  reason  for  rejecting  a  water.  Many  waters, 
especially  artesian  waters,  which  are  in  constant  use,  contain  much 
larger  quantities. 

The  loss  on  ignition  should  never  reach  50  per  cent -of  the  total 
residue. 

Chlorin  in  potable  waters  is  very  largely  derived  from  sodium 
and  potassium  chlorids  of  urine  and  sewage. 

Food  contains  considerable  amounts  of  chlorids,  and  still  more 
is  added  by  way  of  condiment  in  the  shape  of  salt.  The  chlorin  thus 
taken  into  the  system  is  again  thrown  off  in  the  excreta,  and  thus 
appears  in  the  sewage;  hence  the  presence  of  large  quantities  of 
chlorin  in  water  is  taken  as  an  indication  of  pollution.  Urine  contains 
about  500  parts  of  chlorin  per  100,000.  The  average  quantity  found 
in  sewage  is  about  11.5  parts  per  100,000.  Over  5  parts  per  100,000 
of  chlorin  in  a  water  may  be  considered,  in  most  cases,  to  be  due  to 
pollution  of  the  water  by  sewage  or  animal  excretions.  The  chlorin 
itself  is  not  a  dangerous  constituent  of  water,  but  its  presence  in  large 
quantities  is  an  unfavorable  indication.  Nevertheless  too  much 
dependence  must  not  be  put  upon  the  amount  of  chlorin  in  water 
as  a  means  of  judging  of  its  purity,  for  dangerous  vegetable  matter 
may  exist  in  it  without  its  presence  being  indicated  by  chlorin.  The 
maximum  amount  of  chlorin  per  100,000,  given  by  the  Rivers  Pollution 
Commission,  is  21.5  parts,  the  minimum  6.5  parts.  Various  condi- 
tions, however,  which  affect  the  proportion  of  chlorin,  such  as  the 
nature  of  the  strata  through  which  the  water  passes,  proximity  to  the 
sea,  etc.,  must  be  taken  into  account. 


SANITARY   ANALYSIS  OF  WATER  453 

Nitrogen  in  Ammonia.  Ammonium  compounds  are  usually 
the  result  of  spontaneous  putrefactive  fermentation  of  nitrogeneous 
organic  matter;  nitrites  are  then  formed,  and  finally  nitrates.  Ammo- 
nium compounds  may  also  result  from  the  reduction  of  nitrites  and 
nitrates  in  the  presence  of  excess  of  organic  matter.  Therefore  in 
either  case  the  presence  of  ammonia  suggests  contamination. 

This  fact  is  so  generally  conceded  that  the  estimation  of  ammonia 
in  water  is  a  very  important  part  of  the  sanitary  examination. 

In  the  water  from  deep  wells  an  excess  of  ammonia  is  nearly 
always  found,  but  its  presence  here  cannot  always  be  considered  an 
adverse  condition,  since  it  is  derived  largely  from  the  decomposition 
of  nitrates,  and  shows  previous  contamination;  but  the  water  having 
undergone  extensive  nitration  and  oxidation,  its  organic  matter  is 
presumably  converted  into  harmless  bodies. 

Rain-water  often  contains  large  proportions  of  ammonium  com- 
pounds, which  it  dissolves  out  of  the  air  in  its  descent;  but  here  also, 
this  fact  cannot  condemn  the  water,  since  it  does  not  indicate  con- 
tamination with  dangerous  organic  matter. 

An  average  of  71  samples  of  rain-water  collected  in  England 
contained  0.05  parts  per  100,000,  including  an  exceptional  maximum 
of  0.21  parts. 

Fischer  (Chemische  Technologic  des  Wassers)  gives  two  anal- 
yses of  typically  good  wells,  containing  respectively  0.048  and  0.044 
parts  per  100,000,  and  of  two  typically  bad  shallow  wells,  containing 
respectively  0.084  and  2.227  parts  per  100,000. 

Albuminoid  Ammonia.  If  water  yields  no  albuminoid  it  is 
free  from  recently  added  organic  contamination.  If  it  contain  more 
than  o.oi  part  per  100,000  it  is  looked  upon  as  suspicious,  and  when 
it  reaches  0.015  parts  per  100,000  it  is  to  be  condemned.  When  free 
ammonia  is  present  in  considerable  quantity,  then  the  albuminoid 
ammonia  becomes  suspicious  when  it  reaches  0.005  parts  Per  100,000. 
An  opinion  should  not,  however,  be  formulated  without  a  knowledge 
of  the  source  of  the  water;  for,  as  has  been  said  before,  free  ammonia 
may  exist  in  large  quantities  in  deep  wells  without  indicating  con- 
tamination. Wanklyn  gives  the  following  standards: 

High  purity o.ooo    to  0.0041  of  albuminoid  ammonia  per  100,000 

Satisfactory  purity  0.0041  to  0.0082 
Impure./.  ......  over  0.0082 

In  the  absence  of  free  ammonia  he  does  not  condemn  a  water 
unless  the  albuminoid  ammonia  exceeds  0.0082  parts  per  100,000; 
but  he  condemns  a  water  yielding  0.0123  Parts  of  albuminoid  ammonia, 
under  all  circumstances. 


454  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

Nitrogen  as  Nitrates.  Nitrates  are  normally  present  in  all 
natural  waters,  and  are  derived  chiefly  from  the  oxidation  of  animal 
matters.  The  nitrogen  of  organic  matters  liberated  by  putrefaction, 
is  first  converted  into  ammonia;  then  this  is  oxidized  into  nitrous 
and  finally  into  nitric  acid.  These  changes  are  due  partially  to  direct 
oxidation  and  partially  to  certain  micro-organisms  which  have  the 
power  of  converting  nitrogeneous  organic  matter  into  nitrites  and 
nitrates.  Nitrates  and  nitrites  in  themselves,  in  the  quantity  in  which 
they  exist  in  water,  are  perfectly  harmless.  They  are,  however,  an 
indication  of  previous  contamination;  and  many  analysts  believe 
that  a  water  which  has  once  been  contaminated  is  always  open  to 
suspicion.  Others  consider  them  of  little  importance  in  determining 
the  impurity  of  a  water.  Water  which  is  laden  with  organic  matter 
is  purified  by  percolating  through  the  ground,  the  nitrogeneous  matter 
being  converted  into  nitrates;  therefore  deep  wells  may  contain  large 
quantities  of  nitrates  without  being  essentially  impure,  while  the 
water  from  shallow  wells  should  be  condemned  if  the  nitrates  are 
excessive. 

Certain  strata,  as  the  chalk  formation,  yield  large  amounts  of 
nitrates  to  water.  If  the  nitrogen  as  nitrates  exceeds  0.6  parts  per 
100,  ooo  the  water  is  suspected. 

Nitrogen  as  Nitrites.  Some  chemists  regard  the  presence  of 
nitrites  as  an  indication  that  the  oxidation  of  the  dangerous  com- 
pounds has  probably  been  incomplete,  and  accordingly  condemn 
water  in  which  nitrites  are  found.  Leeds  places  the  nitrites  in 
American  rivers  at  0.03  per  100,000.  The  average  in  good  waters  is 
placed  at  about  0.0014  per  100,000.  When  the  quantity  exceeds 
0.02  parts  per  100,000  it  is  considered  an  indication  of  previous  con- 
tamination. 

Oxygen-consuming  Power.  This  is  intended  to  represent  the 
oxidizable  organic  matter  in  the  water.  But  there  are  other  substances 
in  water  besides  organic  matters  which  absorb  oxygen,  namely,  nitrites, 
which  are  thus  oxidized  to  nitrates;  ferrous  salts,  which  are  oxidized 
into  ferric  salts,  etc.  Thus  the  oxygen -consuming  power  does  not 
represent  the  organic  matter  alone.  However,  a  water  having  a  high 
oxygen -consuming  power  may  be  considered  as  polluted. 

The  following  basis  for  interpreting  results  of  this  method  are 
given  by  Frankland  and  Tidy: 

Oxygen  Absorbed  in  3  Hours. 

High  organic  purity 0.05  parts  per  100,000 

Medium  purity 0.05  to  0.15      '*      " 

Doubtful o.i5too.2i      "      "         " 

Impure over  0.2 1       "      " 


SANITARY   ANALYSIS   OF   WATER  455 

Phosphates.  Sewage  contains  large  amounts  of  phosphates,  but 
water  usually  contains  alkali  or  earthy  carbonates,  which  precipitate 
the  phosphates;  therefore  the  absence  of  phosphates  does  not  indicate 
purity.  But  their  presence  may  indicate  .sewage  contamination. 
0.06  parts  per  100,000  is  regarded  with  suspicion  (calculated  as  PO4). 

Hardness.  On  account  of  the  presence  of  considerable  amounts 
of  calcium  compounds  in  our  food,  sewage  is  usually  very  hard,  con- 
taining especially  calcium  sulphate.  The  hardness  of  water,  there- 
fore, has  some  bearing  upon  the  question  as  to  whether  the  water 
is  probably  polluted  with  sewage  or  not.  But  water  may  be  hard, 
yet  otherwise  perfectly  pure.  The  test  for  the  degree  of  hardness  is 
therefore  of  little  importance  in  determining  sewage,  as  the  figures 
below  show  that  water  uncontaminated  by  sewage  may  be  very  hard. 

Temporary.  Permanent. 

Rain-water,  average 0.3  1.7 

Highest  from  different  geological  formations 38.6  48.5 

From  272  samples  of  water  from  shallow  and  polluted 
wells: 

Minimum o  3-8 

Maximum 52  164.3 

Average 19  315 

The  above  figures  are  parts  in  100,000.  The  hardness  has,  how- 
ever, much  significance  from  an  economic  point  of  view.  Hard  water 
is  objectionable  for  domestic  purposes  in  washing,  because  of  its  soap- 
destroying  power,  and  for  manufacturing  purposes  in  boilers.  It 
has  no  bad  effect  upon  the  health,  but  is  by  some  considered  whole- 
some. 

Standards.  Certain  standards  have  been  fixed  by  some  chemists 
for  determining  the  purity  or  impurity  of  water,  according  to  which, 
if  certain  figures  are  exceeded,  the  water  is  to  be  condemned. 

Dr.  Tidy's  classification  depends  upon  the  amount  of  oxygen 
consumed,  from  potassium  permanganate,  after  standing  three  hours: 

1 .  Great  organic  purity o       to  0.05 

2.  Medium  purity 0.05  to  0.15 

3.  Doubtful 0.15  to  0.21 

4.  Impure over  0.2 1 

These  standards  are  applied  to  waters  other  than  upland  surface- 
waters,  in  which  larger  quantities  of  oxygen  may  be  absorbed. 

Wanklyn's  standard  is  based  upon  the  indications  of  the  amounts 
of  free  and  albuminoid  ammonia,  as  follows: 

1.  Extraordinary  purity o        to  0.005  part  albuminoid  NH3. 

2.  Satisfactory  purity 0.005  to  o.oio    '  * 

3.  Dirty over  o.oio    "  "  " 


456 


A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


If  the  albuminoid  ammonia  exceeds  0.005  parts  per  100,000  the 
free  ammonia  must  be  taken  into  account.  If  the  free  ammonia  is 
in  large  quantity  it  is  a  suspicious  sign.  If  it  is  in  small  quantity  or 
altogether  absent,  the  water  should  not  be  condemned,  unless  the 
albuminoid  ammonia  is  something  like  o.oio  parts  per  100,000;  while 
over  0.015  should  condemn  the  water  absolutely. 

The  following  is  a  list  of  analyses  of  waters  which  were  pronounced 
good.  The  results  are  given  in  parts  per  100,000: 


I. 

II. 

III. 

IV. 

Chlorin  

0.877 

I.  333 

9.  2Q4 

1.^78 

Free  ammonia 

o  0004 

none 

O    OO2 

O    OOO2 

Albuminoid  ammonia  . 

none 

o  0006 

o  OQC, 

O    OO22 

Oxygen  absorbed  (in  3  hours)  

o  00^4 

0.0016 

0.02^ 

o  .  0008 

N  in  nitrates  and  nitrites  . 

O    2^2^ 

o  3376 

o  0107 

o  2633 

Total  hardness 

IO    23 

14  oooo 

13    33 

2    O7Q 

Permanent  hardness  

7.711; 

3.934 

3.060 

1.980 

Organic  and  volatile  matters 

i    cr 

I    7 

trace 

2    IOO 

Total  solids  (dried  at  230°  F.)  .  .    . 

24.   4. 

27 

37  40 

0    4O 

The  following  were  pronounced  bad: 


I. 

II. 

III. 

IV. 

Chlorin  .  . 

o  316 

62    43 

4  208 

28    2  3O 

Free  ammonia  

o  0196 

o  278 

none 

o  0105 

Albuminoid  ammonia  

o  0678 

0.0030 

0.0105 

o  O3QS 

o.  2912 

0.  133 

0.0165 

O.  2IIO 

N  in  nitrates  and  nitrites  .  .  . 

o  0283 

none 

O    247 

o  6210 

Total  hardness       

6    O4O 

27    72 

13    068 

CQ    OO 

Permanent  hardness 

3t 

23    76 

2    £74 

32    6?O 

Organic  and  volatile  matters  . 

-.) 

o  ^ 

IO    £. 

trace 

8  oo 

Total  solids  (dried  at  230°  F.)  

15.60 

156.20 

30-5° 

146.50 

I,    Back  of  slaughter-house;    II,    Drive-well  on  beach;     III,  Well;    IV,  Well 
30  feet  deep. 


CHAPTER  XLIV 


MILK 


MILK  is  the  nutritive  secretion  of  glands  (the  mammary  glands) 
which  are  characteristic  of  the  mammalia. 

This  secretion  takes  place  as  a  result  of  pregnancy  and  delivery, 
and  continues  for  a  variable  period,  constituting  the  entire  food  of 
the  young  animal  until  it  is  able  to  live  upon  other  foods. 

The  milk  of  different  animals  contains  qualitatively  identical  or 
analagous  ingredients  to  that  of  the  cow,  namely,  fat  (which  is  held 
in  suspension),  nitrogeneous  matters,  such  as  casein  and  albumin, 
milk  sugar,  inorganic  salts,  and  water. 

The  average  composition  of  cow's  milk  is  as  follows : 

Fat 3 .65  per  cent. 

Proteids 4-4°  "      " 

Lactose 4.25  "      " 

Inorganic  salts 0.75  "      " 


Total  solids  : 13 .05 

Water 86.95 


100.00 


In  the  milk  of  different  animals,  however,  these  ingredients  are  in 
different  proportions,  as  the  following  table  shows: 


Human. 

Goat. 

Mare. 

Ass. 

Fat                 

Per  cent. 
3.40 

Per  cent. 
e.2 

Per  cent. 
I.I 

Per  cent. 

I    O 

Proteids 

2    4.<C 

3  8 

2    2 

2    1 

Lactose  ...                     .               ..... 

e.  71; 

4.  7 

5  8 

£     -1 

Inorganic  salts  

o.2c 

o.  7 

o.  3 

O   d 

Water 

88  o< 

86  o 

oo  6 

oo  6 

100.00 

100   0 

IOO.O 

IOO.O 

Total  solids 

1  1    O^ 

Id.   O 

9       A 

94 

* 

457 


458  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

Milk  is  a  perfect  natural  emulsion.  The  casein  appears  to  be 
the  emulsifying  agent,  a  film  of  which  envelops  each  globule  of  fat, 
thus  preventing  cohesion. 

The  inorganic  salts  are  chiefly  the  phosphates  of  sodium  and 
calcium,  and  the  chlorids  of  sodium  and  potassium,  but  magnesium 
and  iron  are  also  generally  present. 

The  proteids  consist  mainly  of  casein  with  some  albumen,  the 
proportion  being  about  as  6  to  i. 

Besides  the  above-mentioned  constituents  milk  also  contains  a 
very  small  quantity  of  peptone,  kreatin,  leucin,  etc.  Also  gases,  such 
as  CO2,  O,  and  N. 

Colostrum  is  the  milk  secreted  in  the  early  stages  of  lactation; 
it  is  rich  in  proteids,  often  containing  as  much  as  20  per  cent,  and 
contains  a  few  corpuscles  of  a  peculiar  character,  which  look  like 
epithelium  cells,  called  colostrum  corpuscles. 

Reaction.  The  reaction  of  the  milk  of  herbivorous  animals  is 
generally  alkaline,  that  of  carnivora  is  generally  acid.  The  reaction 
of  cow's  milk  is  generally  neutral,  sometimes  slightly  acid,  rarely 
alkaline. 

Specific  Gravity.  This  varies  in  normal  cow's  milk  from  1.029 
to  1.035.  ft  should  not  be  below  1.029. 

An  excess  of  fat  lowers  the  specific  gravity  and  the  removal  of 
fat  raises  it.  Thus  skimmed  milk  has  a  higher  specific  gravity  than 
normal  milk. 

These  facts  are  made  use  of  for  the  detection  of  the  ordinary 
adulterations. 

Determinations  of  the  specific  gravity  of  milk  should  always  be 
made  at  the  temperature  of  60°  F.,  and  may  be  made  by  any  of 
the  ordinary  methods.  See  table  of  corrections  for  temperatures 
other  than  60°  F.,  page  460.  A  special  hydrometer  known  as 
the  lactometer  is,  however,  generally  used.  The  lactometer  is 
graded  from  o  at  the  top  to  120  at  the  bottom.  In  taking  the 
specific  gravity  with  this  instrument  the  temperature  of  the  milk 
must  be  60°  F. 

For  every  2^°  of  temperature  above  the  60°  standard,  one  degree 
is  to  be  added  to  the  reading  of  the  lactometer;  below  60°  F.  a  similar 
subtraction  is  to  be  made. 

On  the  lactometer  scale  0=1.000,  the  specific  gravity  of  pure 
water;  at  60°  F.  100=1.029,  the  specific  gravity  of  the  poorest  allow- 
able milk  at  the  same  temperature. 

If  in  a  sample  of  milk  the  lactometer  stands  at  80  the  sample 
contains  about  80  per  cent  of  standard  milk  and  20  per  cent  of  water. 
If  the  lactometer  stands  at  90,  the  sample  contains  about  10  per  cent 
of  water. 


MILK 


459 


Lactometer  Reading. 

Specific  Gravity. 

Lactometer  Reading. 

Specific  Gravity. 

O 

I  .  0000 

70 

.0203 

10 

1.0029 

80 

.0232 

20 

J  .  0058 

90 

.0261 

3° 

1.0087 

IOO 

.02QO 

40 

1.0116 

no 

.0319 

5° 

1.0145 

120 

.0348 

60 

1.0174 

The  Adulterations  of  Milk.  The  adulterations  usually  prac- 
tised are  the  abstraction  of  cream  (skimming)  or  the  addition  of  water, 
or  both.  OccasionaUy  the  addition  of  some  foreign  substance,  as 
sodium  carbonate,  borax,  common  salt,  or  sugar,  is  met  with;  or 
preservatives,  as  formaldehyde,  boric  or  salicylic  acids. 

The  detection  of  adulterations  usually  depends  upon  the  deter- 
nation  of  the  specific  gravity,  the  fat,  total  solids,  and  the  ash. 

These  ingredients  are,  however,  present  in  milk  in  varying  pro- 
portions, and  hence  certain  limits  of  allowable  variations  have  been 
determined  upon  from  time  to  time. 

The  standard  adopted  in  many  States  in  this  country  is,  for  specific 
gravity,  not  less  than  1.029,  for  total  solids,  not  less  than  12  per  cent, 
for  fat  3  per  cent.  The  total  solids  may  vary  legally  from  12  to  13.13 
per  cent,  and  the  solids  nonfat,  from  8.5  to  9.5  per  cent. 

Estimation  of  Total* Solids  and  Water.  A  small,  shallow 
platinum  or  porcelain  dish  about  i£  inches  in  diameter  is  heated  to 
redness,  allowed  to  cool,  and  weighed.  5  cc.  of  milk  (the  specific 
gravity  of  which  has  been  accurately  determined)  are  then  put  in, 
and  the  dish  and  contents  placed  on  a  water-bath  and  kept  there  until 
there  is  no  further  loss  in  weight.  In  conducting  the  evaporation  on 
the  water-bath,  it  is  a  good  plan  to  place  below  the  dish  a  piece  of 
filter  paper  in  order  to  prevent  depositions  from  the  water  on  the 
bottom  of  the  dish.  The  weight  of  the  dry  residue,  minus  the  tare  of 
the  dish,  equals  the  total  solids. 

Then  by  multiplying  this  by  100,  and  dividing  by  the  weight  of 
milk  taken,  the  percentage  of  total  solids  is  found.  Thus 

Total  solids  X  loo 

— — r     .„ — =per  cent  of  total  solids. 

weight  of  milk 

This  deducted  from  100  gives  the  per  cent  of  water.  The  weight 
of  the  milk  taken  is  the  volume  multiplied  by  the  specific  gravity. 

Fat.  Where  great  accuracy  is  unnecessary  the  fat  may  be  deter- 
mined by  treating  the  total  solid  residue  with  successive  portions  of 
warm  ether  until  the  fat  is  completely  dissolved  out.  The  ethereal 


460 


A   MANUAL   OF   VOLUMETRIC   ANALYSIS 


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MILK 


461 


solution  is  then  evaporated  and  the  fat  which  remains  behind  is  weighed, 
or  the  residue  in  the  dish  may  be  again  weighed.  The  loss  of  weight 
then  represents  the  fat.  The  results  so 
obtained  are  0.3  to  0.5  per  cent  too  low. 

Adams'  Method  is  the  officially  recognized 
method  for  the  accurate  estimation  of  fat  in 
milk. 

This  consists  essentially  in  spreading  the 
milk  over  absorbent  paper,  drying,  and 
extracting  the  fat.  The  paper  used  for  this 
purpose  must  previously  have  been  thoroughly 
exhausted  with  alcohol  and  ether,  and 
should  be  in  long  narrow  strips. 

The  procedure  is  as  follows:  5  cc.  of  the 
milk  are  put  into  a  small  beaker  and  weighed. 
A  strip  of  the  absorbent  paper  which  has 
been  rolled  into  a  coil  is  thrust  into  the 
beaker  containing  the  milk.  In  a  few  minutes 
nearly  the  whole  of  the  milk  will  be  absorbed ; 
the  coil  is  then  withdrawn,  and  stood  dry 
end  down  upon  a  sheet  of  glass. 

It  is  important  to  take  up  the  whole  of 
the  milk  from  the  beaker,  as  the  paper  has 
a  selective  action,  removing  the  watery  con- 
stituents by  preference  over  the  fat.  The 
beaker  is  again  weighed,  and  the  milk  taken 
found  by  difference. 

The  paper  charged  with  the  milk  is  now 
dried  in  a  water  over  and  placed  into  the  tube 
C  of  a  Soxhlet  extraction  apparatus  (Fig.  82). 
About  75  cc.  of  ether  are  introduced  into 
the  tared  flask  A  of  the  apparatus,  the  reflux 
condenser  D  attached  and  heat  applied  to  A 
by  means  of  a  water-bath  and  continued 
until  exhaustion  is  complete.  The  flask  is 
then  detached,  the  ether  removed  by  distilla- 
tion, and  the  flask  and  fat  which  remains  is 
weighed. 

The  vapor  of  the  ether  rises  through  E,  condenses  in  D,  and  drops 
into  the  tube  C  containing  the  paper  charged  with  the  milk.  When 
the  instrument  has  become  filled  with  ether  to  the  level  of  the  top 
of  F,  it  is  automatically  syphoned  back  into  the  flask  charged  with 
part  of  the  fat.  This  process  repeats  itself  until  the  whole  of  the 
fat  is  extracted,  when  the  flask  is  detached  and  the  ether  removed 


FIG.  82. 


462 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


by  distillation  as  above  stated.     The  weight  of  the  tared  flask,  deducted 
from  the  weight  of  the  flask  and  fat,  gives  the  weight  of  the  fat. 

The  paper  may  be  charged  with  the  milk  by  spreading  the  latter 
over  the  surface  of  the  paper  by  means  of  a  pipette. 

The  Werner-Schmid  Method.  This  is  a  satisfactory  and  at  the 
same  time  a  rapid  method  for  the  determination  of  fat,  and  is  espe- 
cially suitable  for  sour  milk. 

10  cc.  of  the  milk  are  put  into  a  long  tube  having  a  capacity  of 
about  50  cc.,  and  10  cc.  of  strong  hydrochloric  acid  are  added;  or  the 
milk  may  be  weighed  in  a  small  beaker  and 
washed  into  the  tube  with  the  acid.  The  liquids 
are  mixed  and  boiled  for  ij  minutes,  or  until 
the  liquid  turns  dark  brown,  but  not  black. 
The  tube  and  contents  are  then  cooled,  and  30 
cc.  of  ether  are  added,  shaken,  and  allowed  to 
stand  until  the  acid  liquid  and  ether  have 
separated.  The  cork  is  now  taken  out  and  the 
wash-bottle  arrangement  inserted  (see  Fig.  83). 
The  stopper  of  this  should  be  of  cork,  not  of 
rubber,  since  the  ether  has  a  solvent  action 
upon  the  latter.  The  lower  end  of  the  exit 
tube,  b,  is  adjusted  so  as  to  rest  immediately 
above  the  junction  of  the  two  liquids.  The 
ethereal  solution  of  fat  is  then  drawn  off,  by 
gently  blowing  into  tube  a,  and  received  in  a 
weighed  beaker.  Two  more  portions  of  10  cc. 
each  are  shaken  successively  with  the  acid 
liquid,  blown  out,  and  added  to  the  first.  The 
ethereal  solution  is  then  heated  on  a  water-bath, 
and  the  residue  of  fat  weighed.  The  results  agree  quite  closely  with 
the  Adams  method. 

The  Babcock  Centrifugal  Method.  This  method  is  easily  and 
quickly  manipulated,  and  gives  very  accurate  results,  which 
pare  very  well  with  those  obtained  by  the  Adams  method.  It  is 
carried  out  as  follows:  Into  the  long-neck  graduated  bottle  which 
accompanies  the  apparatus,  add  from  the  graduated  pipette  exactly 
17.6  cc.  of  the  well  mixed  milk,  and  to  this  add  17.5  cc  (about)  o 
commercial  sulphuric  acid.  The  acid  is  carefully  poured  down  the 
side  of  the  bottle  to  allow  the  air  to  escape,  and  when  all  has  been 
added,  the  contents  of  the  bottle  are  mixed  by  giving  it  a  rotary  motion. 
The  bottle  is  then  placed  in  one  of  the  sockets  of  the  machine.  Another 
bottle  is  then  treated  in  exactly  the  same  way  and  placed  in  the  opposite 
socket,  to  maintain  the  equilibrium,  the  cover  put  on  the  machine, 
and  the  bottle  whirled  at  about  700  to  1200  revolutions  per  minute 


FIG.  83. 


MILK 


463 


for  seven  minutes.  Hot  water  is  then  added  to  the  base  of  the  neck, 
the  bottles  put  back  in  the  machine  and  whirled  for  about  three  minutes. 
After  this  the  bottles  are  removed  and  hot  water  added  to  about  the  7 
per  cent  mark  on  the  stem  of  the  bottle.  After  which  they  are  again 
whirled  for  about  two  minutes  and  taken  out  of  the  machine.  The 
per  cent  of  fat  is  then  read  off  direct  from  the  stem  of  the  bottle.  Each 
time  the  bottles  are  whirled  the  cover  should  be  on  the  machine. 
Immediately  after  using,  the  bottles  should  be  washed  out  with  hot 
water  containing  some  alkali. 

The  readings  should  be  taken  at  a  temperature  between  130° 
and  150°  F.  when  the  fat  is  wholly  liquid.  The  apparatus  is  pictured 
in  Fig.  84. 

Calculation  Method.  This  rests  upon  the  assumption  that  every 
per  cent  of  solids  not  fat,  raises  the  suecific  gravity  by  a  definite 


\ 


FIG.  84. 

amount,  while  every  per  cent  of  fat  lowers  it  by  a  definite  amount. 
An  accurate  determination  of  the  per  cent  of  total  solids  and  of  the 
specific  gravity  therefore  furnish  the  necessary  data  for  calculating 
the  amount  of  fat. 

Hehner  and  Richmond  have  devised  the  following  formula: 

77=0.859  r-o.2i86G; 


in  which  ^=fat,  T=  total  solids,  and  G=the  last  two  units  of  the 
specific  gravity  and  any  decimal.  Thus  if  the  specific  gravity  is  1029, 
G  will  be  29;  if  1029.5,  G  will  be  29.5. 


464  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Example.  Let  us  assume  in  the  examination  of  a  certain  milk 
that  the  specific  gravity  was  1030,  and  that  it  contained  12  per  cent  of 
total  solids.  We  then  have 

Fat  =  o.859X  12— 0.2186X30=3.75  per  cent. 

When  the  per  cent  of  fat  is  known,  the  formula  may  be  transposed 
so  as  to  calculate  the  total  solids,  as  follows: 

F+o.2i86G 

0.859 

Example.  A  sample  of  milk  is  found  to  contain  3.75  per  cent  of 
fat,  and  its  specific  gravity  is  1030;  then 


Total  solids  =  °'/J  '     0        "°   =12  per  cent. 
0.859 

Ash.  The  ash  may  be  determined  by  evaporating  20  gms.  of  milk 
to  dryness  in  the  presence  of  6  cc.  of  nitric  acid,  and  then  heating  at 
a  dull  red  heat  until  the  ash  is  white.  The  organic  matter  is  thus 
all  burned  off,  and  the  residue  is  weighed  and  calculated  as  ash.  The 
ash  should  be  about  0.75  per  cent,  never  below  0.67. 

To  Calculate  the  Per-cent  of  Pure  Milk  and  of  Added 
Water,  the  following  formula  may  be  adopted,  which  is  based  upon 
the  legal  standard  of  the  State  of  New  York,  viz.,  3  per  cent  of  fat, 
12  per  cent  of  total  solids,  and  9  per  cent  of  "solids  not  fat." 

If,  however,  a  milk  has  3  per  cent  of  fat  and  only  8.5  per  cent  of 
"solids  not  fat,"  it  need  not  be  considered  as  definitely  proved  to  be 
adulterated. 

The  quantity  of  added  water  should,  however,  always  be  calcu- 
lated upon  the  average  standard  of  9  per  cent  "solids  not  fat,"  pro- 
vided the  milk  is  certainly  well  below  the  limit  of  8.5  per  cent.  / 

The  "solids  not  fat  "  are  used  as  a  basis  for  the  calculation  because 
they  are  a  fairly  constant  quantity,  the  fat  being  variable.  The  calcu- 
lation is  made  thus: 

"  Solids  not  fa£"X  ioo 

• —  ==per  cent  of  pure  milk  present; 

and  the  difference  between  this  result  and  ioo  will  of  course  give  the 
added  water. 


MILK  465 

Example.     A  sample  of  milk  upon  analysis  was  found  to  contain 
8.1  per  cent  of  solids  not  fat;  then 

8.1X100     810.0 

—  = •  =90  per  cent 


of  pure  standard  milk  and  10  per  cent  of  water. 

Total  Proteids.  The  method  most  generally  employed  is  the 
Kjeldahl  Method.  The  following  reagents  and  apparatus  are  required: 
Decinormal  hydrochloric  acid,  decinormal  ammonia  or  other  alkali, 
saturated  solution  of  sodium  hydrate,  free  from  nitrates  (sulphuric 
acid,  sp.gr.  1.84),  pulverized  potassium  sulphate,  cochineal  or  methyl- 
orange  indicator,  Kjeldahl  flasks,  any  convenient  form  of  distillation 
flask,  those  described  on  page  287,  with  bulbed  outlet  tube,  are  very 
satisfactory. 

The  process  is  conducted  as  follows :  To  5  cc.  of  milk  the  specific 
gravity  of  which  is  known,  contained  in  the  Kjeldahl  flask,  add  10  gms. 
of  pulverized  potassium  sulphate  and  20  cc.  of  sulphuric  acid.  The 
flask  is  placed  in  an  inclined  position  and  heated  over  a  free  flame 
or  on  a  gauze;  at  first  at  a  low  heat  until  frothing  ceases;  then  the 
heat  is  raised  and  continued  until  the  mixture  is  either  colorless  or 
at  most  has  a  faint  straw  color.  Then  cool  and  transfer  to  a  distil- 
lation flask  with  about  200  cc.  of  water,  add  sodium  hydroxid  solution 
to  alkaline  reaction,  using  phenolphthalein  as  indicator.  The  flask  is 
now  connected  with  a  condenser,  the  joint  being  made  tight  and  the 
distillation  carried  on  until  about  150  cc.  have  been  distilled  over  into 
a  known  quantity  of  standard  acid.  To  avoid  bumping  during  the 
distillation,  a  small  piece  of  pumice  stone  or  a  small  amount  of  granu- 
lated zinc  may  be  employed.  Now  add  cochineal  or  methyl-orange 
indicator,  and  titrate  the  unneutralized  acid  by  means  of  decinormal 
alkali. 

This  process  depends  upon  the  destruction  of  the  nitrogeneous 
organic  matter  by  the  sulphuric  acid  and  the  formation  of  ammonium 
sulphate.  This  in  turn  is  broken  up  by  the  sodium  hydroxid  into 
ammonia,  which  is  distilled  over  into  a  measured  quantity  of  standard 
acid.  The  loss  of  acidity  of  the  acid  solution  is  the  measure  of  the 
ammonia  which  distilled  over.  Each  cc.  of  decinormal  acid=. 001393 
gm.  of  nitrogen,  assuming  that  the  percentage  of  nitrogen  in  the  pro- 
teids  of  milk  is  16,  the  quantity  of  nitrogen  found  multiplied  by  6.25 
gives  the  proteids.  Some  chemists  base  their  calculation  upon  the 
assumption  that  the  proteids  of  milk  contain  15.88  per  cent  of  nitro- 
gen, in  which  case  the  nitrogen  is  multiplied  by  6.38  in  order  to  obtain 
the  quantity  of  proteids. 


466  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Rittenhausen1  s  Copper  Process.  The  solutions  required  are:  (i) 
Copper-sulphate  solution,  34.64  gms.  in  500  cc.;  (2)  Sodium-hy- 
droxid  solution,  12  gms.  to  500  cc. 

Ten  grams  of  the  milk  are  diluted  to  100  cc.  with  distilled  water 
and  placed  in  a  beaker;  5  cc.  of  the  copper-sulphate  solution  are  now 
added  and  thoroughly  mixed. 

The  sodium-hydroxid  solution  is  now  added,  drop  by  drop,  stirring 
constantly  until  the  precipitate  settles  quickly,  and  the  liquid  is  neutral 
or  feebly  acid.  It  should  never  be  alkaline,  as  an  excess  of  alkali  pre- 
vents the  precipitation  of  some  of  the  proteids. 

The  precipitate  which  includes  the  fat  carries  down  all  of  the  cop- 
per. It  is  washed  by  decantation  and  collected  upon  a  weighed  dry 
filter,  the  contents  of  the  filter  being  washed  until  the  total  filtrate 
measures  about  250  cc.  This  filtrate,  which  contains  no  copper,  is 
reserved  for  the  determination  of  the  sugar  by  Fehling's  Solution. 

The  precipitate  is  washed  once  by  strong  alcohol  to  remove  adher- 
ing water;  it  is  then  washed  several  times  with  ether  to  remove  the 
fat.  The  residue  on  the  filter,  which  consists  of  the  proteids  and 
copper  hydroxid,  is  dried  at  265°  F.  in  the  air-bath  and  weighed.  It 
is  then  transferred  to  a  porcelain  crucible  and  incinerated,  and  the 
residue  weighed. 

The  weight  of  the  filter  and  contents  less  the  weight  of  the  filter 
and  residue  after  ignition,  gives  the  weight  of  the  proteids, 

The  Milk-sugar  is  estimated  in  the  mixed  filtrate  from  the  pre- 
cipitated proteids  by  the  use  of  Fehling's  Solution  in  the  usual  way. 
(See  Estimation  of  Sugar.) 

Detection  of  Formaldehyde  (Hehner).*  To  the  milk  contained 
in  a  test  tube  add  strong  sulphuric  acid  containing  a  few  drops  of 
ferric  chlorid  solution.  The  acid  should  be  carefulty  added  so  that  it 
does  not  mix  with  the  milk.  At  the  junction  of  the  liquids  a  violet  or 
blue  color  will  appear  if  the  milk  contains  one  or/more  parts  of  formal- 
dehyde per  10,000.  If  a  large  quantity  of  formaldehyde  is  present,  the 
test  does  not  work  so  satisfactorily,  and  it  is  best  to  dilute  the  milk. 

Detection  of  Borax  or  Boric  Acid.  Take  a  small  quantity  of 
milk,  make  it  alkaline  with  lime-water,  and  evaporate  it  to  dryness. 
Ignite  the  residue  to  destroy  organic  matter  and  dissolve  in  a  small 
quantity  of  water  with  the  aid  of  hydrochloric  acid,  adding  about  i  cc. 
of  the  latter  in  excess.  Dip  a  piece  of  turmeric  paper  in  this  solution 
and  dry  it.  If  borax  or  boric  acid  is  present  the  paper  will  acquire  a 
peculiar  red  color,  which  is  changed  to  dark  blue-green  upon  mois- 
tening with  ammonia  water,  but  is  restored  by  acid. 

*  Analyst,  1895,  XX,  155. 


CHAPTER  XLV 

BUTTER 

AN  analysis  of  butter  comprises  the  estimation  of  water,  fat,  casein, 
ash,  sodium  chlorid,  and  also  volatile  fatty  acids. 

The  water  is  determined  by  drying  to  a  constant  weight;  the  fat, 
by  extraction  with  ether;  the  casein,  by  heating  the  residue  after 
extraction  of  fat,  to  just  below  redness,  until  the  former  becomes 
white.  The  loss  of  weight  represents  casein  and  the  residue  ash.  In 
the  ash  the  chlorin  may  be  determined  by  dissolving  in  water  and  titrat- 
ing with  standard  silver  nitrate  solution.  Salt  is  estimated  by  washing 
the  butter  with  several  portions  of  hot  water  and  titrating  the  aqueous 
solution  with  standard  silver  nitrate  solution. 

The  estimation  of  salt  is  conducted  according  to  the  official  method 
of  the  Association  of  Official  Agricultural  Chemists,  as  follows : 

Weigh  in  a  counterpoised  beaker  from  5  to  10  gms.  of  butter,  using 
portions  of  about  i  gm.  from  different  parts  of  the  sample.  Add 
about  20  cc.  of  hot  water,  and  after  the  butter  is  melted  transfer  the 
whole  to  a  separatory  funnel.  Insert  the  stopper  and  shake  for  a  few 
moments.  Let  stand  until  the  fat  has  all  collected  on  the  top  of  the 
water,  then  draw  off  the  latter  into  a  flask,  being  careful  to  let  none 
of  the  fat  globules  pass.  Again  add  hot  water  to  the  beaker  and 
repeat  the  extraction  from  ten  to  fifteen  times,  using  each  time  from 
10  to  20  cc.  of  water.  The  washings  will  contain  all  but  a  mere  trace 
of  the  sodium  chlorid  originally  present  in  the  butter.  Determine  its 
amount  in  the  whole  or  an  aliquot  of  the  liquid  by  the  volumetric 
silver  nitrate  method,  with  potassium  chromate  as  indicator. 

The  Estimation  of  Volatile  Acids.  This  is  undoubtedly  the 
best  process  for  detecting  the  admixture  of  foreign  fats  with  butter. 
This  process  depends  upon  the  fact  that  butter  contains  certain  con- 
stituents which,  when  appropriately  treated,  yield  volatile  acids  in 
much  larger  quantity  than  is  obtained  from  any  of  the  practicable 
substitutes  for  butter.  These  acids  are  principally  butyric  and 
caproic.  The  process  consists  in  saponifying  the  fat  with  an  alkali, 
then  separating  the  fatty  acids  by  neutralization,  and  distilling  off 

467 


468  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

the  volatile  acids  for  titration  with  standard  alkali.  "Reichert's 
number "  is  the  number  of  cc.  of  decinormal  alkali  solution 
required  to  neutralize  the  acids  distilled  from  2.5  gms.  of  fat. 
The  results  are,  however,  often  specified  for  5  or  10  gms.  of 
the  fat. 

The  operations  involved  in  this  process  do  not  admit  of  any  arbi- 
trary variation,  and  reliable  and  comparable  results  can  only  be 
obtained  by  strictly  adhering  to  the  prescribed  details. 

The  following  process  is  adopted  by  the  Association  of  Official 
Agricultural  Chemists.  The  solutions  required  are: 

Sodium  Hydroxid  Solution.  100  gms.  of  sodium  hydroxid  are 
dissolved  in  100  cc.  of  distilled  water.  The  alkali  should  be  as  free 
as  possible  from  the  carbonates,  and  be  preserved  out  of  contact  with 
the  air. 

Potassium  Hydroxid  Solution.  100  gms.  of  the  purest  potassium 
hydroxid  are  dissolved  in  58  cc.  of  hot  distilled  water,  cooled  in  a 
stoppered  vessel,  and  the  clear  liquid  decanted,  and  preserved  out  of 
contact  with  the  air. 

Sulphuric  Acid.  Mix  200  cc.  of  the  strongest  acid  with  1000  cc. 
of  water. 

Alcohol  of  about  95  per  cent,  redistilled  from  caustic  soda. 

Standard  Barium  Hydroxid  Solution.  Accurately  standardized, 
approximately  decinormal. 

Indicator.  Dissolve  r^gmT^bT^henolphthalein  in  100  cc.  of  95 
per  cent  alcohol. 

The  process: 

Weighing  the  Butter.  The  butter  to  be  examined  should  be  melted 
and  kept  in  a  dry,  warm  place  at  about  60°  C.  for  two  or  three  hours, 
until  the  water  and  curd  have  entirely  settled  out.  The  clear  super- 
natant fat  is  poured  off  and  filtered  through  a  dry  filter-paper  in  a 
jacketed  funnel  containing  boiling  water.  Should  the  filtered  fat  in 
a  fused  state  not  be  perfectly  clear,  it  must  be  filtered  a  second  time. 
This  is  to  remove  all  foreign  matter  and  any  trace  of  moisture.  The 
saponification  flasks  are  prepared  by  thoroughly  washing  with  water, 
alcohol,  and  ether,  wiping  perfectly  dry  on  the  outside,  and  heating 
for  one  hour  at  the  temperature  of  boiling  water.  The  flasks  should 
then  be  placed  in  a  tray  by  the  side  of  the  balance  and  covered  with  a 
silk  handkerchief  until  they  are  perfectly  cool.  They  must  not  be 
wiped  with  a  silk  handkerchief  within  fifteen  or  twenty  minutes  of  the 
time  they  are  weighed.  The  weight  of  the  flasks  having  been  accu- 
rately determined,  they  are  charged  with  the  melted  fat  in  the  follow- 
ing way: 

A  pipette  with  a  long  stem,  marked  to  deliver  5.75  cc.,  is  warmed 
to  a  temperature  of  about  50°  C.  The  fat  having  been  poured  back 


BUTTER 


469 


and  forth  once  or  twice  into  a  dry  beaker  in  order  to  thoroughly  mix  it, 
is  then  taken  up  in  the  pipette  and  the  nozzle  of  the  pipette  carried 
to  near  the  bottom  of  the  flask,  having  been  previously  wiped  to  remove 
any  adhering  fat,  and  5.75  cc.  of  fat  are  allowed  to  flow  into  the  flask. 
After  the  flasks  have  been  charged  in  this  way  they  should  be  recov- 
ered with  the  silk  handkerchief  and  allowed  to  stand  fifteen  or  twenty 
minutes,  when  they  are  again  weighed. 

The  Saponification.     Three  methods  may  be  employed: 

1.  Under  Pressure  with  Alcohol.     10  cc.  of  95  per  cent  alcohol  are 
added  to  the  fat  in  the  flask,  and  then  2  cc.  of  the  caustic  soda  solu- 
tion.    A  soft  cork  stopper  is  now  inserted  in  the  flask  and  tied  down 
with  a  piece  of  twine.     The  saponi- 

fication  is  then  completed  by  plac- 
ing the  flask  upon  the  water  or 
steam  bath  (see  Fig.  85).  The 
flask  during  the  saponification, 
which  should  last  one  hour, 
should  be  gently  rotated  from 
time  to  time,  being  careful  not  to 
project  the  soap  for  any  distance 
up  its  sides.  At  the  end  of  an 
hour  the  flask,  after  having  been 
cooled  to  near  the  room  tempera- 
ture, is  opened. 

2.  Under  Pressure  without  the 
Use  of  Alcohol.     Place  2  cc.  of  the 
potassium    hydroxid    in    the   flask 
containing  the  fat,  which  must  be 
round  bottomed  and  made  of  well- 
annealed  glass  to  resist  the  pres- 
sure;   cork,  and  heat    as    in    the 
previous  method.     Rotate  the  flask 
very  gently  during  the  saponifica- 
tion, taking  great  care  that  none  of 


FIG.  85. 


the  fat  rises  on  the  sides  of  the  flask  out  of  reach  of  the  alkali.  Potash 
makes  a  softer  soap  than  soda  and  thus  allows  a  complete  saponifica- 
tion without  the  use  of  alcohol.  This  method  avoids  the  danger  of 
formation  of  esters  and  the  trouble  of  removing  the  alcohol  after 
saponification. 

3.  With  a  Reflux  Condenser  and  the  Use  of  Alcohol.  Place  10  cc. 
of  the  95  per  cent  alcohol  in  the  flask  containing  the  fat,  add  2  cc.  of 
the  sodium  hydroxid  solution  with  a  reflux  condenser  (a  glass  tube 
not  less  than  i  meter  in  length  is  allowable),  and  heat  on  the  steam 
bath  until  the  saponification  is  complete. 


470 


A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


Removal  of  the  Alcohol.  The  stoppers  having  been  laid  loosely  in 
the  mouth  of  the  flask,  the  alcohol  is  removed  by  dipping  the  flask 
into  a  steam  bath.  The'Steam  should  cover  the  whole  of  the  flask 
except  the  neck.  After  the  alcohol  is  nearly  removed,  frothing  may 
be  noticed  in  the  soap,  and  to  avoid  any  loss  from  this  cause  or  any 
creeping  of  the  soap  up  the  sides  of  the  flask,  it  should  be  removed 
from  the  bath  and  shaken  to  and  fro  until  the  frothing  disappears. 
The  last  traces  of  alcohol  vapor  may  be  removed  from  the  flask  by 
waving  it  briskly,  mouth  down,  to  and  fro,  or  better,  by  a  current  of 
carbon  dioxid  free  air. 

Dissolving  the  Soap.  After  the  removal  of  the  alcohol  the  soap 
should  be  dissolved  by  adding  135  cc.  of  recently  boiled  distilled  water 


FIG.  86. 


(or  132  cc.  if  potassium  hydroxid  was  used  in  the  saponification), 
warming  on  the  steam  bath,  with  occasional  shaking  until  solution 
of  the  soap  is  complete. 

Setting  Free  the  Fatty  Acids  and  Distilling.  Cool  to  from  60°  to 
70°  C.,  throw  in  a  few  pieces  of  pumice  stone,  add  5  cc.  of  the  dilute 
sulphuric  acid  (or  8  cc.  if  potassium  hydroxid  was  used  in  the  saponi- 
fication), stopper  as  in  the  method  of  saponification,  and  heat  on  the 
water-bath  Until  the  fatty  acids  form  a  clear,  transparent  layer  on  .top 
of  the  water.  This  may  take  several  hours.  Cool  to  room  tempera- 
ture, add  a  few  pieces  of  pumice  stone,  and  connect  with  a  glass  con- 
denser, as  in  Fig.  86.  / 

Heat  slowly  with  a  naked  flame  until  ebullition  begins,  and  distil, 
regulating  the  flame  in  such  a  way  as  to  collect  no  cc.  of  distillate 
in  as  nearly  thirty  minutes  as  possible. 

Mix  this  distillate,  filter  through  a  dry  filter,  and  titrate  too  cc. 
with  the  standard  barium  hydroxid  solution,  using  0.5  cc.  of  phenol- 


BUTTER  471 

phthalein  as  indicator.  The  red  color  should  remain  unchanged  for 
two  or  three  minutes. 

Increase  the  number  of  cubic  centimeters  of  tenth-normal  alkali 
used  by  one-tenth,  divide  by  the  weight  of  fat  taken,  and  multiply 
by  five  to  obtain  the  Reichert-Meissl  number.  Correct  the  result  by 
the  figure  obtained  in  a  blank  experiment. 

When  treated  as  above  described,  5  gms.  of  genuine  butter  never 

N 
yields  less  acidity  than  is  represented  by  24  cc.  of  —  alkali.     It  is  true, 

however,  that  the  butter  made  from  the  milk  of  a  single  cow,  especially 
towards  the  end  of  her  period  of  lactation,  has  been  known  to  fall 
slightly  below  this  figure,  but  the  average  butter,  as  produced  from 
the  mixed  milk  qf  a  herd,  usually  requires  27  cc.  or  more.  Oleomar- 
garin  requires  about  i  cc.  beef-fat,  and  lard  about  the  same.  Cacao 
butter  requires  about  7  cc. 

The  percentage  of  butter-fat  in  a  mixture  of  fats,  5  gms.  being 
taken:  (n—  0.6) X 3. 65  =  percentage  of  true  butter-fat. 

A  rapid  method  for  detecting  oleomargarin  or  an  admixture  of 
it  with  butter  is  to  heat  the  suspected  substance  in  a  small  tin  dish 
directly  over  a  gas  flame.  If  it  melts  quietly,  foams,  and  runs  over 
the  dish,  it  is  butter;  if  it  sputters  noisily  as  soon  as  heated  and  foams 
but  little,  it  is  oleomargarin. 

Another  way  is  to  heat  the  butter  for  a  moment  with  an  alcoholic 
solution  of  sodium  hydroxid  and  then  empty  into  cold  water.  It 
gives  a  distinct  odor  of  pineapples  (due  to  ethyl  butyrate),  while  oleo- 
margarin gives  only  an  alcoholic  odor. 


CHAPTER   XLVI 


SOME   TECHNICAL    EXAMINATION    METHODS    FOR    FATS,    OILS, 

AND  WAXES 

The  Acid  Value  or  Proportion  of  Free  Fatty  Acids.  This 
indicates  the  number  of  milligrams  of  KOH  required  to  neutralize 
the  free  fatty  acids  in  i  gm.  of  oil,  fat  or  wax.  The  standard  alkali 

N   N 
used  is  in  alcoholic  solution  and  may  be  — ,  — ,  or  weaker,  depending 

upon  the  nature  of  the  fat.  Phenolphthalein  is  the  indicator.  The 
fat  is  dissolved,  according  to  Geissler,  in  ether,  but  alcohol  or  purified 
methylated  spirit,  chloroform  or  a  mixture  of  alcohol  and  ether  may 
be  used.  The  solvent,  whichever  is  used,  must  be  free  from  acidity, 

and  should  Belieufralized  with  —  alkali  if  necessary. 

10 

The  Process.  10  gms.  of  the  oil  are  accurately  weighed  into  a  flask 
and  about  50  cc.  of  solvent  added.  A  few  drops  of  phenolphthalein 

N 
are  then  added  and  the  titration  with  alcoholic  —  potassium  hydroxid 

solution  begun,  shaking  constantly  until  the  first  appearance  of  a  pink 
color.  Care  must  be  taken  not  to  add  too  great  an  excess  of  the 
alkali,  otherwise  saponification  will  occur.  A  small  excess  may,  how- 
ever, be  added  and  retitrated  with  standard  acid,  a  more  distinct  end 
point  is  then  obtained.  In  the  case  of  waxes  or  solid  fats,  the  solvent 
is  added,  heat  applied  until  it  boils,  and  the  titration  at  once  started. 

N 
i  cc.  of  —  KOH  =  0.02787  gm.  of  KOH  or  27.87  mgrns. 

The  number  of  cc.  used,  multiplied  by  27.87  and  then  divided  by 
the  weight  of  oil  taken,  gives  the  milligrams  of  KOH  neutralized  by 
the  free  fatty  acids  of  the  oil,  i.e.,  the  acid  value. 

The  Saponification  Value  (Kottstorfer  Number).*  This  indi- 
cates the  number  of  milligrams  of  KOH  required  for  the  complete 
saponification  of  one  gram  of  fat  or  oil.  Reagents  required  are: 

Alcoholic  Potassium  Hydroxid  Solution,  made  by  dissolving  40 
gms.  of  potassium  hydroxid  in  i  liter  of  95  per  cent  alcohol. 

*  J.  Kottstorfer,  1879,  Zeitschr.  anal.  Chem.,  XVIII,  199. 

472 


FATS,   OILS,   AND   WAXES 


473 


Half -Normal  Hydrochloric  Acid  Solution.  Each  cc.  of  which 
=  27.87  mg.  of  KOH. 

Indicator.     Phenolphthalein  i  gm.  in  100  cc.  of  95  per  cent  alcohol. 

The  Process.  Into  an  Erlenmeyer  flask  capable  of  holding  200  cc., 
accurately  weigh  about  1.5  gms.  of  the  fat  (previously  purified  and 
filtered).  Run  into  this  from  a  burette  25  cc.  of  the  alcoholic  potas- 
sium hydroxid  solution.  Then  place  a  small  funnel  in  the  mouth 
of  the  flask  or  cover  it  loosely  with  a  watch  crystal,  and  set  it  in  a 
steam  bath  for  half  an  hour  or  until  the  fat  is  entirely  saponified. 
The  operation  is  facilitated  by  occasional  agitation.  The  flask  is  then 
removed,  its  contents  cooled,  and  titrated  with  the  half-normal  hydro- 
chloric acid,  using  phenolphthalein  as  indicator.  The  alcoholic 
potassium  hydroxid  solution  is  standardized  by  conducting  a  blank 
experiment  similar  in  every  detail  to  the  above  with  the  exception 
that  the  fat  is  omitted. 

The  Kottstorfer  number  is  then  ascertained  by  subtracting  the 
number  of  cc.  of  the  standard  hydrochloric  acid  used  in  the  analysis 
from  the  number  necessary  to  neutralize  25  cc.  of  the  alcoholic  potas- 
sium hydroxid  solution  in  the  blank  experiment,  multiplying  the  dif- 
ference by  27.87  and  dividing  by  the  weight  of  fat  taken. 

Example.  1.5  gms.  of  the  fat  was  treated  with  25  cc.  of  alcoholic 
potassium  hydroxid  solution,  under  the  above  described  conditions, 

N 
and  in  titrating  the  excess,  21.5  cc.  of  —  hydrochloric  acid  was  required. 

In  the  blank  experiment  25  cc.  of  the  alcoholic  potassium  hydroxid 

N 
required  32  cc.  of  the  —  acid.    Then  32—21.5  =  10.5. 

10.5X27.87 


195  is  the  milligrams  of  KOH  neutralized  by  i  gm.  of  the  oil,  or  the 
Kottstorfer  saponification  number. 

TABLE  SHOWING  U.  S.  P.  REQUIREMENT  AS  TO  SAPONIFICATION 

NUMBER 


Lard  oil 195  to  197 

Almond  oil  (expressed)  ...  191  to  200 

Cottonseed  oil 191  to  196 

Linseed  oil 187  to  195 

Cod-liver  oil 175  to  185 


Olive  oil 191  to  195 

Castor  oil 1 79  to  180 

Theobroma  oil 188  to  195 

Croton  oil 212  to  218 

Yellow  wax 90  to    96 


474  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  Reichert  Number  Volatile  Fatty  Acid  Value.*  This 
indicates  the  number  of  cc.  of  decinormal  KOH  required  to  neutralize 
the  volatile  fatty  acids  distilled  from  2.5  gms.  of  fat.  This  method  is 
conducted  as  follows:  2.5  gms.  of  the  clear  filtered  fat  are  taken  in 
an  Erlenmeyer  flask  of  about  1500:.  capacity  with  i  gm.  of  potassium 
hydroxid  and  20  cc.  of  80  per  cent  alcohol,  and  the  whole  digested  on 
a  water-bath  (rotating  the  flask  frequently)  until  saponification  is  com- 
plete and  the  alcohol  all  removed.  50  cc.  of  water  are  then  added, 
and  20  cc.  of  dilute  sulphuric  acid  (1:10),  and  the  mixture  distilled. 
The  distillate  is  collected  in  a  50  cc.  flask  into  which  is  set  a  funnel 
with  a  wetted  filter  to  collect  any  insoluble  fat  acid.  The  first  10  or 
20  cc.  of  distillate  are  returned  to  the  flask,  and  the  50  cc.  distilled. 
This  is  treated  with  a  few  drops  of  phenolphthalein  and  then  titrated 
with  decinormal  potassium  hydroxid.  The  number  of  cc.  consumed 
is  the  Reichert  Number. 

In  the  Meissl  modification  (the  Reichert-Meissl  method),  5  gms. 
of  the  fat  are  taken  and  a  more  complete  distillation  of  the  volatile 
acids  effected.  The  method  is  particularly  useful  in  determining  the 
genuineness  and  purity  of  butter.  It  is  described  in  detail  in  Chap- 
ter XLV. 

lodin  Absorption  Number  of  Fats  and  Oils  (Hubl's  Number ).f 
This  is  the  percentage  of  iodin  absorbed  by  a  fat  or  an  oil  under  cer- 
tain conditions.  In  other  words  it  is  the  number  of  parts  of  iodin 
absorbed  by  100  parts  of  an  oil. 

Reagents  required: 

(a)  Hubl's  Iodin  Solution.     Dissolve  25  gms.  of  pure  iodin  in  500 
cc.  of  alcohol,  and  mix  this  solution  with  500  cc.  of  alcohol  containing 
30  gms.  of  pure  mercuric  chlorid,  and  set  aside  for  twenty-four  hours. 
The  mercuric  chlorid  solution  should  be  filtered  if  necessary  before 
it  is  mixed  with  the  alcoholic  iodin  solution.     This  solution  loses  its 
strength  rapidly,  and  should  therefore  be  tested  before  using. 

(b)  Decinormal  sodium  thiosulphate. 

(c)  Potassium  iodid  solution,  20  gms.  in  100  cc. 

(d)  Starch  paste  indicator. 

The  Process.  To  about  0.3  gm.J  of  the  fat  or  oil,  accurately 
weighed  and  dissolved  in  10  cc.  of  chloroform,  contained  in  a  glass - 
stoppered  bottle  of  250  cc.  capacity,  add  25  cc.  of  the  iodin  solution. 
Stopper  the  bottle  securely  and  place  in  a  cool,  dark  place  for  four 


*  E.  Reichert,  Zeitschr.  anal.  Chem.,  XVIII,  68. 

t  Dingler's  Polyt.  Jour.,  1884,  281;  Am.  Ch.  Jour.,  VI,  285. 

j  In  the  case  of  drying  oils  which  have  a  very  high  absorbent  power,  as 
linseed  oil,  use  from  0.15  to  o.2Ogm.;  for  oil  of  theobroma  and  similar  fats, 
use  6.80  gm. 


FATS,   OILS,   AND   WAXES  475 

hours.*  At  the  expiration  of  this  time,  the  mixture  must  still  possess 
a  brown  color;  if  it  does  not,  a  further  measured  quantity  of  the 
iodin  solution  must  be  added  and  the  mixture  again  set  aside.  Finally 
20  cc.  of  the  potassium  iodid  solution  are  added  together  with  50  cc. 
of  water,  and  the  mixture  titrated  with  the  decinormal  sodium  thio- 
sulphate until  the. color  is  almost  discharged,  when  a  few  drops  of 
starch  indicator  are  added  and  the  titration  continued  until  the  solu- 
tion is  colorless. 

The  Standardization  of  the  Iodin  Solution  is  effected  by  subjecting 
it  to  the  same  treatment  as  in  the  assay,  but  with  the  oil  omitted,  and 
at  the  end  of  four  hours,  titrating  with  the  decinormal  thiosulphate. 

The  difference  between  the  number  of  cc.  of  thiosulphate  solu- 
tion used  in  the  blank  test  and  the  number  used  in  the  actual  assay, 
is  multiplied  by  12.59,  and  this  divided  by  3,  gives  the  iodin  value  of 
the  oil  under  analysis. 

When  the  quantity  of  the  oil  or  fat  taken  is  not  0.3  gm.,  then  the 
product  is  not  divided  by  3,  but  by  the  figure  corresponding  to  the 
quantity  taken;  thus  if  0.15  gm.  are  taken  the  product  is  divided  by 

Another  way  of  making  the  calculation  is  as  follows: 
The  difference  between  the  cc.  of  thiosulphate  used  in  the  blank 
test,  and  the  cc.  used  in  the  analysis,  is  multiplied  by  0.01259,  ^nen 
by  100,  and  the  product  divided  by  the  weight  in  grams  of  the  oil 
taken  for  analysis. 
Example: 

Number  of  cc.  of  thiosulphate  used  in  blank  test 45 .4 

"    "    "  tf  "     "analysis 26.1 

representing  iodin  absorbed 19.3 

.    19.3X0.01259X100 
lodm  number  is  —     =80.9. 

This  method,  as  is  well  known,  is  based  upon  the  fact  that  the 
unsaturated  glycerids  in  the  oils  form  addition  products  with  the 
iodin.  The  mercuric  chlorid  and  iodin  contained  in  the  alcoholic 
solution  interact  with  a  formation  of  mercuric  chloriodid  and  iodin 
chlorid;  the  latter  is  supposed  to  be  the  active  agent. 

HgCl2+I2=HgClI  +  ICl 

*  The  time  allowed  does  not  give  the  complete  iodin  absorption  power  of 
an  oil  or  fat  and  cannot  be  compared  with  determinations  in  which  six  to  twelve 
hours  have  been  used.  It  gives,  however,  very  satisfactory  comparative  results, 
but  the  time  factor  must  be  very  closely  observed. 


476  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  mercuric  chlorid  also  acts  the  part  of  a  carrier  of  halogen  similar 
to  that  played  by  mercury  in  the  Kjeldahl  process  when  dissolving 
the  substance  in  sulphuric  acid. 

Gill  and  Adams,  J.  A.  C.  S.,  xxn,  12,  call  attention  to  the  fact 
that  not  only  addition  products,  but  also  substitution  products,  are 
formed  in  this  process,  which  vary  in  amount  with  the  time  of  action 
and  the  strength  of  the  solution.  This  is  a  feature  which  interferes 
with  the  accurate  determination  of  the  iodin  number,  giving  as  it  does 
a  higher  figure.  In  order  to  prevent  this  formation  of  substitution 
products,  and  thus  overcome  the  discrepancy  in  results,  these  authors 
suggest  the  use  of  mercuric  iodid  instead  of  mercuric  chlorid,  making 
the  solution  with  methyl  alcohol  (free  from  acetone  and  anhydrous). 
They  claim  that  by  the  use  of  a  solution  so  made  a  truer  iodin  absorp- 
tion number  is  obtained.  A  great  disadvantage  of  the  Hubl  method 
is  that  the  solution  quickly  loses  its  strength;  another  is,  the  length 
of  time  required  for  the  absorption.  As  above  described,  four  hours 
are  required,  but  this  is  not  sufficient  time  to  allow  of  complete  absorp- 
tion of  the  iodin.  It  is,  however,  a  good  practice  to  have  a  fixed 
time  limit;  the/ process  then  gives  very  satisfactory  comparative 
results  ~\ 

The  Harms  Method*  for  the  determination  of  the  iodin  absorp- 
tion number  is  conducted  in  a  manner  similar  to  the  above.  It  differs 
in  the  composition  of  the  iodin  solution  used.  This  is  prepared  as 
follows : 

Dissolve  13.2  gms.  of  iodin  in  1000  cc.  of  glacial  acetic  acid  (99.5 
per  cent),  showing  no  reduction  with  potassium  dichromate  and  sul- 
phuric acid;  add  enough  bromin  to  double  the  halogen  content 
determined  by  titration  (about  3  cc.  is  the  proper  amount).  Heat 
may  be  employed  for  the  solution  of  the  iodin,  but  the  liquid  must  be 
cold  when  the  bromin  is  added. 

The  Absorption  of  Iodin.  Add  25  cc.  of  the  iodin  solution  to  the 
chloroformic  solution  of  the  fat,  allow  to  stand,  with  occasional  shaking, 
for  thirty  minutes.  The  excess  of  iodin  should  be  at  least  60  per 
cent  of  the  amount  added. 

The  titration  of  the  unabsorbed  iodin  is  conducted  as  in  the  HiibPs 
method,  except  that  only  10  cc.  of  the  potassium  iodid  solution  are 
taken,  and  100  cc.  of  water  added. 

The  concensus  of  opinion  among  analysts  is  that  the  Hanus  method 
is  the  most  satisfactory.  Its  principal  advantage  over  the  Hubl 
method  lies  in  the  facts,  (a)  that  the  solution  (which  consists  of  iodin 
bromid  in  acetic  acid),  will  keep  its  strength,  or  at  least  change  very 


*  Zeitschr.  Nahr.  u.  Genus.  (1901),  913. 


FATS,   OILS,   AND   WAXES 


477 


little  in  three  months;  and  (b)  that  the  time  required  for  the  reaction 
with  the  oil  is  comparatively  short,  thirty  minutes  being  usually  allowed. 
Deiter  has  found,  however,  that  ten  to  fifteen  minutes  is  quite  suffi- 
cient. 

The  Wijs  Method.  Wijs  (in  Berichte  [1898]  750)  uses  an  iodin 
solution  which  holds  its  titer  much  longer  than  Hiibl's,  and  acts 
much  more  rapidly.  The  same  results  are,  however,  obtained.  The 
disadvantage  of  Wijs's  method  lies  in  the  difficulty  encountered  in 
preparing  the  solution,  which  consists  of  iodin  monochlorid  in  strong 
acetic  acid.  It  is  prepared  by  dissolving  13  gms.  of  pure  iodin  in  a 
liter  of  99  per  cent  acetic  acid,  and  then  taking  the  titer  by  thio- 
sulphate.  Chlorin  gas  (free  from  HC1),  is  then  passed  into  the  solu- 
tion until  the  titer  is  doubled.  The  proper  ending  is  known  when 
the  color  changes  from  brown  to  light  yellow.  The  process  of  titrat- 
ing the  fat  is  carried  out  exactly  as  in  Hiibl's  method,  except  that 
the  time  required  for  absorption  is  much  less.  For  fats  of  low  iodin 
value,  less  than  five  minutes  is  required;  for  others,  from  five  to  ten 
minutes. 

Usually  fifteen  to  thirty  minutes  are  given.  Like  the  Hanus  method 
its  advantage  lies  in  the  shortness  of  the  time  required  for  reaction 
with  the  oil,  and  also  in  the  fact  that  the  solution  remains  constant  for 
a  very  long  time.  The  results  obtained  with  it  and  the  Hanus  method 
compare  closely  with  those  of  the  Hiibl  method.  In  using  the  Wijs 
solution  an  excess  of  about  forty  per  cent  should  be  added  to  the  oil. 


TABLE  SHOWING  IODIN  ABSORPTION  BY  THE  THREE 
METHODS 


Hiibl's  No. 
4  Hours. 

Wijs'  No. 
30  Minutes. 

Hanus'  No. 
30  Minutes. 

Almond  oil  (expressed)  

08   2 

OO    I 

08   7 

Butter 

?  C.     A 

•7  r    n 

•3  e    * 

Castor  oil                    . 

87     3 

oj-y 
88 

«5o-4 

87    C. 

Cocoanut  oil  

8  4 

O    I 

8  6 

Cod-liver  oil 

14.4.    1 

I4C.    8 

Cottonseed  oil  

I  O4.    3 

106  c. 

*43*v 

106  6 

Lard  oil  • 

60  c 

7O    C. 

Linseed  oil  

170  6 

/wo 

188  T 

wy-  / 
186  2 

^Mustard  oil    . 

T  1  1    r 

118  7 

Olive  oil  

86  i 

86  7 

iiJ'0 

86  8 

Oleornargarin         ... 

C2    I 

e->    7 

C2    2 

Peanut  oil 

06    "? 

J^'O 

Rape  oil  

IO2    4. 

yy  • 
joe  6 

y/-o 

IOC.    2 

Sesame  oil  ...        ..... 

TO6    4. 

106  8 

106  < 

Theobroma  oil  . 

•JC      A 

34.    2 

7  1     C 

OJ'*t 

61  -* 

OJ'J 

478  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 


REFERENCES 

Volte  and  Logan.  J.  A.  C.  S.,  xxin,  156.  "A  Comparison  between  the 
Brorain  and  lodin  Absorption  Figures  of  Various  Oils."  They  allow  twenty- 
four  hours  for  the  absorption,  hence  their  figures  differ  widely  from  those  of 
other  experimenters. 

Wijs.     Chem.  Rev.,  Fett  u.  Hartz  Ind.,  vi,  6  (1899). 

Hanus.     Zeitschr.  f.  Nahrung  u.  Genus.,  iv,  913  (1901). 

Wijs.     Zeitschr.  f.  Nahrung  u.  Genus.,  v,  499  (1902). 

Tolman.    J.  A.  C.  S.,  xxvi,  826. 

Frank  and  Kaminetzky.     Chem.  Centr.,  i,  696  (1905). 

Tolman  and  Munson.     J.  A.  C.  S.,  xxv,  244. 

Marshall.     J.  S.  C.  I.,  213  (1900). 

The  Bromin  Value  (The  Bromin  Absorption  Number).  This 
indicates  the  percentage  of  bromin  absorbed  by  oils,  fats  or  resins. 
—The^  solvent  used  for  both  the  fat  and  the  bromin  is  either  carbon 
disulphid  or  carbon  tetrachlorid.  The  former  is,  however,  seldom 
used  because  of  its  offensive  odor;  the  latter  is  very  satisfactory.  The 
excess  of  bromin  after  absorption  is  found  by  titration  with  standard 
sodium  thiosulphate,  using  potassium  iodid  and  starch  as  the  indicator, 
or  by  retitration  with  a  standard  solution  of  beta-naphthol. 

This  process  has  been  much  improved  by  Mcllhiney,  J.  A.C.S.,  1899, 
1084,  who  determines  both  "the  bromin  addition  "  and  "the  bromin 
substitution."  The  process  consists  briefly  in  adding  to  a  weighed 
portion  of  the  fat  or  resin  in  a  stoppered  bottle,  an  excess  of  standard 
solution  of  bromin  in  carbon  tetrachlorid,  and  after  the  reaction  has 
taken  place,  determining  the  excess  of  bromin  by  adding  an  aqueous 
solution  of  potassium  iodid  and  titrating  with  standard  sodium  thio- 
sulphate. Any  HBr  which  may  have  been  formed  is  determined  in 
the  aqueous  solution.  The  per  cent  of  bromin  found  as  HBr  is  called 
the  substitution  figure,  and  total  per  cent  of  bromin  absorbed,  less 
twice  the  bromin  substitution  figure,  gives  the  bromin  addition  figure. 

The  process  in  more  detail  is  as  follows: 

A  blank  test  is  first  made  to  determine  the  strength  of  the  bromin 
solution  as  follows:  10  cc.  of  carbon  tetrachlorid  are  mixed  with  20 

/N\ 

cc.  of  the  standard  bromin  solution  ( — J  and  titrated  with  the  sodium 

thiosulphate. 

In  the  analysis  itself  0.2  of  oil  to  be  analyzed  is  weighed  into  a 
glass-stoppered  bottle  and  dissolved  in  10  cc.  of  carbon  tetrachlorid. 

N 
20  cc.  of  -—  bromin  in  carbon  tetrachlorid  are  then  added  from  a  pipette, 

o 
and  after  a  minute's  standing  20  to  30  cc.  of  10  per  cent  solution  of 


FATS,   OILS,  AND  WAXES  479 

KI  are  introduced,  the  amount  necessary  depending  upon  the  excess 
of  bromin  present. 

In  order  to  prevent  any  loss  of  bromin  or  HBr  on  removal  of  the 
stopper,  a  short  piece  of  wide  rubber  tubing  is  slipped  over  the  lip  of 
the  bottle  so  as  to  form  a  well  around  the  stopper.  It  is  advisable 
also  to  cool  the  bottle  by  placing  it  in  cracked  ice.  Into  the  well  so 
formed  the  potassium  iodid  solution  may  be  poured  and  the  stopper 
opened  slightly.  If  the  bottle  has  been  well  cooled  the  iodid  solution 
will  be  sucked  into  the  bottle.  No  loss  of  bromin  or  of  HBr  is  sus- 
tained by  this  procedure.  The  bottle  is  now  shaken  and  the  liberated 

N 
iodin  titrated  with  —  thiosulphate.     When  the  titration  is  finished, 

5  cc.  of  a  neutral  2  per  cent  solution  of  potassium  iodate  is  added; 
this  liberates  a  quantity  of  iodin  equivalent  to  the  HBr  formed,  and 

N 
on  titrating  this  with  —  thiosulphate,  the  bromin  substitution  figure 

is  calculated. 

The  principal  advantage  which  this  process  has  over  others  "is 
that  if  the  oil  or  fat  contains  substances  with  which  bromin  reacts  to 
form  substitution  products,  the  fact  will  be  made  known  by  the  pro- 
duction of  HBr,  which  is  determined  in  one  part  of  the  process.  The 
ordinary  animal  and  vegetable  fats,  such  as  olive  oil,  lard,  cottonseed 
oil,  linseed  oil,  etc.,  which  are  practically  pure  glycerids,  absorb  almost 
the  whole  of  the  bromin  which  disappears  by  direct  addition,  that  is 
to  say,  the  bromin  unites  with  pairs  of  carbon  atoms  which  had  pre- 
viously been  connected  by  double  or  triple  bonds,  and  consequently 
but  little  HBr  is  produced  by  the  reaction."  "The  most  frequent 
adulterants  of  fatty  oils  are  petroleum  and  rosin,  both  relatively  cheap. 
Both  of  these  react  with  bromin  to  produce  HBr,  and  thereby  give 
very  distinct  indication  of  their  presence."  (See  the  original  papers 
by  Mcllhiney,  J.  A.  C.  S.,  xvr,  275;  xxi,  1084,  and  xxiv,  1109.) 


CHAPTER  XLVII 
ANALYSIS  OF  SOAP 

(a)  Estimation    of   Water    and    Volatile    Matters.      10   gms.  of 
the  soap  are  dried  to  a  constant  weight  at   100°  C.  and  carefully 
weighed;   the  loss  of  weight  =  water. 

(b)  Free  Fats.    The  dried  soap  contained  as  above,  is  exhausted 
with  petroleum  ether  of  low  boiling  point.     The  petroleum  ether  is 
then  evaporated  off  and  the  residue  weighed;   this  is  the  weight  of 
the  fat  contained  in  10  gms.  of  the  soap. 

(c)  Fatty  Acids.    The  residue  from   (b)  which  is  free  from  fat 
and  which  represents  10  gms.  of  the  soap,  is  weighed  and  half  of  it  dis- 
solved in  water.     Normal  nitric  acid  is  then  added  in  excess  to  liberate 
the  fatty  acids.    These  are  collected  on   a   tared  filter,   dried  and 
weighed.     This  weight  when  doubled  gives  the  amount  of  fatty  acids 
in  10  gms.  of  the  soap.     The  reaction  is  illustrated  by  this  equation: 

NaCi  8H33O2 + HNO3 = HCi  8H33O2 + NaNO2. 

Sodium  oleate.  Oleic  acid. 

The  acid  filtrate  is  now  titrated  with  normal  soda  or  potash,  using 
phenolphthalein  as  an  indicator.  The  difference  between  the  volumes 
of  acid  and  alkali  solutions  used  gives  roughly  the  quantity  of  total 
alkali. 

(d)  Chlorids   and  Sulphates.     The   residual   neutral   liquid   from 
the  above  is  divided  into  two  equal  parts,  in  one  of  which  chlorin  is 

N 
estimated  by  —  AgNO3,  using  potassium  chromate. 

In  the  other  sulphuric  acid  is  estimated  with  barium  chlorid. 

(e)  Free  Alkali  (i.e.,  the  alkali  which  does  not  exist  as  soap).     Ten 
gms.  of  the  soap  are  dissolved  in  hot  alcohol,  and  one  drop  of  phe- 
nolphthalein T.  S.  added;    then  carbonic-acid  gas  is  passed  through 
the  solution  until  the  color  disappears.     The  free  alkali  is  thus  con- 
verted into  sodium  carbonate,  which  is  insoluble  in  alcohol  and  may 
be  separated  by  nitration.     The  residue  on  the  filter  is  washed  with 

N 

hot  alcohol,  and  then  dissolved  in  a  little  water  and  titrated  with  — 

10 

480 


SOAP  481 

acid  in  the  presence  of  methyl-orange.  The  number  of  cc.  used  multi- 
plied by  0.0031  gives  the  grams  of  free  alkali,  as  Na2O,  in  the  10 
gms.  of  soap. 

(/)  Combined  Alkali.  The  alcoholic  solution  from  the  above,  which 
contains  the  combined  alkali  and  the  fatty  acids,  is  diluted  with  a  little 
water,  methyl-orange  added,  and  the  mixture  titrated  with  decinormal 
acid.  The  quantity  of  combined  alkali  is  thus  found.  The  number 
of  cc.  of  acid  consumed  multiplied  by  0.0031  gives  the  quantity  as 
Na20. 

Another  way  is  to  evaporate  the  alcoholic  solution  to  dryness, 
the  residue  then  ignited,  and  the  soap  thus  converted  into  alkali  car- 
bonate. This  is  dissolved  in  water  and  titrated  with  normal  or  deci- 
normal acid  in  the  presence  of  methyl-orange. 

The  fatty  acids  are  found  by  using  the  factor  0.028014  °r  0.28014. 
The  number  of  cc.  of  decinormal  acid  used  in  the  above  titration,  when 
multiplied  by  0.028014,  or  of  normal  acid  when  multiplied  by  0.28014, 
gives  the  quantity  of  fatty  acid  as  oleie.  Soaps,  however,  contain 
various  fatty  acids,  the  molecular  weights  of  which  differ. 

Therefore  in  estimating  the  fatty  acids  volumetrically,  the  neu- 
tralizing power  of  the  acids  liberated  from  soap,  expressed  in  cc.  of 
standard  alkali,  and  called  the  saponification  equivalent,  is  em- 
ployed. 

Geissler  determines  the  free  and  combined  alkali  in  soap  as 
follows : 

Ten  grams  of  the  soap  are  dissolved  in  100  cc.  of  water,  phenol- 

N 
hpthalein  T.  S.  added,  and  the  solution  titrated  with  —  hydrochloric 

N 

acid  until  the  color  is  just  discharged.  The  quantity  of  —  hydro- 
chloric acid  used  represents  the  free  alkali  and  is  calculated  as  car- 
bonate. 

N 
Each  cc.  of  —  acid= 0.05265  gm.  of  Na2COs  or  0.06863  gm.  of 

K2C03. 

N 
The  —  acid  is  now  added  in  excess  in  order  to  liberate  the  fatty 

acids,  and  the  mixture  is  heated  to  melt  the  fatty  acids  and  cause  them 
to  form  a  clear  oily  layer  on  the  surface.  After  the  mixture  has  cooled 
off,  the  watery  layer  is  separated  and  the  fatty  acids  washed  with 
water.  The  washings  are  added  to  the  aqueous  liquid  and  titrated 

N 
with  —  potassium  hydroxid  until  the  red  color  reappears;    this  gives 

the  excess  of  acid,  and  when  deducted  from  the  quantity  of  acid  added 


482  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

after  decolorization  of  the  phenolphthalein,  gives  the  quantity  of  the 
acid  which  combined  with  and  hence  represents  the  combined  alkali 
of  the  soap.  This  is  also  calculated  as  carbonate,  using  the  same 
factors  as  given  above. 

Divine  *  recommends  the  following  method  for  determining  the 
free  alkali  in  soaps:  To  a  solution  2  gms.  of  the  soap  in  50  cc.  of  alco- 
hol, contained  in  a  flask  provided  with  a  reflux  condenser,  an  excess 

N 

of  —  stearic  acid  is  added,  and  the  flask  heated  on  a  water-bath  until 
10 

a  clear  solution  is  obtained.     The  excess  of  stearic  acid  is  then  deter- 

N 
mined  with  —  soda  solution,  the  difference  giving  the  data  for  total 

free  alkali,  both  hydroxid  and  carbonate.  In  a  second  experiment 
with  the  same  quantity  of  soap,  the  carbonate  is  removed  by  means  of 
10  per  cent  solution  of  barium  chlorid,  and  the  remaining  free  alkali 
estimated  as  before.  The  difference  between  the  total  free  alkali  as 
previously  ascertained  and  the  caustic  alkali  as  ascertained  in  the 
second  experiment,  gives  the  amount  of  free  alkali  present  as  carbonate 
in  the  sample. 

'  Determination  of  Glycerin.  E.  Martin  f  recommends  the  follow- 
ing method  for  determining  the  glycerin  in  soaps:  logms.  of  the  sam- 
ple is  dissolved  in  50  cc.  of  hot  water.  The  fatty  acids  are  liberated 
by  the  addition  of  a  slight  excess  of  sulphuric  acid,  the  heat  being 
continued  until  the  fat  is  completely  melted.  The  solution  is  then 
filtered  through  a  wetted  filter,  and  the  fatty  acids  left  on  the  filter 
are  washed  with  boiling  water.  The  filtrate  is  treated  with  excess 
of  basic  lead  acetate  solution,  and,  after  standing  for  half  an  hour, 
the  liquid  is  filtered  into  a  graduated  250  cc.  flask.  The  precipitate 
is  washed,  excess  of  lead  removed  by  the  addition  of  sulphuric  acid, 
and  the  whole  made  up  to  250  cc.  After  subsidence  25  cc.  of  the  liquid 
is  filtered  or  pipetted  off,  introduced  into  a  conical  flask  with  25  cc. 
of  potassium  dichromate  solution  (84.565  gms.  K2Cr2O7  per  liter), 
20  cc.  of  sulphuric  acid  (50  per  cent),  and  heated  on  the  water- 
bath  for  half  an  hour.  After  cooling,  the  excess  of  dichromate  is 
titrated  back  by  means  of  ferrous  ammonium  sulphate  (160  gms. 
FeSO4Ani2SO46H2O  and  20  gms.  H2SO4  per  liter)  in  the  usual  manner 
by  spotting  out  on  a  white  tile,  and  using  potassium  ferricyanid  as  an 
indicator.  These  solutions  should  first  be  "set  "  against  each  other. 
When  V=the  number  of  cubic  centimeters  of  the  solution  of  ferrous 
ammonium  sulphate,  equivalent  to  25  cc.  of  dichromate  solution,  and 


*  Pharm.  Centralh.,  Jan.  3,  1901.  7;    from  Chem.  Ztg.  (Rep.),  1900,  330. 
t  Pharm.  Jour.,  Dec.  31,  1904,  958;  from  Moniteur  Scient.  (4),  17,  797. 


SOAP  483 

' 

v=the  number  of  cc.  of  this  solution  actually  used  to  titrate  back  the 

V-v 
unused  dichromate,  then  X25  =  the  percentage  of  glycerin  in  the 

soap.  If  the  combined  glycerin  also  be  required,  as  in  the  case  of 
superfatted  soap,  another  determination  after  saponification  must  be 
performed.  The  difference  of  the  total  and  free  glycerin  will  be  the 
amount  of  the  combined  glycerin. 


CHAPTER  XL VIII 

ESTIMATION  OF  STARCH  IN  CEREALS,  ETC. 

By  Precipitation  with  Barium  Hydroxid. — The  method  about  to 
be  described  depends  upon  the  fact  that  when  barium  hydroxid  is 
brought  in  contact  with  starch,  an  insoluble  compound  is  formed,  the 
formula  of  which  is  C24H4oO2oBaO.  This  combination  takes  place 
in  definite  proportions,  so  that  if  an  excess  of  barium  hydroxid  solu- 
tion is  added  to  the  starchy  substance,  and  then  the  excess  esti- 
mated, the  quantity  which  combined  with  and  which  consequently 
represents  the  amount  of  starch  present,  is  found.  Solutions  required: 

1.  Decinormal  Hydrochloric  Acid.     (3.618  gms.  to  i  liter.)     Each 
cc,  represents  0.007614  gm.  of  BaO. 

2.  Baryta-water  (barium   hydroxid  solution),  made  by  dissolving 
about  7  gms.  of  pure  crystallized  barium  hydroxid  in  1000  cc.  of  water. 
This  should   be    kept  in   a   special  vessel,  such    as  is  illustrated  in 
Figs.  48  or  49. 

The  Process.  The  sample  is  finely  powdered,  and  i  gm.  weighed 
out  for  analysis.  This  is  rubbed  up  with  successive  portions  of  water 
(using  not  more  than  50  cc.)  and  transferred  to  a  flask  having  a  capacity 
of  about  150  cc.  The  flask  and  contents  are  now  heated  upon  a  water- 
bath  for  half  an  hour  to  thoroughly  gelatinize  the  starch.  If  the  sub- 
stance analyzed  contains  oil,  this  must  first  be  extracted  in  a  "Soxh- 
let  "  apparatus  before  the  water  is  added. 

If  free  starch  is  to  be  experimented  with,  0.2  or  0.3  gm.  instead 
of  i  gm.  should  be  taken. 

When  the  starch  is  gelatinized,  the  solution  is  cooled,  and  25  cc. 
of  the  baryta- water  are  added.  The  flask  is  corked,  and  well 
shaken  for  two  minutes;  proof  spirit  is  then  added  to  make  125  cc., 
the  flask  again  corked,  thoroughly  shaken,  and  set  aside  to  settle. 
While  settling  a  check  is  made  upon  10  cc.  of  the  baryta-water  mixed 
with  50  cc.  of  recently  boiled  distilled  water,  by  titrating  with  deci- 
normal  hydrochloric  acid,  using  phenolphthalein  as  indicator.  The 

N 

number  of  cc.  of  —  hydrochloric  acid  used,  is  noted,  and  when  mul- 
10 

tiplied  by  2^,  the  total  strength  of  the  25  cc.  of  the  baryta-water  em- 
ployed in  the  analysis  is  obtained. 

484 


STARCH  485 

When  the  settling  of  the  insoluble  compound  is  completed,  25  cc. 
of  the  clear  liquid  is  drawn  off  (this  is  -J-  of  the  entire  quantity)  with 

N 
a  pipette  and  rapidly  titrated  with  the  —  acid  V.  S.  in  the  presence  of 

a  few  drops  of  phenolphthalein  T.  S.  The  number  of  cc.  consumed 
is  noted,  multiplied  by  5,  and  then  deducted  from  the  number  repre- 
senting the  total  strength  of  25  cc.  baryta-  water.  The  difference  is 
the  quantity  which  went  into  combination  with  the  starch. 

N 

Each  cc.  of  the  —  hydrochloric  acid  represents  0.007614  gm.  of 
10 

BaO,  which  is  equivalent  to  0.0324  gm.  of  starch. 

Therefore  by  multiplying  the  number  of  cc.  representing  the 
quantity  of  baryta-water  which  combined  with  the  starch  by  0.0324 
gm.,  the  quantity  of  starch  present  in  the  sample  is  obtained. 

Example,  i  gm.  of  substance  was  taken,  mixed  with  50  cc.  of 
water,  25  cc.  of  baryta-  water,  and  sufficient  proof  spirit  to  make  125  cc. 
This  is  set  aside  and  allowed  to  settle. 

The  reaction  which  takes  place  is  as  follows: 


2Ci2H20Oio+BaO,  H2O=C24H4oO2o  .  BaO+H2O. 

Starch. 

2)648  2)152.28 

324  76.14 

While  settling,  the  strength  of  the  baryta-water  is  determined  by 
titrating  with  decinormal  hydrochloric  acid  the  following  equation 
being  applied: 

BaO,  H2O  +  2HCl=BaCl2+2H2O. 

2)152.28  2)72.36 

io)76.jr4_  io)_36._i8_ 

7.614  gins.         3.618  gms.  or  1000  cc.  —  V.  S. 

10 

Thus  each  cc.  represents  0.007614  gm.  of  BaO. 

N 
Ten  cc.  of  the  baryta-water  are  taken,  and  8  cc.  of  the  —  acid 

solution  are  required  to  neutralize  this.     Therefore  25  cc.  of  baryta- 

N 
water  will  require  2^X8  cc.  =  2o  cc.  of  —  acid  V.  S. 

When  the  settling  is  completed,  25  cc.  of  the  clear  solution  is  drawn 

N 
off  and  titrated  with  —  acid  V.  S.     We  will  assume  that  2.5  cc.  of  the 

N  I0. 

—  acid  V.  S.  are  required;    therefore  the  entire  quantity  of  solution 

10 

will  neutralize  5X2.5  cc.=  i2.5  cc. 


486  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  difference  between  12.5  cc.  and  20  cc.  =  ;.5  cc.,  which  is  the 

N 
loss  of  alkalinity  expressed  in  cc.  of  —  acid  V.  S.     Each  cc.  of  alka- 

N 

linity  lost,  expressed  as  —  acid  V.  S.,  indicates  that  0.007614  gm.  of 
10 

BaO  went  into  combination  with  starch;  and  since  0.007614  gm.  of 
BaO  represents  0.0324  gm.  of  starch,  the  substance  analyzed  contains 
7.5X0.0324  gm.  or  0.243  gm.  of  starch. 

0.243X100 

=  24.3  per  cent. 


Estimation  of  Starch  after  Inversion.  This  method  of  esti- 
mating starch  consists  in  converting  it  into  glucose  and  then  esti- 
mating the  glucose  with  Fehling's  solution. 

The  starch  is  weighed  and  boiled  in  a  flask  with  water  containing 
hydrochloric  acid  for  several  hours;  the  solution  is  then  cooled,  neu- 
tralized with  potassium  hydroxid,  and  diluted  so  that  one  part  of  starch, 
or  rather  sugar,  shall  be  contained  in  200  parts  of  water.  This  is  put 
into  a  burette  and  titrated  into  10  cc.  of  Fehling's  solution,  as  described 
under  Sugar. 

In  estimating  the  starch  in  baking  powder,  2  to  5  gms.  of  the  pow- 
der are  introduced  into  an  Erlenmeyer  flask,  150  to  200  cc.  of  a  4  per 
cent  solution  of  hydrochloric  acid  are  added  and  the  solution  gently 
boiled  for  four  hours,  after  which  the  flask  and  contents  are  cooled, 
neutralized  by  adding  sodium  hydroxid,  and  made  up  to  a  definite 
volume.  It  is  then  ready  for  testing  with  Fehling's  solution. 

According  to  O.  Lietz  *  the  estimation  of  starch  in  substances 
containing  but  small  quantities  of  cellulose,  may  be  made  as  follows: 
into  a  flask  of  about  500  cc.  capacity  put  2  to  10  gms.  of  the  sub- 
stance, according  to  the  quantity  of  starch  present.  Add  75  cc.  of 
alcoholic  potassium  hydroxid  solution  (containing  5  per  cent  of  KOH 
and  prepared  with  90  per  cent  alcohol);  connect  with  an  upright 
condenser  and  warm  on  asbestos  over  a  naked  flame  or  on  a  water- 
bath,  so  that  the  alcohol  boils  gently  for  twenty  minutes.  Then  cool 
somewhat  and  filter  by  suction  through  a  plug  of  asbestos.  Wash 
with  hot  alcohol  and  return  the  residue,  together  with  the  asbestos,  to 
the  flask,  rinsing  into  the  flask  any  particles  which  may  be  adhering 
to  the  funnel.  Add  200  cc.  of  water  and  20  cc.  of  hydrochloric  acid, 
and  invert  the  starch  by  heating  on  a  water-bath  for  two  and  a  half 
hours.  Nearly  neutralize  with  potassium  hydroxid,  leaving  the  liquid 
faintly  acid,  and  dilute  to  300  cc.  Of  this  fluid  remove  25  cc.,  esti- 

*  Berichte  D.  Pharm.  Gesellschaft,  1902,  153. 


STARCH  487 

mate  the  dextrose  present  (by  Allihn's  or  Fehling's  method),  and  cal- 
culate it  into  starch;  100  parts  of  glucose= 90.85  parts  of  starch. 

If  a  larger  proportion  of  cellulose  is  present,  the  residue  which  is 
left  after  treating  with  alcoholic  potassium  hydroxid  is  returned  to 
the  orignial  flask  and  30  to  60  cc.  of  a  3  to  5  per  cent  aqueous  potas- 
sium hydroxid  solution  added.  Warm  until  the  mass  is  almost 
entirely  dissolved.  Dilute  to  400  cc.,  filter  off  200  cc.,  neutralize 
with  hydrochloric  acid,  add  20  cc.  more  of  the  acid,  invert  by  heating, 
and  proceed  as  above. 

Estimation  of  Starch  after  Inversion  by  Means  of  Diastase. 
The  treatment  of  starch  with  malt  infusion  or  pure  diastase  at  a  tem- 
perature not  above  70°  C.,  readily  converts  the  starch  into  maltose, 
but  the  solution  also  contains,  besides  maltose,  various  dextrins  in 
proportions  varying  with  the  temperature  at  which  the  diastase  acts. 
The  digestion  may  vary  from  fifteen  rpinutes  to  fifteen  hours.  Com- 
plete conversion  of  the  starch  may  be  determined  by  testing  occasion- 
ally with  iodin.  A  blank  experiment  should  be  made,  especially  if 
the  digestion  is  carried  on  beyond  half  an  hour.  A  like  quantity  of 
the  same  diastase  solution  thould  be  digested  at  the  same  temperature 
and  for  the  same  time,  and  the  amount  of  sugar  found  deducted  from 
the  total  quantity  found  in  the  analysis.  Faulenbach  *  makes  use  of 
the  following  solution  of  diastase:  Crush  3.5  kilos  of  fresh  green 
malt,  treat  with  a  mixture  of  two  liters  of  water  and  four  liters  of 
glycerin  and  let  stand  for  one  week,  stirring  occasionally;  then  express 
and  filter.  This  solution  is  very  stable.  Five  drops  of  it  will  dis- 
solve i  gm.  of  starch;  15  drops  of  it  contain  a  quantity  of  carbohydrate 
=to  o.ooi  gm.  of  glucose. 

A  quantity  of  the  substance  to  be  tested  (containing  about  2  gms. 
of  starch)  is  boiled  to  gelatinize  the  starch.  Fifteen  drops  of  the  dias- 
tase solution  are  then  added,  and  the  mixture  digested  at  63°  C.  It 
is  then  filtered  to  separate  the  undissolved  cellulose,  etc.,  and  heated 
with  20  cc.  of  hydrochloric  acid  in  a  water-bath  for  three  hours.  The 
acidity  is  then  just  destroyed  by  means  of  caustic  soda,  the  glucose 
determined,  o.ooi  gm.  deducted,  and  the  starch  calculated  from  the 
glucose. 

O 'Sullivan  f  employs  pure  diastase,  prepared  as  follows:  Pour 
sufficient  water  over  2  or  3  kilos  of  finely  crushed,  pale  malt,  to  just 
cover  it.  Let  stand  for  three  or  four  hours,  then  express  and  filter 
the  solution.  Add  alcohol  (sp.gr.  0.83)  until  the  liquid  above  the 
flocculent  precipitate  becomes  opalescent.  Collect  the  precipitate, 


*  Zeitschr.  f.  physiol.  Chetn.,  VII,  5 10;    and  Chem.  Centralh.,  1883,  632. 
f  Jour.  Chem.  Soc.,  XLV  (1884),  i. 


488  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

wash  it  first  with  alcohol  (sp.gr.  0.86  to  0.88)  then  with  absolute  alco- 
hol, and  press  it  between  linen.  Finally  dry  it  in  a  vacuum  over 
sulphuric. 

The  Determination  of  Starch  in  Cereals  is  effected  as  follows:  5  gms. 
of  the  finely-ground  substance  is  treated  successively  with  ether,  with 
alcohol  at  35°  to  40°  C.,  and  with  water  at  the  same  temperature,  so 
as  to  remove  the  fat,  sugar,  soluble  albuminates  and  soluble  carbo- 
hydrates. The  residue  is  then  boiled  at  63°  C.  to  gelatinize  the  starch, 
and  allowed  to  cool.  0.025  to  0.035  gm-  °f  the  diastase  dissolved  in 
a  small  quantity  of  water  are  now  added  and  the  mixture  kept  at 
62°  to  63°  C.  for  an  hour.  It  is  then  heated  to  boiling,  filtered,  the 
insoluble  residue  washed  with  hot  water,  and  the  filtrate  diluted  to 
100  cc.  In  this  solution  are  determined  on  the  one  hand  the  .maltose 
by  Fehling's  solution,  and  on  the  other  hand  the  dextrin  by  polar- 
ization, deducting  from  the  total  polarization  that  due  to  maltose. 
Both  maltose  and  dextrin  are  then  calculated  into  starch  and  the 
results  added. 


CHAPTER  XLIX 
ESTIMATION  OF  SUGARS 

By  Fehling's  Solution.  If  a  solution  of  grape  sugar  is  warmed 
with  caustic  potash  or  soda,  together  with  a  cupric  salt,  the  grape  sugar 
is  oxidized  and  the  cupric  oxid  is  reduced  to  cuprous  oxid,  which 
separates  as  a  yellow  or  reddish  precipitate.  If  there  is  more  cupric 
oxid  present  than  the  grape  sugar  can  reduce,  a  black  precipitate  of 
cupric  oxid  falls,  which  hides  the  red  cuprous  oxid,  and  hence  the  pres- 
ence of  grape  and  other  reducing  sugars  may  be  overlooked. 

In  order  to  overcome  this  difficulty  certain  substances  may  be 
added  to  the  alkaline  solution  which  will  prevent  the  precipitation  of 
cupric  oxid,  but  not  cuprous  oxid;  among  such  substances  may  be 
mentioned  glycerin  and  the  neutral  tartrates.  The  former  is  used 
in  Haines'  solution,  while  the  double  tartrate  of  sodium  and  potas- 
sium is  used  for  this  purpose  in  Fehling's  solution. 

If  to  a  portion  of  hot  Fehling's  solution  some  grape  sugar  or  other 
reducing  sugar  is  added,  a  quantity  of  the  cupric  oxid  is  reduced  cor- 
responding to  the  quantity  of  sugar  present.  The  unreduced  cupric 
oxid  remaining  in  solution  while  a  distinct  yellow  or  reddish  precipi- 
tate of  cuprous  oxid  is  produced.  The  solution  above  this  precipitate 
is  a  clear  blue.  If,  however,  the  quantity  of  grape  sugar  added  is 
sufficient  to  completely  reduce  the  copper  present,  the  supernatant 
liquid  will  be  colorless. 

Preparation  of  Fehling's  Solution,  (a)  The  Copper  Solution. 
34.67  gms.  of  carefully  selected  small  crystals  of  pure  cupric  sulphate 
are  dissolved  in  sufficient  water  to  make,  at  or  near  15°  C.  (59°  F.), 
exactly  500  cc.  Keep  in  small  well-stoppered  bottles. 

(b)  The  Alkaline-tar tr ate  Solution.  173  gms.  of  potassium  and 
sodium  tartrate  (Rochelle  salt)  and  75  gms.  of  potassium  hydroxid, 
U.  S.  P.,  are  dissolved  in  sufficient  water  to  make,  at  or  near  15°  C. 
(59°  F.),  exactly  500  cc.  Keep  in  small  rubber-stoppered  bottles. 

For  use,  equal  quantities  of  the  two  solutions  should  be  mixed  at 
the  time  required. 

One  molecular  weight  of  water-free  glucose  will  reduce  five  mole- 
cular weights  of  cupric  oxid,  i.e.,  178.74  gms.  of  glucose  will  reduce 
1239.25  gms.  of  crystallized  copper  sulphate  (CuSO4+5H2O).  If 

489 


490  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

pure  chemicals  are  used  in  the  preparation  of  this  solution  there  will 
be  no  need  of  standardizing  it,  but  if  the  solution  is  old  and  its  titer 
doubtful,  the  following  method  of  standardization  may  be  employed. 

Dissolve  0.95  gm.  of  pure  cane  sugar  in  50  cc.  of  water,  add  2  cc. 
of  hydrochloric  acid,  and  heat  to  70°  C.  for  ten  minutes.  Then  neu- 
tralize with  sodium  carbonate  and  dilute  to  i  liter. 

Fifty  cc.  of  this  solution  should  exactly  reduce  10  cc.  of  Fehling's 
solution.  10  cc.  of  the  mixed  Fehling's  solution  is  equivalent  to 

Glucose 0.050 

Maltose 0.082 

Inverted  cane-sugar 0.0475 

Inverted  starch 0.045 

Lactose 0.0678 

The  Process.  0.5  gm.  or  less  of  the  sugar  is  dissolved  in  100  cc. 
of  water.  This  liquid  is  placed  in  a  burette.  10  cc.  of  the  Fehling's 
solution  are  mixed  with  40  cc.  of  water  and  placed  in  a  porcelain 
dish  over  a  Bunsen  burner  and  heated  to  boiling.  The  sugar  solu- 
tion is  then  run  in  from  the  burette,  until  all  blue  color  is  destroyed. 

The  Calculation.  10  cc.  of  Fehling's  solution  are  always  taken; 
and  whatever  the  quantity  of  glucose  or  sugar  solution  is  required  to 
effect  reduction,  that  quantity  contains  the  equivalent  of  10  cc.  of 
Fehling's  solution.  Thus  if  12  cc.  of  the  sugar  solution  were  required 
to  reduce  10  cc.  of  Fehling's  solution,  the  12  cc.  contain  0.05  gm.  of 
glucose  or  0.082  gm.  of  maltose,  etc.  100  cc.  of  the  solution  there- 
fore contain  x  gm.  of  glucose. 

0.05X100 

—  =  0.416  gm.  glucose. 

In  order  to  obtain  reliable  results  it  is  important  that  the  process 
be  carried  out  exactly  as  laid  down  in  the  above  directions,  and  that 
the  quantity  of  sugar  present  in  solution  be  no  greater  than  one  per 
cent.  The  degree  of  heat  and  the  time  occupied  in  the  process,  as 
well  as  the  concentration  of  the  Fehling's  solution,  have  a  very  impor- 
tant bearing  upon  the  accuracy  of  the  results.  The  complete  reduc- 
tion of  the  copper  (using  undiluted  Fehling's),  after  the  addition  of 
the  requisite  quantity  of  sugar,  does  not  take  place  instantly.  The 
time  required  varies  somewhat  with  the  different  sugars.  For  instance, 
with  glucose,  invert  sugar  and  levulose,  the  reduction  is  not  com- 
plete until  after  heating  two  minutes;  with  maltose,  four  minutes, 
and  with  lactose  six  minutes  are  required. 


SUGARS  491 

It  is  always  advisable  to  complete  the  titration  in  as  short  a  time 
as  possible.  A  preliminary  test  should  always  be  made,  in  which 
the  approximate  quantity  of  the  solution  required  is  found;  then  a 
second  and  more  accurate  titration  can  be  done  in  which  the  sugar 
solution  may  be  added  more  boldly,  and  the  time  of  boiling  and  expos- 
ure of  the  copper  solution  to  the  air  much  lessened. 

Determination  of  the  End-point.  It  is  always  somewhat 
difficult  to  determine  the  exact  point  at  which  the  blue  color  dis- 
appears, owing  to  the  presence  of  the  precipitated  suboxid  of  cop- 
per. This  difficulty  may  be  overcome  by  the  addition  of  some 
substance  which  will  prevent  the  precipitation  of  the  cuprous  oxid, 
such  as  ammonium  hydroxid  or  potassium  ferrocyanid.  When  the 
latter  is  used  the  disappearance  of  the  blue  color  can  then  be  readily 
seen,  as  the  solution  remains  clear  to  the  end,  turning  from  blue  to 
green,  and  finally  brown,  which  indicates  the  end  of  the  reaction. 

Professor  Bartley  reports  this  method  as  accurate,  reliable,  and 
rapid,  provided  the  solution  be  not  boiled  during  the  reduction.  He 
recommends  to  add  to  the  Fehling's  solution  in  the  porcelain  basin 
10  cc.  of  a  10  per  cent  freshly  prepared  solution  of  potassium  ferro- 
cyanid and  30  cc.  of  water.  The  ferrocyanid  does  not  precipitate 
the  copper  in  alkaline  solution. 

L.  Beulaygue  (Compt.  rend.,  138,  51)  suggests  the  following 
method  for  determining  the  end-reaction  when  titrating  sugar  solu- 
tion in  the  usual  manner  with  Fehling's  reagent,  using  solution  of 
sodium  monosulphid  as  the  indicator.  When  the  end  of  the  reaction 
is  near,  a  little  of  the  hot  solution  is  applied,  by  means  of  a  glass  rod, 
to  two  superimposed  white  filter  papers.  The  upper  one  acts  as  a 
filter,  retaining  the  particles  of  cuprous  oxid.  The  lower  paper  is 
withdrawn,  and  the  moist  spot  touched  with  a  drop  of  the  sodium 
monosulphid  reagent,  when  an  immediate  black  stain  of  cupric  sul- 
phid  is  formed  if  the  reaction  is  not  complete.  By  successive  spotting 
out  and  testing  in  this  manner,  a  very  accurate  reading  of  the  end- 
reaction  may  be  obtained.  It  is  important,  when  standardizing  the 
Fehling's  solution,  that  the  same  indicator  should  be  employed. 
Potassium  ferrocyanid  in  a  solution,  acidified  by  either  hydrochloric  or 
acetic  acid,  may  be  employed  in  a  similar  manner.  The  end-reaction 
is  then  indicated  by  the  disappearance  of  the  red  color  from  the  last  spot. 

Ley  and  Dichgens  (Pharm.  Ztg.,  48,  No.  68  P.,  689-670)  employ 
Fehling's  solution  in  excess,  heating  as  usual,  filtering  off  the  reduced 
cuprous  oxid,  and  treating  an  aliquot  part  of  the  filtrate  with  a  stan- 
dardized solution  of  potassium  ferricyanid,  also  in  excess.  This  latter 
excess  is  then  titrated  in  the  well-known  manner  with  potassium  iodid 
and  sodium  thiosulphate  (see  page  280),  in  a  medium  acidulated  with 
HC1,  using  starch  paste  as  indicator — the  reaction  here  being  sharp 


492  A    MANUAL  OF   VOLUMETRIC   ANALYSIS 

and  easily  recognized.  The  figures  thus  obtained  permit  the  ac- 
curate calculation  of  the  quantity  of  Fehling's  solution  originally  con- 
sumed, and  thus  of  the  percentage  of  glucose  in  the  substance 
examined. 

S.  A.  Vasey  *  finds  that  the  addition  of  a  quantity  of  precipitated 
calcium  carbonate  or  finely  powdered  barium  sulphate  facilitates  the 
recognition  of  end-point  in  sugar  determinations  with  Fehling's  solu- 
tion. The  mixture  of  a  measured  quantity  of  this  solution  with  about 
two  teaspoonfuls  of  either  of  the  compounds  named,  is  heated  to  the 
boiling  point  and  the  sugar  solution  is  run  in,  the  mixture  being  con- 
stantly stirred  until  the  supernatant  fluid  becomes  colorless — the 
exact  point  being  easily  recognized  by  the  complete  and  rapid 
precipitation  of  the  cuprous  oxid,  which  is  carried  down  by  the 
chalk  or  barium  sulphate,  leaving  the  liquid  limpid  and  trans- 
parent. 

E.  F.  Harrison,f  who  has  employed  Fehling's  solution  somewhat 
extensively  in  quantitative  sugar  estimations,  has  found  the  indicators 
usually  recommended  to  determine  the  end-point  of  reaction  to  be 
unsatisfactory.  It  was  suggested  by  him  that  the  action  of  cupric 
salts  in  liberating  iodin  from  iodid  might  be  utilized  for  this  purpose 
with  advantage,  and  his  experiments  determine  the  superiority  of 
this  over  the  other  indicators  heretofore  proposed.  The  indicator  is 
prepared  by  boiling  0.05  gm.  of  starch  with  a  few  cc.  of  water,  adding 
10  gm.  of  potassium  iodid  and  diluting  to  100  cc.  This  indicator 
should  be  prepared  as  required.  In  use  0.5  to  i.o  cc.  of  this  solution 
is  acidified  with  about  5  or  10  drops  of  acetic  acid,  and  one  drop  or 
more  of  the  liquid  in  process  of  titration  added.  As  long  as  unreduced 
copper  is  present,  a  color  is  produced,  varying  from  red  to  blue,  and 
of  greater  or  less  intensity,  according  to  the  nearness  of  the  end-point. 
The  production  of  no  color  marks  the  end  of  the  reaction.  The  indi- 
cator is  available  with  one  drop  of  a  solution  containing  one  part  of 
cupric  sulphate  in  twenty  thousand  parts. 

//  the  sugar  to  be  examined  be  either  glucose,  maltose,  or  lactose, 
it  may  be  titrated  directly;  but  if  it  be  cane-sugar,  it  must  first  be 
inverted.  This  is  done  by  dissolving  the  sugar  (0.475  gm-)  m  about 
100  cc.  of  water,  adding  3  or  4  drops  of  strong  hydrochloric  acid, 
and  boiling  briskly  for  ten  or  fifteen  minutes.  This  is  then  allowed 
to  cool,  neutralized  with  potassium  hydroxid,  and  made  up  to  100  cc. 
with  water. 

The  sugar  in  urine  may  be  estimated  by  this  process.  The  urine 
is  placed  in  the  burette  and  run  into  the  boiling  Fehling's  solution  in 

*  Lancet,  184  (1903),  i,  137.         f  Trans.  Brit.  Pharm.  Conf.,  1903   568,  569. 


SUGARS  493 

the  usual  manner.     If  it  contain  a  large  quantity  of  sugar,  it  must  be 
diluted  two  or  three  times. 

In  estimating  with  Fehling's  solution  it  is  well  to  attach  a  rubber 
tube  eight  to  twelve  inches  in  length  to  the  lower  end  of  the  burette, 
so  that  the  boiling  need  not  be  done  directly  under  the  burette,  and 
thus  cause  incorrect  readings  through  the  expansion  of  the  liquid 
therein. 

Pavy's  Method.  This  consists  in  adding  ammonia  water  to  the 
ordinary  Fehling's  solution,  in  order  to  prevent  the  precipitation  of 
cuprous  oxid,  which  has  a  tendency  to  hide  the  end-reaction.  Thus 
the  disappearance  of  the  blue  color  which  constitutes  the  end-reaction 
is  distinctly  seen. 

Pavy's  solution  is  made  by  dissolving  170  gms.  of  Rochelle  salt 
and  170  gms.  of  potassium  hydroxid  in  sufficient  water.  Then 
34.65  gms.  of  copper  sulphate  are  separately  dissolved  in  water 
with  the  aid  of  heat,  and  the  two  solutions  are  mixed  and  diluted  to 
i  liter. 

120  cc.  of  this  solution  are  now  taken  and  mixed  with  400  cc.  of 
ammonia  water  (sp.gr.  0.88)  and  diluted  with  water  to  i  liter.  This 
constitutes  Pavy's  solution,  or  rather  Pavy-Fehling  solution,  of  which 
10  cc.=  i  cc.  of  Fehling's  solution,  i.e.,  10  cc.  of  Pavy's  solution  =0.005 
gm.  of  glucose. 

The  process  is  conducted  as  follows:  10  cc.  of  the  Pavy's  solution 
are  diluted  with  20  cc.  of  water  and  placed  in  a  small  flask,  and  heated 
to  and  kept  at  the  boiling  point,  while  the  glucose  solution  properly 
diluted  is  added  from  a  burette.  The  glucose  solution  should  be 
added  at  about  the  rate  of  100  drops  per  minute  until  the  blue  color  is 
just  destroyed.  The  sugar  solution  should  be  so  diluted  that  not  less 
than  4  nor  more  than  7  cc.  are  required  to  produce  the  decolora- 
tion. 

In  order  to  avoid  the  nuisance  of  filling  the  laboratory  with  ammo- 
niacal  vapors,  the  titration  may  be  performed  in  a  small  flask  pro- 
vided with  a  well-fitting  cork,  having  two  holes,  through  one  of  which 
the  spit  of  the  burette  is  passed,  and  through  the  other  an  escape- 
tube  which  conducts  the  vapors  into  a  vessel  containing  water  or 
diluted  hydrochloric  acid. 

Several  titrations  should  always  be  made  in  order  to  obtain  exact 
results,  and  it  is  advisable  to  check  the  solution  against  a  sugar  solu- 
tion of  known  strength,  since  the  ratio  of  reduction  is  seriously  influenced 
by  the  amount  of  potassium  hydroxid  present  and  the  strength  of  the 
ammonia  water. 

The  calculation  is  exactly  the  same  as  that  in  the  use  of  Fehling's 
solution,  except  that  10  cc.  of  Pavy= 0.005  gm.  glucose. 


494  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

The  Soxhlet-Fehling  Method.  The  solutions  required  are: 
(a)  Copper  Sulphate  Solution.  Dissolve  34.639  gms.  of  CuSO4 .  5H2O 
in  water  and  dilute  to  500  cc. 

(b)  Alkaline   Tartrate   Solution.     Dissolve    175   gms.  of    Rochelle 
salts  and  50  gms.  of  sodium  hydroxid  in  water  and  dilute  to  500  cc. 

(c)  Mixed  Solution.     Mix  equal  volumes  of  solutions  (a)  and  (b) 
immediately  before  use. 

A  preliminary  titration  is  made  to  determine  the  approximate 
per  cent  of  reducing  sugar  in  the  material  as  follows:  100  cc.  of  the 
mixed  solution  are  placed  in  a  porcelain  dish,  heated  to  boiling,  and 
the  sugar  solution  added  in  small  portions  at  a  time  until  the  liquid, 
after  sufficient  boiling  (the  duration  of  which  must  depend  upon  the 
kind  of  sugar,  see  page  490),  no  longer  appears  blue.  From  this  pre- 
liminary test  calculate  the  approximate  quantity  of  sugar  correspond- 
ing with  100  cc,  of  Soxhlet-Fehling  solution,  as  per  the  following 
table. 

On  boiling  one  per  cent  solutions  of  the  different  sugars  with 
undiluted  Soxhlet-Fehling  solution,  the  following  results  are  obtained: 

Grape  sugar,  C6H12O6 0.475    gm- 

Invert  sugar,  CeH^Oe 0.494      * ' 

Levulose,  CeH^Oe 0.5144    ' ' 

Maltose,  Ci2H22On 0.778      " 

Lactose,  Ci2H22Oii,  H20 0.676      ' ' 

Then  a  solution  of  the  sugar  to  be  examined  is  made,  which  contains 
approximately  one  per  cent.  Place  into  a  beaker  100  cc.  of  the  mixed 
copper  solution  and  add  approximately  the  amount  of  the  sugar  solu- 
tion for  its  complete  reduction.  Boil  for  two  minutes.  Filter  through 
a  folded  filter  and  test  a  portion  of  the  filtrate  for  copper  by  the  use  of 
acetic  acid  and  potassium  ferrocyanid.  Repeat  the  test,  varying  the 
volume  of  sugar  solution,  until  two  successive  amounts  are  found 
which  differ  by  o.i  cc.,  one  giving  complete  reduction  and  the  other 
leaving  a  small  amount  of  copper  in  solution.  The  mean  of  these 
two  readings  is  taken  as  the  volume  of  the  solution  required  for  the 
complete  precipitation  of  100  cc.  of  the  copper  reagent. 

Under  these  conditions  100  cc.  of  the  mixed  copper  reagent  require 
0.475  Sm-  °f  anhydrous  dextrose  or  0.494  gm.  of  invert  sugar  for  com- 
plete reduction.  Calculate  the  percentage  by  the  following  formula: 

V=the  volume  of  the  sugar  solution   required  for  the  complete 

reduction  of  100  cc.  of  the  copper  reagent. 
W=the  weight  of  the  sample  in  i  cc.  of  the  sugar  solution. 


THE    PERMANGANATE    METHOD  495 


100X0.475 
Then  —       —  =per  cent  of  dextrose, 


ioo  X  0.494 
or  — 7777T —  =  per  cent  of  invert  sugar. 

The  Permanganate  Method.*  Place  60  cc.  of  the  mixed  Fehling's 
alkaline  copper  solution  in  a  beaker,  add  60  cc.  of  water,  and  heat  to  boil- 
ing. Add  25  cc.  of  the  sugar  solution,  which  must  not  contain  more  than 
0.250  gm.  of  dextrose,  and  boil  for  two  minutes.  Filter  immediately 
through  asbestos  without  diluting,  then  filter  through  a  Gooch  crucible, 
wash  the  beaker  and  precipitate  thoroughly  with  hot  water,  without 
any  effort  to  transfer  the  precipitate  to  the  filter.  Wash  the  asbestos 
film  and  adhering  cuprous  oxid  into  a  beaker,  add  about  30  cc.  of 
hot  water,  and  heat  the  precipitate  and  asbestos  thoroughly.  Rinse 
the  crucible  with  50  cc.  of  hot  saturated  solution  of  ferric  sulphate 
in  20  per  cent  sulphuric  acid,  receiving  the  rinsings  in  the  beaker  con- 
taining the  precipitate.  After  the  cuprous  oxid  is  dissolved,  wash 
the  solution  into  a  large  Erlenmeyer  flask  and  immediately  titrate 
with  standard  solution  of  potassium  permanganate.  One  cc.  of  the 
permanganate  solution  should  equal  o.oio  gm.  of  copper.  In  order 
to  determine  the  strength  of  this  solution,  make  six  or  more  determi- 
nations with  the  same  sugar  solution,  titrating  one  half  of  the  pre- 
cipitates obtained,  and  determining  the  copper  in  the  others  by  electrol- 
ysis. The  average  weight  of  copper  obtained  by  electrolysis,  divided 
by  the  average  number  of  cubic  centimeters  of  permanganate  solu- 
tion required  for  the  titration,  is  equal  to  the  weight  of  copper  equiva- 
lent to  one  cc.  of  the  standard  permanganate  solution.  A  solution 
standardized  with  iron  or  oxalic  acid  will  give  too  low  results. 

METHODS    DEPENDING   UPON  THE  REDUCTION   OF  MERCURY  COMPOUNDS 

Knapp's  Method.  The  Standard  Mercuric  Cyanid  Solution. 
10  gms.  of  pure  dry  mercuric  cyanid  are  dissolved  in  water,  ioo  cc. 
of  caustic  soda  solution  (sp.gr.  1.145)  are  added,  and  the  liquid  diluted 
to  measure  I  liter. 

The  method  as  originally  used  by  Knapp  consisted  in  adding  the 
sugar  solution  gradually  to  the  mercurial  solution,  was  found  by  Soxh- 
let  and  others  to  be  inaccurate.  Brumme,  and  also  Soxhlet,  assert 
that  the  reducing  effect  of  sugar  is  greater  when  the  requisite  quantity 

*  One  of  the  methods  of  the  Assoc.  of  Official  Agricultural  Chemists,  Bulletin 
No.  107. 


496  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

of  the  sugar  solution  is  added  all  at  once,  in  a  manner  analogous  to 
that  of  the  Soxhlet-Fehling  method.  100  cc.  of  the  mercury  solution 
are  taken,  and  as  near  as  can  be  judged  the  entire  requisite  quantity 
of  the  0.5  or  i  per  cent  sugar  solution  added  all  at  once,  the  liquid 
boiled  for  two  or  three  minutes  and  then  tested  to  see  if  it  still  con- 
tains mercury.  Several  tests  are  made  with  larger  and  smaller  quan- 
tities of  sugar  solutions  until  two  tests  are  made  in  which  the  quantity 
of  sugar  differs  but  very  little,  one  of  which  containing,  however,  a 
slight  quantity  of  mercury  and  the  other  being  free  from  it. 

The  end  of  reaction  is  found  by  placing  a  drop  of  the  clear  liquid 
above  the  precipitate  on  a  piece  of  white  filter  paper  and  holding  this 
over  an  open  bottle  of  fuming  hydrochloric  acid,  and  then  over  strong 
sulphureted  hydrogen  water.  A  light  brown  stain  so  produced  indi- 
cates mercury. 

According  to  Soxhlet,  the  following  relations  exist  between  Knapp's 
solution  and  the  various  sugars.  100  cc.  of  Knapp's  solution  are 
reduced  by  the  following  quantities  of  sugar,  using  a  0.5  per  cent 
sugar  solution: 


" 


Grape  sugar,  CeH^Oe  ....................  0.202  gm. 

Invert  sugar,  CeH^Oe  ....................  0.200 

Levulose,  C6Hi2O6  .......................  0.198 

Maltose,  Ci2H22On  .......................  0.308    " 

Milk  sugar,  Ci2H22Oii  .  H20  ..............  0.311    " 

Sachsse's  Method.  The  Standard  Mercuric  lodid  Solution. 
18  gms.  of  pure  dry  mercuric  iodid  and  25  gms.  of  pure  potassium  iodid 
are  dissolved  in  water,  and  to  this  solution  is  added  80  gms.  of  caustic 
potash  dissolved  in  water,  and  then  the  whole  diluted  to  i  liter. 

In  using  this  solution  more  mercury  seems  to  be  reduced  if  the 
sugar  is  added  gradually  than  if  added  all  at  once.  The  reverse  is 
true  in  the  case  of  Knapp's  solution.  It  is,  however,  necessary,  in 
order  to  obtain  comparable  results,  to  proceed  in  the  manner  pre- 
scribed by  Soxhlet  for  Knapp's  solution.  It  is  also  to  be  remembered 
that  the  reducing  action  of  sugar  upon  this  solution  differs  according 
as  a  i  per  cent  or  a  0.5  per  cent  solution  is  used.  It  is  therefore  import- 
ant to  have  the  sugar  solution  as  nearly  0.5  per  cent  as  possible. 

The  operation  is  carried  out  by  using  100  cc.  of  Sachsse's  solution 
and  proceeding  in  the  same  manner  as  described  under  Knapp's 
method.  An  alkaline  stannous  oxid  solution  is  used  as  indicator. 
The  end-reaction  is  found  by  removing  a  few  drops  of  the  clear  super- 
natant liquid  and  mixing  it  with  the  alkaline  stannous  oxid  solution 
on  a  porcelain  plate.  A  black  precipitate  indicates  a  large  quantity 
of  mercury,  a  brown  precipitate  indicates  a  very  small  quantity.  The 


SUGARS  497 

alkaline  stannous  oxid  solution  may  be  prepared  by  simply  super- 
gaturing  a  stannous  chlorid  solution  with  caustic  soda.  100  cc.  of 
Sachsse's  solution  are  reduced  by  the  following  quantities  of  sugar, 
when  in  0.5  per  cent  solution,  according  to  Soxhlet,  under  the  above 
conditions. 

Grape  sugar 0-325  gm. 

Invert  sugar 0.269    ' ' 

Levulose 0.213    " 

Maltose 0.491    ' ' 

Milk  sugar 0.387    ' ' 

REFERENCES 

Munson  and  Walker,  "  Unification  of  Reducing  Sugar  Methods."    J.  A. 
C.  S.,  xxvin,  663. 

Oerum,  Zeitschr.  anal.  Chem.,  XLIII,  365. 
Rosenthal,  Zeitschr.  anal.  Chem.,  XLIII,  252. 
Craven  and  Hill.     J.  S.  C.  I.,  xvi,  981;  and  xvii,  124. 
Knapp,  Zeitschr.  anal.  Chem.,  ix,  395. 
Sachsse,  Zeitschr.  anal.  Chem.,  xvi,  121. 


CHAPTER  L 
ESTIMATION  OF  ALKALOIDS  (VOLUMETRICALLY) 

IN  making  alkaloidal  assays  of  drugs  it  has  long  been  the  custom 
to  evaporate  the  final  ethereal  or  chloroformic  extract,  and  to  weigh 
the  residue  as  alkaloid.  This  residue  seldom,  if  ever,  consists  of  the 
pure  alkaloid,  and  the  impurities — i.e.,  non-alkaloidal  matter — is  varia- 
ble in  amount  and  difficult  to  entirely  remove,  consequently  gravi- 
metric results  are  in  many  cases  very  wide  of  the  truth,  and  hence 
unreliable. 

The  volumetric  methods  are  in  most  cases  much  more  satisfactory. 

While  the  results  of  'the  titration  of  the  total  alkaloids  of  drugs 
cannot  be  called  absolutely  accurate,  nevertheless  experience  has 
shown  that  they  are  nearer  the  truth  than  those  obtained  by  the  gravi- 
metric method. 

In  estimating  an  alkaloid  by  titration  it  is  essential  to  know  the 
formula  and  molecular  weight  of  the  alkaloid,  as  well  as  the  equiva- 
lent of  acid  with  which  it  will  combine. 

The  quantity  of  alkaloid  present  is  easily  calculated  when  we 
know  that  a  molecular  weight  of  a  monobasic  acid  or  half  a  molecular 
weight  of  a  dibasic  acid  will  combine  with  and  neutralize  a  molecular 
weight  of  an  alkaloid,  provided  the  alkaloid  is  a  monacid  base.  If 
the  alkaloid  is  a  diacid  base,  one  molecular  weight  will  combine  with 
two  molecules  of  a  monobasic  acid  or  one  molecular  weight  of  a  dibasic 
acid. 

Sparteine  and  emetine  ( ?)  are  diacid  alkaloids ;  most  of  the  others 
are  monacid  bases. 

Examples.     Monacid  alkaloids: 

C2oH24N2O2  +  HCl=C2oH24N2O2  .  HC1; 

Quinine. 

(C17H19N03)2+H2S04=(C17H19N03)2  -  H2SO4; 

Morphine. 

(C2iH22N202)2  +  H2S04=(C2iH22N202)2  -  H2SO4; 

Strychnine. 

498 


ESTIMATION   OF    ALKALOIDS  499 

Diacid  alkaloids: 

Ci5H26N2  +  2HC1=  Ci5H26N2(HCl)2. 

Sparteine. 


.  H2SO4. 

Emetine  (Kunz). 

The  quantity  of  alkaloid  present  in  the  substance  is  easily  calcu- 
lated, as  illustrated  by  this  equation: 

C20H24N2O2  +  HC1  =  C20H24N2O2  .  HC1. 

Quinine. 

N 
321.82  gms.     36.18  gms.  =  1000  cc.  —  V.  S.; 

N 

32.182  gms.     3.618  gms.  =  1000  cc.  —  V.  S. 

10 

N 
Thus  i  cc.  of  --  V.  S.  =  o.o32i82  gm.  of  quinine. 

Ci5H26N2  -f  2HC1  =  Ci5H26N2(HCl)2. 

Sparteine. 

N 
2)232.51  gms.     2)73.36  gms.  =  1000  cc.  —  V.  S.; 

116.25     "          36.18     " 

N 

11.625"  3-6i8"    =iooocc.  —  V.  S.; 

10 

N 
i  cc.  of  —  V.  S.  hence=  0.011625  gm.  of  sparteine. 

10  N 

Thus  1000  cc.  of  -  -  hydrochloric  acid  will  combine  with  TV  of 
10 

the  molecular  weight  in  grams  of  a  monacid  alkaloid,  or  -fa  of  the 
molecular  weight  of  a  diacid  alkaloid. 

In  the  case  of  drugs  where  two  or  more  alkaloids  are  present,  accu- 
rate results  can  only  be  obtained  by  determining  how  much  of  each 
alkaloid  is  present  by  a  separate  assay.  But  often  it  is  assumed  that 
the  alkaloids  are  present  in  equal  quantities,  and  the  mean  of  their 
molecular  weights  is  taken  as  the  basis  for  the  calculation. 

It  must  be  borne  in  mind,  however,  that  in  titrating  alkaloids  the 
greatest  care  must  be  exercised  and  all  precautions  closely  observed 
in  order  to  attain  any  degree  of  accuracy.  The  volumetric  solutions 
must  be  prepared  with  the  greatest  care  and  must  be  absolutely  accu- 


500  A    MANUAL    OF    VOLUMETRIC   ANALYSIS 

rate.  The  eye  must  be  trained  (as  it  can  only  be  through  practice), 
to  distinguish  the  end-colors  of  the  indicators  employed,  a  matter  of 
some  difficulty.  Furthermore,  all  measuring  instruments  used  must 
be  accurate,  or  they  should  be  carefully  calibrated  in  order  to  find 
the  necessary  factor  for  correction. 

The   Volumetric   Solutions  usually  employed  in  titrating  alka- 

N     N    N     N  N 

loids  are  — ,  — ,  — ,  — ,  and  .     The  weaker  solutions  give  more 

10    20    25    50  100  N 

accurate  results,  as  will  be  understood  if  we  remember  that  a  —  is 

N  N 

10  X  stronger  than  a  —    and    100  X  stronger   than     —  V.  S.     Then 
N  10  100 

if  —  solution  be  used,  one  drop  may  overstep  the  neutral  point,  while 

1  N 

if  the  same  solution  were  treated  with  —  solution  5  drops  would  be 

10 

required  to  neutralize,  which  is  equivalent  to  using  one  half  a  drop  of 

N  N 

—  solution.     A  solution  will  of  course  be  capable  of  even  more 

i  100  N 

delicate  work.     In  the  case  just  mentioned  45  drops  of  the  —  may 

100 

N 
exactly  neutralize  the  solution,  hence  less  than  half  a  drop  of  —  and 

N 

between  4  and  5  drops  of  —  are  represented.     Therefore  in  all  deli- 

10 

cate  alkaloidal  titrations  weak  standard  solutions  should  be  used — 
in  fact,  in  all  titrations  where  great  accuracy  is  required. 

If  the  alkaloid  be  from  a  recent  extraction  and  is  in  the  form  of 

N 

a  free  alkaloid,  it  is  dissolved  in  a  measured  excess  of  —  acid  solu- 

10 

tion  and  the  excess  of  acid  solution  then  determined  by  residual  titra- 

N 
tion  with  alkali  solution. 

.  I0°    N 
In  this  the  —  sulphuric  acid  solution  is  preferred,  except  in  the 

10  N 

case  of  quinine  or  cinchonine,  in  which  -  -  hydrochloric  acid  gives 
better  results. 

The  process  in  detail  is  as  follows:    Place  2  gms.  of  the  alkaloid 

N 
into  a  beaker,  add  75  cc.  of  —  sulphuric  acid  solution,  and  warm  on 

a  water-bath  until  the  alkaloid  is  completely  dissolved.  The  solution 
is  then  allowed  to  cool  and  diluted  to  100  cc. 

locc.  of  the  solution  (containing  0.2  gm.  of  the  alkaloid  and  7.5  cc. 

N 

of  —  sulphuric  acid  solution)  are  removed  by  means  of  a  pipette  and 
10 


ESTIMATION    OF    ALKALOIDS  501 

N 

retitrated   with    potassium  hvdroxid   solution.     One  tenth  of  the 

100 

N  N 

quantity  of  the  alkali  used  is  deducted  from  the  7.5  cc.  of  the  — 

acid  solution,  and  the  remainder  is  the  quantity  of  the  latter  which 
combined  with  and  hence  represents  the  alkaloid  present.  This,  if 
multiplied  by  the  factor,  gives  the  weight  of  the  alkaloid. 

Either  hasmatoxylin  solution  or  Brazil-wood  T.  S.  may  be  employed 
as  the  indicator. 

//  the  alkaloid  is  soluble  in  alcohol,  as  are  quinine  and  codeine,  it 
may  be  treated  as  follows: 

Place  2  gms.  of  the  alkaloid  in  a  graduated  cylinder,  dissolve  in 
alcohol,  and  dilute  the  solution  up  to  100  cc.  with  alcohol.  Remove 
10  cc.  of  this  solution  (containing  0.2  gm.  of  the  alkaloid)  and  place 
in  a  beaker,  add  the  indicator  and  run  in  the  decinormal  acid  solu- 
tion to  slight  excess.  Rotate  the  beaker  several  times,  let  stand  for  a 
few  minutes,  wash  down  the  sides  of  the  beaker  with  distilled  water, 

N 
using  about  40  cc.,  and  retitrate  the  excess  of  acid  with potassium 

hydroxid  solution,  until  end-color  is  given  by  the  indicator.     Deduct 

N 
one  tenth  of  the  quantity  of  the solution  used  from  the  quantity 

N" 

of  —  acid  solution  added,  and  the  remainder  is  the  quantity  of  the 
10 

latter,  which  combined  with  and  hence  represents  the  alkaloid  present. 

The  indicators  best  suited  for  most  alkaloids  are  Hamatoxylin, 
Brazil-wood,  Cochineal,  lodeosin,  and  Litmus. 

A.  H.  Allen  states:  "In  titrating  an  alkaloid  with  methyl-orange 
as  indicator  it  is  rarely  convenient  to  employ  an  aqueous  solution  of 
the  base. 

"A  solution  in  proof -spirit  can  be  employed,  but  the  indicator 
is  much  less  sensitive  under  such  conditions. 

"I  have  found  it  preferable,  especially  when  an  alkaloid  is  much 
colored,  as  is  frequently  the  case  in  assaying  bases  directly  extracted 
from  their  sources,  to  dissolve  the  alkaloid  in  a  little  chloroform, 
ether,  amylic-alcohol,  or  other  suitable  immiscible  solvent. 

"The  solution  is  placed  in  a  small  stoppered  cylinder,  together 
with  a  few  cc.  of  water  colored  with  a  drop  or  two  of  methyl-orange. 
Then  on  gradually  running  in  the  standard  acid  from  a  burette,  and 
agitating  thoroughly  after  each  addition,  it  is  easy  to  observe  the  end 
of  the  reaction,  as  the  coloring  matter  remains  in  the  immiscible 
layer  and  presents  a  marked  contrast  to  the  red  color  of  the  aqueous 
liquid." 

Allen  has  obtained  satisfactory  results  with  aconitine  and  its  allies, 


502 


A    MANUAL    OF   VOLUMETRIC    ANALYSIS 


even  when  working  on  as  little  as  0.030  gm.,  by  using  ether  as  a 

N 
solvent  and  titrating  with  —  hydrochloric  acid. 

In  the  titration  of  cinchona  alkaloids  such  anomalous  results  are 
obtained  that  there  is  some  doubt  as  to  whether  the  relation  of  these 
alkaloids  to  acids  is  thoroughly  understood.  When  quinine  is  titrated 
with  an  acid,  almost  twice  as  much  of  the  latter  is  used,  when  methyl- 
orange  is  the  indicator,  as  when  Brazil-wood  is  employed  as  indicator. 
This  is  probably  due  to  the  fact  that  ordinary  quinine  sulphate  is 
slightly  alkaline  to  methyl-orange,  and  the  end-reaction  with  this 
indicator  is  not  reached  until  the  acid  sulphate  is  formed,  while  with 
Brazil-wood  as  indicator  the  end-reaction  is  reached  sooner,  that  is, 
when  the  normal  sulphate  is  formed,  which  is  practically  neutral  to 
this  indicator. 

Quinine  sulphate  is  also  neutral  to  cochineal,  but  distinctly  alkaline 
to  litmus;  hence  the  latter,  like  methyl-orange,  is  inapplicable  in  the 
titration  of  quinine.  These  anomalies  should  be  had  in  mind  when 
working  upon  the  cinchona  bases. 

In  titrating  alkaloids  the  personal  equation  plays  an  important 
part.  It  is  generally  correct  to  titrate  to  the  point  where  a  change 
of  color  is  developed,  though  there  is  no  agreement  among  authorities 
as  to  the  proper  end-reaction  tints,  and  each  operator  relies  upon 
his  own  judgment. 

Lyman  F.  Kebler  says:  "In  order  to  obtain  standard  end-reaction 
tints  for  alkaloids  it  will  be  necessary  to  prepare  some  absolutely  pure 
alkaloid.  Treat  a  molecular  quantity  of  the  alkaloid  with  an  equiv- 
alent of  the  acid  in  question  to  form  a  neutral  salt,  then  add  one  drop 
more  of  the  decinormal  acid  for  an  acid  color-reaction. 

"  For  alkaline  tints  add  one  drop  of  the  centinormal  alkali  solution 
to  a  solution  of  neutral  alkaloidal  salt  theoretically  prepared." 

The  color  changes  produced  by  the  principal  indicators  used  in 
alkaloidal  titrations  are  as  follows: 


Acid. 

Alkali. 

Hsernatoxylin                         •    . 

Yellow 

Blue 

Brazil-wood               

Purplish-red 

Yellowish-red 

Purplish 

Red 

Blue 

Yellow 

Rose-  red 

Red 

Blue 

Green  fluorescence 

No  fluoresc.,  yellowish 

Blue 

Red 

Red 

Straw-yellow 

Phenolphthalein      .      

Colorless 

Red 

ESTIMATION    OF    ALKALOIDS 


503 


TABLE  SHOWING  BEHAVIOR  OF  SOME  OF  THE  ALKALOIDS  WITH 

INDICATORS 


Name. 

Formula. 

• 

Methyl- 
orange. 

Phenolph- 
thalein. 

Litmus. 

C34H47NOU 
C17H23N03 
C23H26N204 

Alk 

iline 

Neutral 
Alkaline 

Neutral 
«  < 

<  « 

Alkaline 
<  « 

Faintly  acid 
Alkaline 

Neutral 

i  i 

Alk 

iline 

Atropine          

Cinchona  bases                     .    • 

Cocaine 

C17H21N04 
C81H21N03 
C8H17N 
C17H19N03 
C5H7N 
C20H24N202 
C21H22N202 

Codeine  .       

Coniine 

Morphine  

Nicotine 

Ouinine    .       •  

Strychnine 

TITRATION  OF  ALKALOIDAL   SALTS 

Prof.  Plugge  made  a  number  of  experiments  with  a  view  to  deter- 
mine the  possibility  of  estimating  volumetrically  the  amount  of  acid 
contained  in  alkaloidal  salts,  and  from  this  determining  the  amount 
of  alkaloid.  He  found — 

(1)  That  in  the  salts  of  the  weak  opium  bases  narcotine,  papa- 
verine,  and  narceine  the  amount  of  acid  can  be  volumetrically  estimated 
with  either  litmus  or  phenolphthalein,  the  reaction  being  as  precise 
and  well  defined  as  if  no  alkaloid  were  present. 

(2)  That  in  the  salts  of  alkaloids  in  general,  the  acid  can  be  readily 
determined  by  the  use  of  phenolphthalein  the  volatile  alkaloids  coniine 
and  nicotine  being  exceptions ;  and  that  in  the  case  of  morphine,  brucine, 
codeine,  and   thebaine,   phenolphthalein   may  be   used  with  certain 
restrictions. 

(3)  That  the  free  acid  in  solutions  of  alkaloidal  salts  can  be  deter- 
mined by  the  use  of  litmus,  but  in  solutions  of  weak  opium  bases  litmus 
cannot  be  used.     The  entire  quantity  of  acid,  both  free  and  combined, 
may  be  determined  by  the  use  of  phenolphthalein.      The  difference 
between  the  two  titrations  gives  the  quantity  of  acid  united  to  the  base. 

Thus  he  estimates  the  alkaloid  by  titrating  the  acid  of  the  salt 
of  the  alkaloid  with  standard  alkali,  and  from  the  result  calculates  the 
quantity  of  alkaloid  present.  He  first  determines  the  uncombined 
(free)  acid  by  titrating  with  standard  alkali  in  the  presence  of  litmus. 

He  then  titrates  another  portion  of  the  solution  in  the  presence 
of  phenolphthalein  to  determine  the  total  quantity  of  acid  (both  free 
and  combined)  present,  and  from  this,  indirectly,  the  quantity  of 
alkaloid  is  calculated. 

For  the  estimation  of  the  alkaloid  in  a  commercial  salt,  such  as 
quinine  sulphate,  strychnine  sulphate,  etc.: 


504 


A    MANUAL   OF    VOLUMETRIC   ANALYSIS 


N 
Dissolve  the  salt  in  hot  water  and  titrate  with  —  sodium  hydroxid 

solution,  using  phenolphthalein,  methyl-orange,  or  some  other  suitable 
indicator. 

The  acid  in  combination  with  the  alkaloid  acts  as  though  it  were 
a  free  acid,  and  may  be  readily  estimated  by  this  method. 

Phenolphthalein  should  be  used  with  caution,  as  an  indicator,  in 
titrating  morphine  salts,  as  this  alkaloid  has  a  faint  acid  reaction  with  it. 

It  is  generally  preferable  to  titrate  the  solution  of  the  salt  of  an 

N 

alkaloid  with  —  potassium  hydroxid  to  exact  neutrality,  using  phenol- 
phthalein as  indicator.  The  alkaloid  which  is  thus  set  free  and  in  a 

N 
neutral  liquid  may  be  titrated  in  the  same  by  means  of  —  hydrochloric 

acid,  using  Brazil-wood  T.  S.  as  indicator.  This  gives  very  good 
results,  and  the  two  titrations  are  a  check  upon  each  other. 

TABLE  SHOWING  THE  FACTOR  FOR  VARIOUS  ALKALOIDS  WHEN 

TITRATING  WITH  —  ACID   OR  ALKALI 

10 


Name. 

Formula. 

Molecular 
Weight. 

Factor. 

Aconitine 

C,4H<7NOn 

640.  55 

0.06406 

Atropine                               

C17H23NO3 

287.04 

0.02870 

C23H26N2O4 

391.31 

0.03913 

Cephaeline 

C14H1BNOj 

231.43 

0.02314 

Cinchona  alkaloids  (combined) 

o  03060 

Cinchonine 

.  . 

C19H22N2O 

202.03 

0.02920 

Cinchonidine 

C19H22N2O 

202.03 

0.02920 

Cocaine    ...        

C17H21NO4 

300.92 

0.03009 

Codeine 

C18H21NO3 

296.9? 

c.  02060 

Coniine                         .          .... 

C8HnN 

126.21 

0.01262 

C30H44N2O4  (Glenard) 
C,nH,ftN,O-  (Kunz) 

492.68 
504.  <6 

0.02463 
0.02522 

C15H21N02  fU.  S.  P.) 
C21H21NO6 

245-34 
380.32 

0.02453 

0.03803 

C|VB^NQj 

300.92 

0.03009 

Hyoscyamine    ..         

C^H-aNO, 

287.04 

0.02870 

Ipecac  alkaloids  (combined)  . 

0.02384 

Morphine                      •-• 

C17Hi9NO, 

283.04 

0.02830 

C5H7N 

80.48 

0.008048 

Physostisrmine                              .... 

C1VH21N,O2 

273.  20 

0.02732 

Pilocarpine  .             

CnH18N2O2 

206.63 

0.020663 

C,nH24N,O2 

321.82 

0.03218 

Sparteine  .  .                      

Ci5H26N2 

232.  51 

0.001625 

Strychnine  .  . 

C21H22N202 

3.3i  73 

0.03317 

ESTIMATION    OF    ALKALOIDS  505 

New  Acidimetric  Method  for  Alkaloidal  Assay.  Elie  Falieries 
(Compt.  rend.,  129,  no)  proposes  to  avoid  the  difficulty  which 
surrounds  the  determination  of  the  end-reaction  when  titrating  colored 
solutions  of  alkaloids  by  titrating  back  the  excess  of  acid  with  ammo- 
niacal  cupric  oxid  solution.  The  end  of  the  reaction  is  sharply  denned 
by  the  precipitation  of  cupric  oxid,  which  produces  a  very  clearly 
marked  precipitate.  The  cupric  oxid  solution  is  obtained  by  dis- 
solving 10  gms.  of  cupric  sulphate  in  one  half  liter  of  water  and  adding 
ammonia  until  the  precipitate  formed  at  first  is  almost  entirely  redis- 
solved,  filling  up  to  1000  cc.  with  distilled  water,  filtering  and  deter- 
mining the  amount  of  alkali  present  by  titration  with  decinormal 
sulphuric  acid.  The  estimation  itself  is  carried  out  as  follows:  o.io  gm. 
of  the  alkaloid  is  placed  in  a  small,  narrow  cylinder,  20  cc.  of  deci- 
normal sulphuric  acid  added,  and  when  solution  has  been  effected 
the  excess  of  sulphuric  acid  titrated  back  by  means  of  the  cupric 
oxid  solution.  The  sulphuric  acid  combined  with  the  alkaloid  takes 
no  part  in  the  reaction. 


ESTIMATION  OF  ALKALOIDS  BY  MAYER'S  REAGENT 

The  results  of  titrating  with  Mayer's  solution  have  only  an  approxi- 
mate value,  being  influenced  to  a  large  extent  by  various  conditions, 
such  as  degree  t)f  dilution,  mode  of  conducting  the  operation,  and  the 
length  of  time  allowed  for  precipitation  after  each  addition  of  the 
reagent. 

The  Mayer's  solution  is  added  from  a  burette,  and  the  precipitate 
allowed  to  subside  after  each  addition  until  no  further  precipitation 
takes  place,  which  can  be  seen  by  bringing  a  drop  of  the  clear  super- 
natant liquid  in  contact  on  a  watch-glass,  with  two  or  three  drops 
of  the  reagent. 

A  more  common  practice  is  to  filter  the  solution  after  each  addition 
of  the  reagent,  using  the  same  filter.  When  10  cc.  of  the  filtered 
liquid  are  no  longer  affected  by  two  drops  of  the  reagent,  the  titration 
is  complete. 

If  a  considerable  length  of  time  is  allowed  to  elapse  after  each 
addition  of  reagent,  it  is  found  that  the  results  of  a  titration  will 
coincide  more  nearly  with  what  theory  requires ;  but  the  principal 
advantage  which  volumetric  analysis  has  over  gravimetric,  namely, 
rapidity  of  execution,  is  thereby  forfeited. 

The  presence  of  alcohol,  free  acetic  acid,  or  ammonia  vitiates  the 
result;  but  gum,  albumen,  glucose,  or  extractives  in  moderate  quan- 
tities have  no  effect  upon  the  reaction. 

In  all  comparative  titrations  with  this  reagent  the  dilution  of  the 


506 


A    MANUAL   OF   VOLUMETRIC    ANALYSIS 


alkaloidal   solution   should   be   the   same.     The   solution   should   be 

slightly  acid,  and  its  strength  about  i  :  200. 

In  titrations  where  the  end-reaction  can  only  be  ascertained  by 

the  cessation  of  the  formation  of  a  precipitate,  it  is  often  necessary 
to  filter  a  portion  of  the  turbid  solution  at  intervals 
during  the  titration,  and  test  it  to  see  whether  the 
process  is  completed.  In  such  cases  Beale's  filter, 
Fig.  87,  may  be  used.  Over  the  lower  end  of  this 
instrument  a  piece  of  filter-paper  is  tied,  and  over 
that  a  piece  of  thin  muslin  to  keep  the  paper  from 
being  broken.  When  dipped  into  a  turbid  mixture 
the  clear  liquid  rises,  and  may  be  poured  out  of 
the  little  spout  for  testing.  If  the  process  is  shown 
to  be  unfinished,  the  contents  are  washed  back  to 
the  bulk  of  the  liquid,  and  small  portions  filtered 
ou*  a*  mtervals  until  the  process  is  found  to  be  com- 
pleted. 

N 

The  Decinormal  Mayer's  Solution,  —  Mercuric  Potassium  lodid 

10 

(Hgl2  +  21^1  =  783.98.  39.  2  gms.  in  a  liter),  is  made  as  follows: 

Dissolve   13.44  gms.  of  pure  mercuric  chlorid  in  600  cc.  of  water, 

and  49.8  gms.  of  potassium  iodid  in  100  cc.  of  water. 

Mix  the  two  solutions,  and  then  add  enough  water  to  make  a 

mixture  measure  at  or  near  15°  C.  (59°  F.)  exactly  1000  cc. 

The   reaction   which   takes   place   when   these   two  solutions   are 

mixed  is 


FIG  87 


A.  B.  Lyons  and  many  others  prefer  to  use  a  solution  of  half  the 
above  strength. 

Each  cc.  of  the  decinormal  solution,  according  to  Dr.  Mayer,  pre- 
cipitates of — 


Aconitine 0.0267 

Atropine 0.0145 

Brucine 0.0233 

Cinchonine  . .       .0.0102 


grn. 


Coniine 0.00416 

Morphine 0.0200 

Narcotine 0.0213 

Nicotine 0.00405 


gm. 


Quinidine 0.0120 

Quinine 0.0108 

Strychnine 0.0167 

Veratrine 0.0269 


The  precipitates  are  hydriodates  of  the  alkaloids,  respectively, 
with  iodid  of  mercury;  but  Lyons  finds  that  they  are  not  of  definite 
composition,  though  the  variation  is  very  slight.  This  reagent  will 
give  similar  precipitates  with  all  of  the  alkaloids,  except  perhaps  col- 
chicine,  caffeine,  and  the  glucoside  digitalin. 


ESTIMATION  OF  ALKALOIDS  Bi    WAGNER'S  REAGENT     507 


ESTIMATION  OF  ALKALOIDS  BY  WAGNER'S  REAGENT 

Wagner's  reagent  has  long  been  known  to  have  the  power  of 
completely  precipitating  most  alkaloids,  even  in  dilute  solutions.  It 
is  a  very  delicate  reagent,  and  is  said  to  precipitate  ^Tinr  of  a  grain 
of  the  alkaloid  in  i  grain  of  water. 

The  use  of  a  potassium  iodid  solution  of  iodin  as  an  alkaloidal 
precipitant  was  first  recommended  by  Bouchardat  as  early  as  1839, 
but  it  was  not  until  1861  that  Wagner  proposed  the  decinormal 
solution  for  use  in  volumetric  analysis. 

Because  of  the  many  difficulties  that  attended  its  use,  through 
imperfect  knowledge  of  the  composition  of  the  precipitate  formed  and 
the  variations  noticed  with  different  alkaloids,  it  dropped  into  disuse 
for  a  time. 

Recently  Wagner's  reagent  has  been  again  brought  into  notice  as 
a  volumetric  reagent  for  alkaloids.  This  reagent  is  a  solution  of  iodin 
in  potassium  iodid  (see  Decinormal  Iodin  V.  S.,  page  186).  Its  use 
in  volumetric  analysis  depends  upon  precipitating  the  alkaloids  in 
the  form  of  definite  periodids,  and  its  advantages  over  other  methods 
are  its  sharp  end-reactions  and  accurate  results.  The  operations 
being  performed  in  acid  solutions,  the  presence  of  ammonia  or  other 
alkalies  does  not  interfere. 

The  alkaloid  in  acidulated  solution  is  treated  with  Wagner's 
reagent,  added  in  excess.  The  precipitate  is  allowed  to  settle,  and 
an  aliquot  portion  of  the  clear  liquid  decanted,  and  titrated  with  deci- 
normal thiosulphate  solution,  to  determine  the  excess  of  iodin.  This, 
deducted  from  the  quantity  of  iodin  added,  gives  the  quantity  of  the 
latter  which  combined  with  the  alkaloid. 

The  difficulty  that  presents  itself  is  that  different  alkaloids  when 
treated  under  apparently  the  same  conditions  give  periodids  of  entirely 
different  composition.  Thus  morphine  is  said  to  give  with  Wagner's 
reagent  Alkaloid  HI  .  13;  codeine  gives  Alkaloid  HI .  I4;  caffeine  gives 
Alkaloid  HI  .  I4. 

Thus  in  the  first  case  one  equivalent  of  morphine  equals  three  of 
iodin,  in  the  case  of  codeine  and  caffeine  one  equivalent  of  the  alkaloid 
equals  in  each  case  four  of  iodin;  therefore  we  must  ascertain  exactly 
the  composition  of  the  precipitate  in  a  particular  case  before  we  can 
make  use  of  the  reagent  for  volumetric  analysis.  When  the  com- 
position of  the  different  periodids,  as  produced  under  the  conditions 
of  titrations,  is  exactly  known,  this  method  may  be  placed  upon  a  sound 
basis. 

The  General  Method  of  Procedure  in  the  estimation  of  the 
strength  of  an  aqueous  solution  of  an  alkaloidal  salt  is  as  follows 
(Prescott  and  Gordin,  J.  A.  C.  S.,  xx,  722):  To  about  10  cc.  of  the 


508 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


decinormal  iodin  solution,  diluted  with  a  little  water,  i  cc.  of  the 
acidulated  alkaloidal  solution  is  added,  and  the  mixture  well  shaken 
for  a  few  minutes.  Should  the  precipitate  separate  out  very  quickly 
and  the  supernatant  liquid  be  clear  and  of  a  light  yellowish  or  greenish 
color,  or  altogether  colorless,  the  alkaloidal  solution  is  too  strong  and 
must  be  diluted,  till  after  a  few  trials  the  clear  supernatant  liquid 
retains  a  very  dark-red  iodin  color.  The  acidulated  alkaloidal  solution 
is  then  made  up  to  a  given  volume,  and  10  or  15  cc.  of  it  are  run  from 
a  burette  into  a  graduated  cylinder,  into  which  has  been  previously 
put  25  or  30  cc.  of  the  decinormaj  iodin  solution  diluted  with  a  little 
water.  The  mixture  is  then  made  up  to  a  given  volume  and  shaken 
till  the  supernatant  liquid  is  perfectly  transparent  and  is  of  a  very 
dark-red  iodin  color.  This  point  is  very  important,  and  should  the 
clear  liquid  not  have  this  dark-red  color,  the  experiment  with  most 
alkaloids  (except  morphine  and  possibly  some  others)  should  be  re- 
peated, putting  more  decinormal  iodin  solution  into  the  graduated 
vessel,  or  decreasing  the  number  of  cc.  of  the  alkaloidal  solution.  It 
would  not  do  to  add  more  decinormal  iodin  solution  to  the  same  mix- 
ture, as  the  iodin  must  be  in  large  excess  during  the  whole  operation 
in  order  to  prevent  the  formation  of  lower  periodids,  which,  once 
formed,  might  not  take  up  fresh  iodin  and  form  higher  periodids. 
Only  with  morphine,  which  in  such  solutions  forms  but  one  periodid, 
these  precautions  are  not  necessary.  When  the  liquid  has  become 
perfectly  clear,  an  aliquot  portion  of  it  is  filtered  off  and  the  excess  of 
iodin  determined  by  standard  sodium  thiosulphate.  From  these  data 
is  obtained  the  quantity  of  iodin  consumed.  This  quantity  of  iodin 
multiplied  by  the  "  ratio  "  of  alkaloid  to  the  one  of  iodin  gives  the 
quantity  of  alkaloid  sought.  If  preferred,  but  generally  with  less 
convenience  to  the  chemist,  the  number  of  cc.  of  decinormal  solution 
of  iodin  consumed  may  be  multiplied  by  the  "alkaloidal  factor  of 

N 

i  cc.  —  iodin  V.  S." 
10 

IODIN-FACTORS  OF  THE  ALKALOIDS  SO  FAR  DETERMINED 


Higher  Periodid 
Formed. 

Ratio  of  Alka- 
loid to  One 
of  Iodin. 

Alkaloidal 
Factor 
,            N 
of  i  cc.  — 

10 

Iodin. 

CnHaNOsHI.I,, 

o   2840 

o  0036048 

Strychnine  

C21H22N2O2HT.I6 

0.4300 

o  00^^467 

Brucine                          

C23H26N2O4HI  16 

O    !\I  7O 

o  006^^200 

Morphine       

C17H1BNO,HI.I, 

o  74018 

O    OOQ47Q37 

C33H45NO2HI.I6 

Caffeine                         

C8Hlf)N4O,HI.L 

o  3834 

o  0048  ^ 

ESTIMATION    OF    ALKALOIDS  509 


ESTIMATION  OF  CAFFEINE  BY  WAGNER'S  REAGENT 

Caffeine  may  be  titrated  as  follows:  o.i  gm.  of  caffeine  is  dissolved 
in  30  cc.  of  water  and  acidulated  with  5  or  6  drops  of  hydrochloric 

N 

acid.     —  iodin  solution  is  then  run  in  from  a  burette,  a  few  cc.  at  a 
10 

time,  until  30  cc.  have  been  used;  this  is  a  little  over  one  and  one  third 
the  quantity  required  theoretically  to  precipitate  o.i  gm.  of  caffeine. 
The  precipitated  caffeine  periodid  is  separated  by  nitration  through  a 
dry  asbestos  filter  after  five  minutes'  standing,  and  an  aliquot  portion 

N 

of  the  filtrate  then  titrated  with  —  sodium  thiosulphate  solution,  in 

10 

order  to  determine  the  excess  of  iodin.  The  difference  between  the 
quantity  of  thiosulphate  used  for  the  whole  filtrate  and  the  quantity 
of  iodin  solution  originally  added,  gives  the  quantity  of  the  latter 
which  reacted  with  the  caffeine. 

N 

Each  cc.  of  the  —  iodin  solution  =  0.00485  gm.  of  caffeine. 
10 

The  calculation  in  detail  is  as  follows: 

Caffeine  solution 30  cc. 

N  .   ,.       .    . 

—  iodin  solution 30  cc. 

ic  

Total .  .  60  cc. 


N 
30  cc.  of  filtrate  required    5.8  cc.  —  thiosulphate; 

whole       "      "  "        n.6cc.  " 

N 
30  cc.  —  iodin  — 1 1. 6  cc.  " 

N 

=  18.4  cc.  of  —  iodin  consumed  by  caffeine, 
10 

18.4X0.00485  =  0.08924  gm.  caffeine. 


510  A    MANUAL  OF    VOLUMETRIC   ANALYSIS 


GORDIN'S  MODIFIED  ALKALIMETRIC  METHOD,  USING  PHENOLPHTHALEIN 

AS  INDICATOR 

The  alkaloidal  residue  obtained  by  any  of  the  extraction  methods 

N 

in  use  is   dissolved  in  a  measured  excess   of  —  hydrochloric  acid 

20 

solution.  Wagner's  or  Mayer's  reagent  is  then  added,  a  little  at  a 
time,  with  frequent  shaking,  until  the  alkaloids  are  completely  pre- 
cipitated. The  mixture  is  then  diluted  with  water  to  100  cc.  and 
shaken  until  the  double  salt  of  the  alkaloid  and  reagent  completely 
separate.  When  allowed  to  stand  a  few  minutes,  the  supernatant 
liquid  should  be  clear,  and  if  Wagner's  reagent  has  been  used,  this 
will  be  of  a  dark-red  color.  The  liquid  is  now  filtered,  and  50  cc  of 
the  filtrate  (representing  one  half  of  the  alkaloid)  is  treated  with  a 
10  per  cent  solution  of  sodium  thiosulphate  added,  drop  by  drop, 
until  the  color  of  the  free  iodin  disappears.  This  discolorization  is 
not  needed  if  Mayer's  reagent  has  been  used.  A  few  drops  of  the 
indicator  phenolphthalein  are  now  introduced,  and  the  excess  of  acid 

N 
estimated  by  retitration  with  —  potassium  hydroxid  solution.     This, 

deducted  from  one  half  of  the  volume  of  the  standard  acid  solution 

N 
employed,  indicates  the  number  of  cc.  of  —  hydrochloric  acid  solution, 

which  combined  with  the  alkaloid  in  50  cc.  of  the  solution.  This 
number,  multiplied  by  two  and  then  by  the  factor  for  the  alkaloid 
present,  gives  the  total  quantity  of  alkaloid. 

Example.     An  alkaloidal  residue  consisting  of  morphine  was  dis- 

N 
solved  in  30  cc.  of  —  HC1  V.  S.     Then  Wagner's  reagent  was  added  in 

excess  as  described,  and  the  mixture  made  up  to  100  cc.  with  water. 
50  cc.  of  this  were  filtered  off,  decolorized  as  directed,  and  titrated  with 

N 
-  KOH  V.  S.     10  cc.  were  required,  which  corresponds  to  20  cc.  for 

20  N 

the  entire  quantity.     Then  20  cc.  deducted  from  the  30  cc.  of  —  HC1 

20 

V.  S.  added,   leaves    10  cc.,   the   quantity  required  to  neutralize   the 

N 

morphine.  The —  factor  for  morphine  (0.0137  gm->  Gordin),  multi- 
plied by  10=0.137  gm.,  the  quantity  of  alkaloid  in  the  residue  exam- 
ined. 

N 

The  —  factor  for  morphine  here  given  is  somewhat  lower  than  the 
10 


ESTIMATION    OF    ALKALOIDS  511 

theoretical    equivalent.     It    was    ascertained    by    experiment    with   a 
sample  of  the  anhydrous  alkaloid. 

The  following  factors  (by  Gordin)  were  obtained  by  comparing 
their  molecular  weights  with  that  of  morphine,  which  factor  was  deter- 
mined by  experiment : 

N 
Morphine 0.0137  gm-==  l  cc-  —  HC1  V.  S. 

Hydrastine 0.0184  ' '  = 

Strychnine 0.0160  '  *  = 

Caffeine,  cryst 0.0102  •*•'  = 

Cocaine 0.0146  *  *  = 

Atropine 0.0139  "  =  " 


CHAPTER  LI 

VOLUMETRIC  ASSAYING  OF  VEGETABLE  DRUGS 
EXTRACTION  OF  THE  ALKALOIDS 

Selection  of  the  Sample.  Care  must  be  taken  to  secure  a  fairly 
representative  sample. 

If  the  drug  is  in  small  pieces  or  consists  of  seeds  or  leaves,  mix 
it  well,  take  a  portion,  pulverize  it,  and  of  the  powder  take  a  sufficient 
quantity  for  the  assay.  If  the  drug  be  in  large  lumps,  which  vary  in 
quality,  select  a  few  representative  lumps  and  cut  from  each  a  fairly 
representative  section;  pulverize  these,  mix  well,  and  weigh  off  a  suffi- 
cient quantity  for  the  assay. 

If  drying  is  necessary,  the  loss  of  weight  in  drying  must  be  made 
note  of. 

The  Exhaustion  of  the  Drug  is  usually  effected  by  maceration 
in  a  suitable  menstruum,  although  percolation,  boiling,  and  hot  reper- 
colation  must  be  employed  in  some  cases. 

The  Choice  of  Solvent  depends  upon  the  nature  of  the  drug. 
Water  dissolves,  besides  the  alkaloids,  so  much  inert  matter  that  the 
subsequent  steps  in  the  assay  are  liable  to  be  interfered  with.  Alcohol 
dissolves  too  much  of  the  resinous  matter,  and  besides  does  not  pene- 
trate the  drug  very  well.  Acidulated  water  has  been  much  used,  but 
chloroform  and  ether,  separately  and  in  various  combinations,  are 
now  most  generally  employed  for  exhausting  drugs  in  conjunction 
with  alcohol  and  ammonia.  Petroleum  benzin  has  of  late  been 
recommended. 

Prollius'  fluid,  or  some  modification  of  it,  is  very  satisfactory. 

Prollius'  Fluid  consists  of  ether  325  cc.,  alcohol  25  cc.,  and 
concentrated  ammonia  water  10  cc. 

Modified  Prollius1  Fluid  consists  of  ether  250  cc.,  chloroform  80  to 
100  cc.,  alcohol  25  cc.,  concentrated  ammonia  water  10  cc. 

Alkaloidal  Assay  by  Immiscible  Solvents.  The  following  is 
the  U.  S.  P.  description  of  this  procedure: 

"  Nearly  all  alkaloids  are  practically  insoluble  in  water,  but  they 
are  soluble  in  alcohol,  chloroform,  ether,  amyl-alcohol,  benzene,  petro- 
leum benzin,  or  mixtures  of  several  of  these.  The  salts  of  these  alka- 

512 


ALKALOID AL    ASSAY    BY    IMMISCIBLE    SOLVENTS       513 


loids,  however,  are  soluble  in  water,  but  practically  insoluble  in  the 
above-mentioned  solvents.  The  process  of  assay  by  immiscible  solvents, 
which  is  generally  known  as  the  "shaking-out  "  process,  is  based  on 
this  property  of  alkaloids,  and  it  is  carried  out  by  treating  liquid  extracts 
that  have  been  freed  from  alcohol  with  an  immiscible  solvent  in  the 
presence  of  an  excess  of  alkali.  This  liberates  the  alkaloid,  and,  on 
becoming  free,  if  not  so  previously,  it  is  dissolved  by  the  immiscible 
solvent.  This  solution  is  then  separated,  transferred  to  another  con- 
tainer, shaken  with  an  excess  of  acid 
largely  diluted  with  water.  The  acid 
combining  with  the  free  alkaloid  forms 
a  salt,  which  now  leaves  the  immiscible 
solvent  and  is  found  in  the  aqueous 
solution.  This  process  is  sometimes  re- 
peated, in  case  the  alkaloidal  solution 
is  still  colored.  The  apparatus  used  in 
this  operation  of  shaking-out  is  termed 
a  "separator,"  (see  Fig.  88)  and  consists 
of  an  oval  or  pear-shaped  glass  vessel, 
with  an  opening  at  the  top  supplied  with 
a  well-ground  glass  stopper,  and  an 
outlet  tube  at  the  bottom,  provided  with 
an  accurately  fitting  glass  stop-cock. 
The  solvents  directed  to  be  used  in 
this  Pharmacopoeia  are  alcohol,  chloro- 
form, ether,  and  various  mixtures  of  both 
containing  at  least  75  parts  of  ether  in 

100  parts  of  solvent  by  volume.  In  the  case  of  chloroform,  the  solvent 
will  collect  at  the  bottom  of  the  separator,  and  can  be  drawn  off,  but 
the  ethereal  or  ether-chloroform  mixture  will  form  the  upper  portion 
of  the  liquid  in  the  separator,  and  the  aqueous  layer  must  first  be  drawn 
off  into  a  suitable  vessel,  and  the  ethereal  layer  then  transferred  to 
another  vessel.  It  is  not  necessary  or  desirable  to  shake  the  mixture 
of  immiscible  solvent  and  water  violently,  for  a  rotation  of  the  separator 
or  a  gentle  shaking  for  about  a  minute  will  answer  all  purposes.  At 
times,  an  emulsion  of  the  water  and  the  solvent  is  formed,  especially 
if  the  shaking  is  too  violent,  and  in  order  to  separate  this,  it  is  advisable 
to  proceed  as  follows:  If  the  solvent  is  heavier  than  the  water,  add 
more  of  the  former,  a  little  water,  and  a  slight  amount  of  alcohol;  if 
the  solvent  is  lighter  than  the  water,  add  sufficient  saturated  sodium 
chlorid  solution  or  crystals  of  sodium  chlorid.  A  safe  procedure  to 
avoid  the  forming  of  emulsions  is  to  invert  the  separator  several  times, 
and  then  to  at  once  begin  rotating  to  keep  the  solvents  well  mixed. 
To  insure  a  complete  extraction  of  the  alkaloid,  it  is  desirable  to  treat 


Squibb's  Pattern 

FIG.  88. 


514  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

the  liquid  three  times  with  the  immiscible  solvent,  and  this  is  to  be 
followed  by  a  rinsing  of  the  empty  separator  with  repeated  small 
portions  of  the  same  solvent.  The  separator  should  not  be  filled  to 
more  than  two  thirds  of  its  capacity  at  any  time,  and  if  its  contents 
should  become  heated  by  the  neutralization  of  acid  by  alkali,  or  vice 
versa,  it  should  be  cooled  to  the  temperature  of  the  room,  before 
opening  the  stopper,  by  immersing  it  in  running  water.  The  final 
operation  must  always  be  the  collection  of  the  free  alkaloid  by  the  use 
of  a  portion  of  the  immiscible  solvent,  drawing  this  off  into  a  beaker, 
rinsing  with  small  portions  of  the  solvent  to  prevent  possible  loss. 
The  beaker  is  then  placed  on  a  water-bath  and  gently  heated,  to  remove 
the  solvent  by  evaporation,  leaving  the  alkaloids  in  the  beaker  in  the 
dry  form,  and  usually  in  the  condition  of  a  resinous  or  varnish-like 
mass.  It  is  then  either  weighed  as  such  or  dissolved  in  volumetric 
acid  solution,  delivered  in  measured  quantity  from  a  burette,  and  the 
excess  of  the  acid  titrated  with  volumetric  alkali  solution  with  the 
use  of  an  indicator.  Should  the  final  residual  alkaloids  still  be  slightly 
colored,  it  is  preferable  to  employ  iodeosin  as  the  indicator,  as  the 
alkaloidal  solution  contains  ether  and  the  ethereal  layer  retains  in 
solution  coloring  matter  or  impurity  which  may  be  present.  If  the 
alkaloids  are  not  colored,  haematoxylin  or  cochineal  may  safely  be 
used. 

"The  quantity  of  alkaloid  is  found  by  multiplying  the  number  of 
cc.  of  volumetric  acid  consumed  by  a  constant  factor,  depending  upon 
the  molecular  weight  of  the  individual  alkaloid." 

"The  factor  in  each  case  represents  the  weight  in  grams  of  the 
alkaloid  required  to  neutralize  i  cc.  of  volumetric  acid. 

In  connection  with  this,  attention  is  called  to  the  fact  that  some 
of  the  most  important  methods  of  isolating  alkaloids  in  drug  assays 
and  in  toxicological  investigations  depend  upon  the  solubility  of  the 
free  alkaloids  in  ether,  chloroform,  benzene,  etc.,  and  the  relative 
insolubility  of  alkaloidal  salts  in  the  same  solvents.  There  are, 
however,  a  number  of  exceptions  to  this  rule,  and  special  attention 
is  called  to  the  fact,  that  in  many  cases  alkaloids  pass  from  decidedlv 
acid  aqueous  solutions  (in  which  they  certainly  occur  as  alkaloidal 
salts)  into  chloroform  and  ether,  in  the  shaking-out  methods.  While 
the  amount  of  alkaloid  so  dissolved  by  these  solvents  is  never  very 
great,  still  the  quantity  is  an  appreciable  one.  This  behavior  occurs 
in  the  case  of  caffeine,  colchicine,  and  narcotine;  and  under  certain 
conditions  also  in  the  case  of  strychnine,  atropine,  veratrine,  and  other 
bases.  This  transfer  of  alkaloid  to  chloroform  occurs  more  particularly 
in  the  case  of  alkaloids  of  weak  basic  character  and  when  the  solution 
is  neutral  or  only  feebly  acid,  or  when  the  alkaloid  is  in  combination 
with  a  comparatively  weak  acid,  as  citric,  tartaric,  etc.  Furthermore, 


GENERAL    METHODS    OF   ASSAYING    DRUGS  515 

some  alkaloidal  salts,  notably  those  of  weak  bases,  are  transferred  as 
such  to  chloroform,  especially  the  salts  of  hydrochloric  acid,  hydro- 
bromic  acid,  and  nitric  acid.  In  the  case,  however,  of  the  sulphates, 
phosphates,  tartrates,  and  citrates  of  strongly  basic  alkaloids,  no 
transfer  occurs,  or  at  most,  only  minute  quantities  of  the  alkaloids 
pass  over.  In  order  to  prevent  a  transfer  of  alkaloid  or  alkaloidal 
salt  out  of  an  aqueous  solution  to  an  immiscible  solvent,  the  use  of 
sulphuric  acid  is  to  be  recommended,  and  in  all  toxicological  investi- 
gations due  regard  should  be  paid  to  the  above  named  conditions. 
For  more  details,  see  the  paper  by  Edward  Schaer,  Proc.  A.  Ph.  A., 
1906,  425. 

GENERAL  METHODS  OF  ASSAYING  DRUGS 

No  rule  can  be  formulated  as  to  the  method  of  extraction,  or  the 
solvent  to  be  employed,  which  can  be  applied  to  all  drugs;  each 
must  be  dealt  with  in  accordance  with  the  properties  of  the  contained 
alkaloids  and  their  state  of  combination.  Several  methods  are,  how- 
ever, in  use  which  may  be  applied  to  a  large  number  of  different  drugs, 
and  with  slight  special  modifications  to  many  more. 

The  Keller  Method  *  has  been  widely  accepted,  and  in  its  various 
modifications  is  certainly  the  most  practical. 

In  brief  it  is  as  follows:  A  convenient  quantity  of  the  drug  in  fine 
powder  is  introduced  into  a  flask  with  about  ten  times  its  weight  of 
menstruum,  usually  a  mixture  of  one  part  of  chloroform  and  about 
eight  parts  of  ether.  This  is  allowed  to  stand  for  about  ten  minutes, 
a  small  quantity  of  ammonia  water  added,  the  flask  stoppered  and 
shaken  frequently  during  several  hours.  A  sufficient  quantity  of 
water  is  then  added  to  cause  the  powder  to  cake  together  and  leave 
the  ethereal  fluid  quite  clear.  Of  this  ethereal  fluid  an  aliquot  portion 
is  removed  by  decantation  and  the  alkaloid  extracted  by  shaking  out 
with  dilute  acid.  By  this  procedure  the  alkaloids  are  almost  wholly 
dissolved  out  of  the  drug,  even  when  the  latter  is  in  a  coarse  powder. 
A  more  detailed  description  of  this  method  is  given  in  the  assay  of 
aconite  root. 

A  serious  objection  to  this  method  lies  in  the  taking  of  a  so-called 
aliquot  part:  First,  because  of  the  well-known  solubility  of  ether  in 
water,  and  conversely  of  water  in  ether,  as  a  result  of  which  the 
volume  of  the  ethereal  stratum  is  materially  changed.  Furthermore, 
commercial  ether  contains  variable  quantities  of  alcohol,  hence  the 
change  in  volume  will  not  be  always  the  same. 

*  C.  C.  Keller,  Schweitz.  Wochenschr.  f.  Chem.  u.  Pharm.,  XXX,  501-509; 
A.  J.  Ph.,  LXV,  78,  and  LXVI,  42. 


516  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Another  source  of  error  in  the  aliquot  part  is  found  in  the  volatile 
nature  of  the  solvents  used.  In  warm  weather  it  is  impossible  to 
avoid  loss  by  volatilization,  hence  the  aliquot  part  taken  is  too  large. 

W.  A.  Puckner  *  has  described  a  modification  of  the  Keller  method 
which  avoids  the  use  of  the  aliquot  part.  He  uses  only  one  half  of 
the  ethereal  solvent  for  the  maceration,  and  after  the  usual  maceration 
transfers  the  drug  to  a  small  percolator  in  which,  after  the  ethere-1 
solution  has  been  well  drained  off,  the  marc  is  percolated  with  the 
same  menstruum  to  complete  exhaustion.  The  quantity  of  eihere:il 
solvent  required  is  not  materially  greater  than  in  the  Keller  method, 
while  the  quantity  of  alkaloid  obtained  for  weighing  or  titrating  is 
larger  because  it  represents  the  whole  of  the  sample  taken  for  the 
assay.  In  the  case  of  drugs  containing  a  very  small  proportion  of 
alkaloid  this  is  an  important  advantage. 

The  objection  to  this  plan  is  that  the  transfer  of  the  mass  from 
the  flask  in  which  the  maceration  has  been  conducted  to  a  suitable 
percolator,  which  should  not  be  more  than  3  cm.  in  diameter,  requires 
very  dextrous  manipulation,  or  it  will  be  attended  with  loss  of  alkaloid. 

A.  B.  Lyons  f  recommends  the  following  procedure:  Provide  a 
cylindrical  percolator  about  20  cm.  in  length  and  2  to  2.5  cm.  in 
internal  diameter,  ending  in  a  tube  5  cm.  long  and  about  3  mm.  in 
internal  diameter.  A  glass  stop-cock  in  the  tube  would  be  a  very 
desirable  improvement.  In  absence  of  this,  the  rate  of  flow  of  the 
percolate  must  be  controlled  by  packing  the  tube  more  or  less  firmly 
with  absorbent  cotton.  Since  the  solvent  is  to  be  a  very  mobile  fluid, 
the  packing  should  generally  be  quite  firm. 

Having  prepared  the  percolator,  moisten  the  drug  (5,  10,  15,  20  gms. 
or  more,  according  to  richness  in  alkaloid — the  finer  the  powder  the 
better)  with  the  mixture  of  ammonia,  alcohol,  and  ether-chloroform, 
the  proportions  of  which  will  be  somewhat  varied  to  suit  different 
drugs.  If  10  gms.  of  such  a  drug  as  belladonna  leaf  are  to  be  used 
for  the  assay,  the  mixture  may  consist  of:  Stronger  water  of  ammonia, 
ice.;  alcohol,  4cc.;  ether-chloroform  (6:1  vol.),  5  cc.  Moisten  in 
a  small  evaporating-dish,  transfer  quickly  to  the  percolator,  pressing 
the  powder  down  firmly  with  a  glass  rod.  The  small  amount  of 
powder  that  remains  adhering  to  the  dish,  spatula,  and  glass  rod  can 
be  easily  transferred  to  the  percolator  by  aid  of  a  little  absorbent  cotton, 
which  is  firmly  pressed  down  upon  the  powder.  The  percolator  is 
then  to  be  covered  and  allowed  to  stand  five  to  ten  minutes,  so  that 
the  ammonia  may  thoroughly  permeate  the  drug.  A  mixture  of 


*  Ph.  Rev.,  XVI,  180,  and  XX,  457. 

f  Ph.  Rev.,  Nov.,  1903,  and  Proc.  A.  Ph.  A.,  1903,  254. 


GENERAL    METHODS    OF    ASSAYING    DRUGS  517 

ether  and  chloroform,  or  whatever  solvent  is  best  suited  to  the  ex- 
traction of  the  alkaloid  present,  is  next  added  and  the  powder  per- 
colated with  it  to  exhaustion.  It  is  easy  generally  to  secure  a  rate 
of  flow  of  one  drop  per  second,  which  will  insure  thorough  exhaustion 
by  the  time  that  50  to  75  cc.  of  percolate  has  passed.  When  it  is 
believed  that  the  exhaustion  is  complete,  test  this  by  collecting  15  or 
20  drops,  stirring  this  with  a  drop  of  normal  sulphuric  acid,  evapo- 
rating off  the  ethereal  solvent  and  testing  the  acid  solution  with 
Meyer's  or  Wagner's  reagent. 

From  this  point  the  assay  is  to  be  carried  on  in  the  usual  manner. 

H.  M.  Gordin  *  proposes  a  new  method  in  which  special  forms 
of  apparatus  are  used,  including  a  combination  percolator  and  shaking 
tube  in  which  the  distillation  of  the  ethereal  solution  to  dryness  is 
avoided;  another  feature  of  which  is  the  use  of  fixed  alkali  hydroxid 
or  carbonate  instead  of  ammonia  for  the  liberation  of  the  alkaloids. 
By  this  method  the  loss  occasioned  through  the  transferring  of  the 
material  from  one  vessel  to  another  is  avoided,  and  the  injury  to 
certain  alkaloids  by  heat  prevented.  The  process  and  apparatus  are 
described  under  Aconite,  page  523. 

Kebler's  Modification  of  the  Keller  Method.f  Treat  10  gms. 
of  the  dry  powdered  drug  in  a  250-0:.  flask  with  25  gms.  of  chloro- 
form and  75  gms.  of  ether;  stopper  the  flask  securely,  agitate  well 
for  a  few  minutes  and  add  10  gms.  of  ammonia-water  U.  S.  P.,  and 
shake  frequently  during  one  hour.  Then  on  adding  5  gms.  more  of 
the  ammonia-water  and  shaking,  the  suspended  powder  agglutinates 
into  a  lump  and  leaves  the  solution  clear  after  a  few  minutes'  standing. 
Then  proceed  by  A  or  B. 

A.  When  the  mixture  has  completely  separated,  50  gms.   (repre- 
senting 5  gms.  of  the  drug)  are  poured  off  into  a  beaker  and  heated 
on  a  water-bath  until  the  solvent  is  evaporated.     10  cc.  of  ether  are 
then  added  and  again  evaporated.     The  varnish-like  residue  is  then 
dissolved  in  15  cc.  of  warm  alcohol  and  water  added  to  slight  perma- 
nent turbidity,  then  the  indicator  is  added,  followed  by  an  excess  of 
standard  acid  solution  and  the  mixture  retitrated  with  standard  alkali. 

B.  When  the  mixture  has  completely  separated  pour  off  50  gms. 
into  a  separatory  funnel,  and  add  20  cc.  of  acidulated  water,  agitate, 
and  when  the  liquids  have  separated  draw  off  the  aqueous  solution 
into  a  second  separatory  funnel.     Repeat  this  operation  with  two  more 
portions  of  15  cc.  of  acidulated  water.     Now  render  the  contents  of  the 
separatory  funnel  alkaline  by  adding  ammonia-water.    This  liberates 
the  alkaloids,  which  are  then  separated  by  treatment  with  a  mixture  of 

*  Proc.  A.  Ph.  A.,  1906,  377.  t  J.  A.  C.  S.,  XVII,  828. 


518  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

chloroform  three  parts  (by  volume)  and  ether  one  part,  using  three 
successive  portions,  first  20  cc.,  then  twice  15  cc. 

The  chloroform-ether  solution  is  heated  on  a  water-bath  until  the 
solvent  is  evaporated,  and  then  the  varnish-like  residue  treated  twice 
with  8  cc.  of  ether  and  again  evaporated. 

The  residue  is  then  dissolved  in  15  cc.  of  alcohol,  water  added  to 
slight  permanent  turbidity,  and  then  the  indicator.  Titrate  in  usual 
way  with  decinormal  acid  and  centinormal  alkali. 

Gordin  and  Prescott's  Method  (J.  A.  C.  S.,  xxi,  232).  This 
may  be  used  when  the  alkaloid  is  to  be  estimated  by  Wagner's  reagent, 
or  by  any  other  method,  i  to  4  gms.  of  the  finely  powdered  drug  is 
weighed  into  a  low  wide-mouthed  vessel  with  a  round  bottom,  hold- 
ing 8  to  10  ounces  and  having  a  well-fitted  cork,  such  as  a  screw-top 
ointment  jar.  The  powder  is  rubbed  up  with  a  small  pestle  to  a  fine 
paste  by  adding  a  little  of  a  solvent  mixture  composed  of  stronger 
ammonia-water  and  alcohol  each  5  cc.,  chloroform  10  cc.,  and  ether 
20  cc.  Then  a  few  more  cc.  of  this  mixture  are  added,  so  as  to  have 
the  drug  well  covered  with  the  liquid,  using  in  all  about  five  times 
the  amount  of  the  drug  taken.  The  vessel  is  corked  with  the  pestle 
inside  and  is  set  aside  for  about  four  or  five  hours,  taking  care  to 
agitate  by  rotating  very  frequently  during  that  interval.  After  that 
time  the  cover  is  removed  and  the  vessel  kept  in  a  current  of  air, 
stirring  frequently  till  all  odor  of  ammonia  has  disappeared.  With  a 
good  draught  and  frequent  stirring  the  powder  will  be  almost  per- 
fectly dry  in  about  two  hours.  The  vessel  is  then  put  in  a  vacuum 
desiccator  over  sulphuric  acid  for  about  four  or  five  hours.  An  amount 
of  powdered  sodium  chlorid  equal  to  about  five  or  six  times  the  amount 
of  drug  employed  is  then  carefully  mixed  in,  with  the  use  of  the  pestle 
and  the  whole  thrown  into  a  small  percolator  having  a  plug  of  cotton 
at  the  bottom.  The  vessel  is  then  cleaned  out  several  times  with  small 
quantities  of  sodium  chlorid  and  the  cleanings  added  to  the  percolator. 
The  mixture  in  the  percolator  is  then  covered  with  a  piece  of  cotton, 
which  is  pressed  down  with  a  piece  of  glass,  and  a  suitable  menstruum, 
usually  chloroform,  is  poured  slowly  into  the  percolator  till  the  men- 
struum begins  to  drop  from  the  bottom  of  the  percolator;  the  flow  is 
stopped,  the  percolator  covered  and  set  aside  for  five  or  six  hours. 
After  that  time  the  stop-cock  is  opened  and  the  drug  exhausted  with 
the  menstruum  percolating  until  ten  drops  of  the  percolate  evaporated 
on  a  watch-glass  and  the  residue  taken  up  with  a  few  drops  of  acidu- 
lated water,  the  solution  shows  no  turbidity  whatever  on  adding  a 
few  drops  of  Wagner's  reagent.  When  finished,  the  percolate  which 
is  received  in  an  evaporating-dish,  is  placed  in  a  good  draught  at  a 
temperature  of  about  30°  C.  When  the  liquid  is  reduced  to  a  very 
small  volume  10  cc.  of  acidulated  water  are  added,  and  then  a  few  cc. 


LITERATURE    ON    ALKALOID AL    ASSAYING  519 

of  ether,  so  as  to  have  the  ethereal  liquid  cover  the  aqueous  solution. 
(If  an  alkalimetric  assay  is  intended  the  acidulated  water  should  be 

N 

—  sulphuric  acid,  and  taken  in  definite  quantity.)     Then  the  whole 

is  stirred  with  a  glass  rod  till  all  the  ethereal  liquid  is  evaporated  off. 
The  liquid  is  then  filtered  and  the  evaporat ing-dish  and  filter  washed 
several  times  with  the  acidulated  water.  In  this  way  a  colorless 
solution  of  the  alkaloid  is  obtained,  which  can  be  worked  up  for 
any  method  of  assay. 

LITERATURE  ON  ALKALOIDAL  ASSAYING 

1890.  O.  Schweissinger  and  G.  Sarnow.     A.  J.  Ph.,  LXIII,  96. 

1891.  O.  Schweissinger.     Ph.  Centrh.,  xxxii,  583. 
1891.  A.  H.  Allen.     Analyst,  xvii,  186  and  215. 

1891.  H.  Beckurts.     A.  J.  Ph.,  LXIV,  25. 

1892.  L.  Barthe.     A.  J.  Ph.,  LXV,  638. 
1892.  A.  H.  Allen.     Ch.  News,  LXVI,  259. 

1892.  C.  C.  Keller.     Schweiz.  Wochenschr.  f.  Chem.  u.  Ph.,  xxx,  501-509; 

A.  J.  Ph.,  LXV,  78. 

1893.  C.  C.  Keller.     A.  J.  Ph.,  LXVI,  42. 

1893.  c-  c-  Keller.     Schweiz.  Wochenschr.  f.  Chem.  u.  Ph.,  xxxi,  473. 

1893.  C.  Caspari  and  A.  R.  L.  Dohme.     A.  J.  Ph.,  LXVI,  473. 

1894.  H.  Beckurts.     Zeitschr.  Oesterreich.  Apoth.  Ver.,  XLVIH,  624;    and 

Am.  Druggist,  xxv,  328. 
1894.  K.  Schwickerath.     Bull.  Ph.,  vm,  56  and  246. 

1894.  L.  F.  Kebler.     Proc.  A.  Ph.  A.,  XLII,  193. 

1895.  L.  F.  Kebler.    A.  J.  Ph.,  LXVII,  398, 

1895.  R.  A.  Cripps.     Abstr.  Chem.  and  Drugg.,  XLVII,  198. 

1898.  C.  Kippenberger.     Apoth.  Ztg.,  page  664. 

1898.  N.  Rusting.     Pharm.  Centrh.,  Aug.  18,  page  603. 

1898.  Prescott  and  Gordin.     J.  A.  C.  S.,  xx,  722. 

1898.  W.  Lenz.     Pharm.  Ztg.,  Sept.  24,  page  683. 

1809.  Gordin  and  Prescott.     Proc.  A.  Ph.  A.,  261  and  271. 

1899.  Gordin  and  Prescott.     A.  J.  Ph.,  Jan.,  pages  14-18. 
1899.  Gordin  and  Prescott.     Ph.  Rev.,  Nov.,  page  495. 

1899.  E.  Failieres.     Ph.  Jour.,  Sept.,  page  295. 

1900.  E.  Schmidt.     Apoth.  Ztg.,  Jan.  6,  page  13. 

1900.  C.  Kippenberger.     Ph.  Rev.,  Aug.,  page  373. 

1901.  H.  Proelss.     Apoth.  Ztg.,  No.  88. 
1903.  A.  B.  Stevens.     Pharm.  Arch.,  vr,  49. 
1903.  A.  B.  Lyons.     Ph.  Rev.,  Nov. 

1905.  G.  Fromme.     Geschaftsbericht,  Caesar  and  Loretz,  page  51. 

1905.  H.  M.  Gordin.     A.  J.  Ph.,  page  464. 

1906.  H.  M.  Gordin.     Proc.  A.  Ph.  A.,  page  377. 
1906.  E.  Schaer.     Proc.  A.  Ph.  A.,  page  425. 
1908.  W.  A.  Puckner.     A.  J.  Ph.,  LXXX,  page  66. 


520  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 


THE  ASSAYING  OF  CRUDE  DRUGS 

The  Extraction  of  the  Alkaloids.  This  may  be  effected  by 
numerous  methods.  The  following  are  those  most  frequently  recom- 
mended at  the  present  time.  Though  these  methods  are  of  general 
application,  yet  owing  to  slight  differences  in  the  solvent  action  of  the 
fluids  used,  and  for  other  reasons,  one  method  is  better  adapted  fcr 
a  particular  drug  than  another. 

The  Keller  method,  as  modified  by  Puckner,*  in  which  the  drug 
is  exhausted  by  percolation  after  maceration,  and  the  use  of  an  aliquo' 
part  avoided,  is  one  of  the  best  general  methods.  The  percolation 
method  of  Lyons, f  also  gives  very  excellent  results.  Gordin's  {  method, 
in  which  a  special  combination  percolator  and  shaking  tube  is  em- 
ployed, and  fixed  alkali  used  instead  of  ammonia,  obviates  the  neces- 
sity for  evaporation  of  the  ethereal  solution  to  dryness,  thus  prevent- 
ing injury  to  certain  alkaloids.  The  methods  suggested  by  Gordin 
and  Prescott,§  and  by  KeblerJ  are  also  useful  in  some  cases. 

ASSAY   OF   ACONITE 

Assay  of  Aconite  Root  (C.  C.  Keller).  Place  12  gms.  of  the 
root  (in  No.  80  powder)  in  a  25o-cc.  flask,  add  30  gms.  of  chloroform 
and  90  gms.  of  ether;  stopper  securely  and  shake  the  flask  for  five  or 
ten  minutes;  then  add  10  gms.  of  ammonia-water  U.  S.  P.  and  shake 
frequently  during  half  an  hour,  and  introduce  20  gms.  of  water  and 
again  shake;  this  causes  the  drug  to  gather  in  lumps  and  permits  the 
chloroform-ether  mixture  to  separate  so  that  it  can  be  easily  poured  off. 

When  the  mixture  has  completely  separated,  pour  off  100  gms.  of 
the  chloroform-ether  solution  (representing  10  gms.  of  the  drug)  into 
a  separatory  funnel  and  treat  at  once  with  25  cc.  of  a  i  per  cent  solu- 
tion of  hydrochloric  acid.  Agitate,  and  when  the  liquids  have  sepa- 
rated, draw  off  the  aqueous  solution  into  a  second  separatory  funnel, 
and  repeat  the  operation  with  two  more  portions  of  the  hydrochloric 
acid  solution,  using  first  15  and  then  10  cc. 

The  acidulated  solution  is  then  rendered  alkaline  by  adding 
ammonia -water  and  the  reprecipitated  alkaloid  removed  by  treatment 
with  successive  portions  of  a  mixture  of  chloroform  3  parts  and  ether 
2  parts  (by  weight),  using  in  all  about  100  gms.  of  the  mixed  solvent. 
It  is  best  to  add  a  portion  of  the  solvent  before  the  ammonia. 

The  chloroform-ether  solution  is  collected  in  a  beaker  or  flask  and 
the  solvent  distilled  off.  The  residuum  is  treated  with  two  or  three 
small  portions  of  ether  and  the  latter  removed  by  heating  on  a  water- 
bath.  The  alkaloids  are  then  dissolved  in  10  cc.  of  absolute  alcohol, 

*  Page  516.         f  Page  516.         t  Page  523.         §  Page  518.  ||Page  517. 


ASSAY    OF    ACONITE  521 

and  water  added  to  slight  permanent  turbidity.     The  solution  is  then 

N 
titrated  with  —  hydrochloric  acid  in  the  presence  of  haematoxylin  as 

indicator. 

N 
i  cc.  of  -  •    acid  equals   0.0640  gm.   of    aconitine,  the    principal 

alkaloid. 

Assay  of  Aconite  Leaves.  This  assay  is  the  same  as  that  of  the 
root.  Use  the  following  quantities:  aconite  leaves  (No.  80  powder) 
25  gms.,  ether  100  gms.,  chloroform  25  gms.,  ammonia-water  U.  S.  P. 
10  gms.  Pour  off  105  cc.  of  the  chloroform-ether  solution,  and  after 
it  has  stood  for  a  few  minutes  to  settle,  transfer  100  gms.  of  it  into 
a  separatory  funnel  and  proceed  as  above.  In  this  way  a  clearer  solu- 
tion is  obtained. 

The  German  Pharmacopoeia  Method.  This  is  a  modification 
of  the  foregoing.  In  it  the  precipitation  of  the  alkaloid  is  effected 
by  means  of  sodium  hydroxid,  and  iodeosin  is  the  indicator.  The 
method  is  as  follows: 

(a)  Place  12  gms.  of  the  drug  (in  fine  powder,  and  dried  at  100°  C.) 
into  a  250-cc.  bottle,  add  30  gms.  of  chloroform  and  90  gms.  of  ether, 
stopper  the  bottle,  and  shake  it  briskly. 

(b)  Add   10  cc.  of  a  mixture  of  two  parts   (by  weight)  of  soda 
solution  (Pharm.  G.  15  per  cent)  and  one  part  of  water,  and  shake 
frequently  during  three  hours ;   then  add  10  cc.  of  water,  or  as  much 
more  as  may  be  necessary  to  cause  the  powder  to  aggultinate  into 
lumps  upon  shaking.     The  liquid  becomes  clear  after  a  few  minutes' 
standing   and    can    be   decanted    almost   completely   from  the  sedi- 
ment. 

(c)  After  standing  for  one  hour,   decant    100  gms.   of  the  clear 
chloroform-ether  solution    (representing    10  gms.   of  the   drug),   and 
pass  it  through  a  small,  dry,  well-covered  filter  into  a  flask  and  distil 
off  about  half  of  it.     Introduce  the  rest  into  a  separating  funnel,  rinsing 
the  flask  three  times  with  a  mixture  of  chloroform  and  ether  (1:3), 
using  5  cc.  each  time  and  adding  the  rinsings  to  the  contents  of  the 
separator.  -^ 

(d)  Then  add  25  cc.  of hydrochloric  acid;   shake  thoroughly, 

and  set  aside  until  the  liquids  have  separated.     If  necessary,  add  a 
little  more  ether  to  insure  more  complete  separation. 

(e)  Draw  off  the  aqueous  layer,  pass  it  through  a  filter  moistened 
with  water  into  a   loo-cc.  flask,  wash  the  chloroform-ether   mixture 
in  the  separator  with  three  portions  of  water  (10  cc.  each),  passing 
the  washings  through  the  same  filter  into  the  flask  and  further  dilute 
with  water  to  100  cc. 


522  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

(/)  Place  50  cc.  of  this  solution  into  a  white  glass  flask,  add  50  cc. 
of  water  and  sufficient  ether  to  form  a  layer  i  cm.  in  depth;  then 

N 
five  drops  of  iodeosin  solution  and  titrate  with potassium  hydroxid, 

shaking  the  flask  after  each  addition  of  the  standard  solution  until 
the  lower  aqueous  layer  assumes  a  pale  red  color.     Not  more  than 

N 
8.5  cc.  should  be  required.     The  factor  for  aconitine  is  0.0064  gm. 

N 
(g)  The  Calculation.     The   25  cc.   of hydrochloric  acid  take 

up  the  alkaloids  representing  10  gms.  of  the  drug.     One  half  of  this 

N 
when  titrated  with  -  -  KOH  requires,  we  will  assume,  8.5  cc.  which 

N 
represents    the    quantity   of   free   -    -   acid   solution    in   the    quantity 

100  N 

titrated.     Twice  this  quantity,  i.e.,  17  cc.  of  the alkali  then  repre- 

N 

sents   17  cc.  of  acid  solution  in  a  free  state.     This  subtracted 

100 

from  the  25  cc.  originally  added  leaves  8  cc.,  which  latter  is  the  quan- 

N 

titv  of  acid  which  was  taken  up  by  the  alkaloid  present.     This, 

100        N 

multiplied  by factor  for  aconitine  (0.0064  gm-)>  gives  us   0.0512 

100 

gm.,   the   quantity  of  aconitine   in   10  gms.   of    the  drug   (0.512    per 
cent). 

The  Method  of  the  U.  S.  P.  VIII.*  Introduce  10  gms.  of  the 
aconite  root  (in  No.  40  powder)  into  a  2Oo-cc.  Erlenmeyer  flask,  add 
75  cc.  of  a  mixture  of  alcohol,  seven  parts,  and  distilled  water,  three  parts 
(by  volume),  stopper  the  flask  securely,  and  agitate  it  at  intervals 
during  four  hours.  After  placing  a  pledget  of  cotton  in  the  bottom 
of  a  small  cylindrical  glass  percolator  (25  mm.  in  diameter),  carefully 
transfer  the  contents  of  the  flask  to  the  percolator.  When  the  liquid 
has  all  passed  through,  continue  the  percolation  with  more  of  the  same 
mixture  until  150  cc.  of  percolate  have  been  obtained.  Pour  the 
percolate  into  a  shallow  porcelain  evaporat ing-dish,  and  evaporate 
it  to  dryness  at  a  temperature  not  exceeding  60°  C.  (140°  F.).  Add 
5  cc.  of  tenth-normal  sulphuric  acid  V,  S.  and  10  cc.  of  distilled  water. 
When  the  extract  is  dissolved,  filter  the  liquid  into  a  separator,  wash- 
ing the  dish,  and  filter  with  about  40  cc.  of  distilled  water,  and  add 
the  washings  to  the  separator.  Add  25  cc.  of  ether  and  2  cc.  of 
ammonia-water  to  the  separator  and  agitate  it  for  one  minute.  Draw 

*  Proposed  by  A.  B.  Stevens,  Pharm.  Arch.,  1903,  VI,  49. 


ASSAY    OF    ACONITE  523 

off  the  lower  layer  into  a  flask  and  filter  the  ether  solution  into  a 
beaker.  Return  the  contents  of  the  flask  into  the  separator,  add 
15  cc.  of  ether,  and  again  agitate  for  one  minute.  Draw  off  the  lower 
layer  into  the  flask  and  filter  the  ether  solution  into  the  beaker.  Repeat 
the  shaking  out,  with  two  other  portions  of  10  cc.  each  of  ether.  Evap- 
orate the  combined  ether  solutions  to  dryness  and  dissolve  the  residue 
in  3  cc.  of  tenth-normal  sulphuric  acid  V.  S.  Add  to  the  solution 
five  drops  of  haematoxylin  T.  S.,  and  then  carefully  run  in  fiftieth- 
normal  KOH  V.  S.  until  a  violet  color  is  produced,  the  transition 
stages  being  as  follows:  First,  yellow,  then  green,  finally  passing 
into  violet.  Divide  the  number  of  cc.  of  fiftieth-normal  KOH  V.  S. 
used  by  5;  subtract  this  number  from  3  (the  3  cc.  of  tenth-normal 
sulphuric  acid  V.  S.  taken),  multiply  the  remainder  by  0.06406,  and 
this  product  by  10,  which  will  give  the  percentage  of  aconitine  in  the 
aconite. 

N 
Each   cc.    of  —  sulphuric  acid  represents  0.06406  gm.  of  aconitine 

(CaArNOn). 

H.  M.  Gordin,*  in  commenting  upon  the  U.  S.  P.  alkaloidal  assay 
processes,  points  out  that  in  this  process,  as  well  as  in  several  others, 
the  filtration  of  the  first  acid  liquid,  as  directed,  is  attended  with  some 
difficulty  in  that  the  liquid  being  thicker  and  slimy  soon  clogs  up  the 
filter.  He  proposes  a  new  method  in  which  special  forms  of  apparatus 
are  used,  and  in  which  the  distillation  of  the  ethereal  solution  to 
dryness  is  avoided,  and  furthermore  the  liberation  of  the  alkaloid 
effected  by  means  of  fixed  alkali  hydroxid  or  carbonate.  In  the 
presence  of  the  impurities  which  always  accompany  the  alkaloid 
obtained  in  an  assay,  the  distillation  to  dryness  of  the  ethereal  or 
chloroformic  solution  frequently  causes  a  partial  resinification  of  the 
alkaloid,  which  renders  it  difficultly  soluble  in  dilute  acids  without 
heating.  Heat  in  the  presence  of  free  acid  is  liable  to  injure  the 
alkaloid. 

If  ammonia  is  employed  to  liberate  the  alkaloid,  it  is  necessary 
to  distil  the  ethereal  solution  to  dryness,  or  at  least  to  a  small  bulk, 
in  order  to  drive  off  the  ammonia  which  is  dissolved  in  the  ether  and 
which  would  vitiate  the  results  of  the  titration.  If,  on  the  other  hand, 
sodium  carbonate  or  hydroxid  is  used  for  liberating  the  alkaloid,  there 
will  be  no  need  for  distillation  to  dryness  inasmuch  as  the  fixed 
alkalies  are  not  taken  up  by  the  immiscible  solvents.  The  latter  will 
therefore  contain,  besides  the  alkaloid,  no  other  basic  substance 
except  minute  traces  of  ammonia  formed  by  the  action  of  the  fixed 

*  Proc.  A.  Ph.  A.,  1906,  377. 


524  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

alkali  upon  the  albuminous  matter  of  the  drug.  Such  traces  of  ammo- 
nia can  be  easily  and  quickly  removed  either  by  drawing  air  over 
the  surface  of  the  ethereal  liquid  or  more  quickly  by  concentrating 
the  ethereal  solution  upon  a  warm  water-bath  to  about  one  half  of 
its  original  volume. 

Ether  alone  cannot  be  used  as  an  immiscible  solvent  in  these  methods, 
because,  since  it  dissolves  water,  it  takes  up  some  fixed  alkali  when 
shaken  with  an  alkaline  solution.  But  if  instead  of  ether  alone,  chlo- 
roform alone,  or  a  mixture  of  three  volumes  of  ether  and  one  volume 
of  chloroform,  or  a  mixture  of  two  or  three  volumes  of  ether  and  one 
volume  of  petroleum  ether  be  shaken  with  a  solution  of  a  fixed  alkali, 
no  trace  of  alkali  goes  into  the  immiscible  solvent  even  if  the  alkaline 
solution  contains  50  to  60  per  cent  of  alcohol.  This  can  be  shown 
by  filtering  the  immiscible  solvent,  after  shaking  it  with  the  alkaline 
solution,  through  a  plain  filter  of  ordinary  filter-paper,  having  four 
folds  on  each  side  and  previously  moistened  with  ether,  and  then 
shaking  up  the  ethereal  liquid  with  a  little  water.  Neither  phenol- 
phthalein  nor  any  other  delicate  indicator  will  show  the  presence  of 
alkali  in  the  aqueous  liquid. 

In  order  to  eliminate  the  sources  of  error  involved  in  transferring 
ethereal  liquids  *  from  one  vessel  to  another,  Gordin  suggests  the  use 
of  special  apparatus.  These  are  described  and  illustrated  below. 

The  combination  percolator  and  shaking  tube  (Fig.  89)  is  designed 
to  avoid  the  necessity  of  transferring  the  mixture  of  drug  and  men- 
struum from  one  vessel  to  another.  Both  the  shaking  and  the  per- 
colating of  the  drug  may  be  carried  out  in  it. 

This  apparatus  is  described  by  Gordin  in  A.  J.  Ph.,  1905,  464,  as 
follows:  "The  apparatus  consists  of  a  cylindrical  tube  drawn  out  at 
both  ends  so  that  it  has  the  shape  of  an  ordinary  percolator,  but  a 
neck  like  an  ordinary  bottle.  The  main  body  of  the  tube  has  an 
inner  diameter  of  25  mm.  and  is  200  mm.  in  length.  The  lower, 
longer  but  narrower,  drawn-out  part  has  an  inner  diameter  of  14  mm., 
and  is  30  mm.  long.  At  the  juncture  of  this  narrow  tube  to  the  main 
body  of  the  tube  there  are  three  rather  deep  indentations  in  the  narrow 
tube.  The  upper  bottle-neck  shaped  end  of  the  tube  has  an  inner 
diameter  of  17  mm.  and  is  10  mm.  long.  The  whole  apparatus  is 
made  of  strong  glass  of  about  i  mm.  in  thickness. 

The  tube  is  used  in  the  following  way:  A  piece  of  cotton  is  placed 
in  a  piece  of  cheese  cloth  and  then  pushed  up  from  below,  with  the 
cloth  upwards,  into  the  tube  so  that  the  plug  reaches  the  indentations 
and  closes  the  tube  rather  tightly.  The  plug  is  then  followed  by  more 

*  Ethereal  liquids  have  a  tendency  to  " creep"  to  the  outer  sides  of  the  vessels. 


ASSAY    OF    ACONITE 


525 


cotton,  so  as  to  nearly  fill  the  narrow  tube,  and  the  latter  is  closed 
by  a  good  perforated  cork  through  which  passes  a  thin  glass  stop- 
cock. After  closing  the  stop-cock,  the  weighed  out  drug  is  introduced 
through  the  open  end  of  the  tube,  and  after  adding  the  menstruum 
the  apparatus  is  closed  with  a  good  cork. 

The  tube  can  now  be  shaken  without  the  plug  becoming  dislodged. 
After  shaking  the  tube  the  prescribed  length  of  time,  it  is  set  aside 
with  the  stop-cock  downwards  till  the  drug  has  well  settled,  and  the 
percolation  then  finished  in  the  usual  way. 

By  means  of  separating  funnel  (Fig.  90),  aqueous  and  ethereal 
liquids  can  be  drawn  off  through  separate  outlets  and  the  contamination 
of  the  immiscible  liquids  by  each  other  completely  avoided.  By  means 


FIG.  89. 


FIG.  90. 


FIG.  91. 


of  separator  (Fig.  91)  the  concentration  of  an  ethereal  liquid  can  be 
accomplished  by  placing  the  funnel  in  warm  water  and  connecting 
the  long  goose  neck  with  a  condenser,  and  the  necessity  for  trans- 
ferring the  liquid  from  a  distilling-flask  to  a  separatory  funnel  thus 
avoided. 

Gordin's  Modified  Assay  for  Aconite  Root.  Put  10  gms. 
of  aconite  root  in  No.  60  powder  into  the  percolator  shaking 
tube,  add  50  cc.  of  a  mixture  of  three  volumes  ether  and  one 
volume  chloroform  and  5  cc  of  a  10  per  cent  solution  of  sodium 
carbonate.  After  closing  the  tube  shake  the  whole  thoroughly  during 
one  hour,  then  percolate  with  the  same  immiscible  solvent  to  exhaus- 
tion. In  order  to  obtain  a  perfectly  clear  liquid  free  from  any  trace  of 
fixed  alkali  pass  the  stop-cock  of  the  percolator  through  a  very  wide 
cork  placed  over  a  small  funnel  which  contains  a  small  plain  double 


526  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

filter  of  ordinary  filter-paper  having  four  folds  on  each  side.  The 
cork  serves  as  a  cover  to  prevent  evaporation.  The  percolate  can  be 
received  into  any  vessel,  or  better  into  special  separatory  funnel  (Fig. 
91).  The  percolate  is  then  concentrated,  by  placing  the  funnel  in 
warm  water,  to  about  one  half  of  the  original  volume  to  remove  traces 
of  ammonia  and,  when  cold,  diluted  again  with  ether  to  approximately 
the  original  volume.  The  ethereal  liquid  is  now  shaken  out  once  with 
excess  of  standard  acid  and  then  washed  twice  with  water.  The 
excess  of  acid  is  then  titrated  in  the  usual  way.  As  it  is  always  advisable 
to  control  the  acidimetric  estimation  by  a  gravimetric  one  the  acid 
liquid  can  be  received  into  a  separating  funnel  of  suitable  capacity 
and  the  titration  carried  out  directly  in  this  funnel.  After  the  titration 
the  liquid  is  made  strongly  alkaline  with  sodium  hydroxid  and  shaken 
out  three  times  with  chloroform.  The  indicator  added  all  remains 
in  the  alkaline  liquid  and  the  colorless  chloroformic  solution,  after 
evaporation,  leaves  the  alkaloid  to  be  weighed  and  identified  by  special 
reactions.  Assayed  by  this  method  a  sample  of  good  aconite  root  gave 
(volumetrically)  1.02  per  cent  alkaloids. 

Method  of  C.  Kippenberger  (Apoth.  Ztg.,  1898,  664).  18  gms. 
of  the  root  in  fine  powder  are  extracted  with  180  cc.  of  alcohol  con- 
taining 3  gms.  of  tartaric  acid.  120  cc.  of  the  filtrate  are  evaporated 
to  dryness,  avoiding  high  temperatures.  The  residue  is  dissolved  in 
60  cc.  of  water  containing  2  cc.  of  dilute  hydrochloric  acid  (G.  Ph.), 
and  50  cc.  of  the  filtrate  (representing  10  gms.  of  the  drug)  are 
precipitated  with  an  excess  of  iodin  solution,  20  to  40  cc.  being 
used. 

The  iodin  solution  is  made  as  follows:  Iodin  20  gms.,  potassium 
iodid  60  gms.,  water  to  make  1000  cc.  The  mixture  is  let  stand  about 
half  an  hour  in  order  to  insure  complete  precipitation  of  the  periodid. 
The  precipitate  is  then  collected  upon  a  small  plaited  filter,  washed 
twice  with  water  containing  a  few  drops  of  the  iodin  solution,  and 
dissolved  in  pure  acetone  (of  which  15  to  20  cc.  are  required),  observing 
that  it  be  completely  dissolved,  and  that  there  be  no  waste.  The 
acetone  solution  is  then  treated  with  alkali  hydroxid  solution  in  excess, 
to  precipitate  the  pure  alkaloid,  then  diluted  hydrochloric  acid  is 
added  in  excess  to  take  up  the  alkaloid,  and  the  mixture  diluted  with 
water  and  shaken  up  with  petroleum-ether  to  remove  the  acetone, 
the  last  traces  of  which  are  removed  by  warming  on  a  water-bath. 
The  liquid  is  now  supersaturated  with  alkali  hydroxid,  and  shaken  out 
with  chloroform.  The  chloroformic  solution  of  the  pure  alkaloid  so 
obtained  is  evaporated  and  the  amount  of  alkaloid  determined  either 
by  direct  weighing  or  by  titration.  This  method  may  be  applied  to 
most  other  drugs,  but  the  amount  of  solvents  and  precipitant  must, 
of  course,  vary  with  the  nature  of  the  drug  assayed. 


MYDRIATIC   DRUGS  527 

G.  Fromme's  Method.*  7  gms.  of  aconite  of  medium  fineness, 
70  gms.  ether,  and  5  gms.  15  per  cent  sodium  hydroxid  solution  are 
shaken  frequently  and  vigorously  during  half  an  hour  and  then  as 
much  as  possible  poured  through  a  pledget  of  cotton  into  a  flask; 
i  cc.  of  water  is  added  to  the  ethereal  liquid,  the  mixture  well  shaken, 
put  aside  until  perfectly  clear,  and  then  50  gms.  or  as  much  as  possible 
(iogms.=  igm.  of  drug)  poured  off.  This  is  extracted  with  15, 
10,  and  10  gms.  of  i  per  cent  hydrochloric  acid.  Then  the  acid 
extractions  are  just  neutralized  with  ammonium  hydroxid,  and  the 
alkaloid  abstracted  •  with  15,  10,  and  10  gms.  of  chloroform,  and 
successively  passed  through  a  3  to  4  cm.  plain  filter  into  a  loo-cc. 
tared  Erlenmeyer  flask.  The  chloroform  is  distilled  from  a  water- 
bath,  the  residue  twice  dissolved  in  ether,  5  gms.  each  time,  brought 
to  dryness,  the  residue  dried  to  constant  weight  in  a  desiccator  and 
weighed.  As  a  check  the  residue  is  dissolved  in  a  few  cc.  of  absolute 
alcohol,  about  20  cc.  of  water,  and  a  few  drops  of  haematoxylin  solution 

N 
added  and  its  alkalinity  determined  with  —  hydrochloric  acid. 

*  10 


THE  MYDRIATIC  DRUGS 

These  drugs,  namely,  belladonna,  hyoscyamus,  scopola,  and 
stramonium,  are  assayed  according  to  the  U.  S.  P.  by  the  Keller 
method,  as  modified  by  W.  A.  Puckner  (Ph.  Rev.,  xvr,  180;  and  xx, 
457).  (Seepage  516). 

Assay  of  Belladonna  (U.  S.  P.).  Place  10  gms.  of  belladonna 
(leaves  or  root  in  No.  60  powder)  in  an  Erlenmeyer  flask,  and  add 
50  cc.  of  a  mixture  of  chloroform  one  part  and  ether  four  parts  (both 
by  volume).  After  inserting  the  stopper  securely,  allow  the  flask  to 
stand  ten  minutes,  then  add  2  cc.  of  ammonia  water  mixed  with  3  cc. 
of  distilled  water,  and  shake  the  flask  well  at  frequent  intervals  during 
one  hour.  Then  transfer  as  much  as  possible  of  the  contents  of  the 
flask  to  a  small  percolator  which  has  been  provided  with  a  small 
pledget  of  cotton  packed  firmly  in  the  neck  and  inserted  in  a  sepa- 
rator containing  6  cc.  of  normal  sulphuric  acid,  diluted  with  20  cc. 
of  distilled  water.  When  the  liquid  has  passed  through  the  cotton, 
pack  the  drug  firmly  in  the  percolator  with  the  aid  of  a  glass  rod, 
and  having  rinsed  the  flask  with  10  cc.  of  the  chloroform-ether  mixture, 
transfer  the  remaining  contents  of  the  flask  to  the  percolator,  by  the 
aid  of  several  small  portions  (5  cc.)  of  the  chloroform-ether  mixture, 
and  continue  the  percolation  with  successive  small  portions  of  the 

*  Geschaftsbericht,  Caesar  and  Loretz,  1905,  51. 


528  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

same  liquid  (using  in  all  50  cc.).  Next  shake  the  separator  well  for 
one  minute  after  securely  inserting  the  stopper,  and  when  the  liquids 
have  completely  separated,  draw  off  the  acid  solution  into  another 
separator.  Add  to  the  chloroform-ether  mixture  10  cc.  of  sulphuric 
acid  mixture  of  the  same  strength  as  that  previously  used,  agitate  well, 
and  again  draw  off  the  acid  solution  into  the  second  separator;  repeat 
this  operation  once  more,  drawing  off  the  acid  solution  as  before. 
Introduce  into  the  acid  solution  contained  in  the  second  separator  a 
small  piece  of  red  litmus  paper,  then  add  ammonia-water  until  the 
liquid  is  distinctly  alkaline,  and  shake  out  with  three  successive 
portions  of  chloroform  15,  15,  and  5  cc.;  collect  the  chloroform  solu- 
tions in  a  beaker,  place  it  on  a  water-bath  containing  warm  water, 
and  allow  the  chloroform  to  entirely  evaporate.  Dissolve  the  residue 
in  3  cc.  of  ether,  and  let  this  also  entirely  evaporate.  To  the  alka- 

N 
loidal  residue  add  3  cc.  of  —  sulphuric  acid  and  5  drops  of  hasmatoxylin 

N 
T.  S.  (or  iodeosin  T.  S.);  then  titrate  the  excess  of  acid  with  —  KOH 

N  5o 

Divide  the  number  of  cc.  of  —  KOH  used  by  5,  subtract  the  quotient 

N 

from  3  (the  3  cc.  of  —  sulphuric  acid  taken),  and  multiply  the  re- 
mainder by  0.0287,  and  this  product  by  10;  the  result  will  be  the 
percentage  of  total  mydriatic  alkaloids  contained  in  the  belladonna. 
The  pharmacopceial  requirement  is,  not  less  than  0.35  per  cent  for 
the  leaves  and  not  less  than  0.5  per  cent  for  the  root  of  mydriatic 
alkaloids. 

It  is  suggested  that  in  carrying  out  this  process,  the  percolation 
be  continued  to  exhaustion,  as  indicated  by  Wagner's  reagent,  and 
that  double  the  quantity  of  drug  be  taken  for  the  assay. 

Assay  of  Belladonna  Leaves  (Lyman  F.  Kebler,  J.  A.  C.  S., 
XVH,  828).  Place  10  gms.  of  the  powder  in  a  25o-cc.  flask,  add  25  gms. 
of  chloroform  and  75  gms.  of  ether;  stopper  the  flask  securely,  agitate 
well  for  several  minutes,  add  10  gms.  of  10  per  cent  ammonia-water, 
then  agitate  frequently  and  during  one  hour.  On  adding  5  gms.  more 
of-the  ammonia-water  and  shaking  well,  the  suspended  powder  agglu- 
tinates into  a  lump,  and  the  liquid  becomes  clear  after  standing  a  few 
minutes  and  can  be  poured  off  almost  completely. 

When  the  mixture  has  completely  separated,  pour  50  gms.  into  a 
separatory  funnel,  and  treat  it  at  once  with  20  cc.  of  acidulated  water. 
After  thorough  agitation  and  complete  separation  remove  the  aqueous 
solution  into  a  second  separatory  funnel.  Repeat  the  above  operation 
twice  more  successively  with  15  cc.  of  slightly  acidulated  water. 

The  acidulated  aqueous  solution  in  the  second  separatory  funnel 


ASSAY  OF  HYOSCYAMUS  LEAVES  529 

is  then  rendered  alkaline  with  ammonia-water  and  the  reprecipitated 
alkaloid  removed  by  adding  successively  20  cc,  15  cc.,  and  15  cc.  of 
a  mixture  of  3  parts  (by  volume)  of  chloroform  and  i  part  of  ether. 
Collect  the  chloroform-ether  mixture  in  a  beaker  and  distil  off  the 
solvent.  The  varnish-like  residue  is  next  dissolved  in  15  cc.  of 
alcohol  with  heat,  water  is  added  to  slight  permanent  turbidity,  a 

N 
few  drops  of  haematoxylin  solution  added,  then  a  slight  excess  of  — 


N 


sulphuric  acid,  and  retitrate  with  —  alkali  solution. 

N  20 

Each  cc.  of  —  acid= 0.01445  gm.  of  alkaloid  as  atropin. 

The  use  of  decinormal  sulphuric  acid  and  of  centinormal  alkali 
solution  is  preferred  by  many;  one  tenth  the  number  of  cc.'s  of  the 
alkali  used  is  then  deducted  from  the  quantity  of  decinormal  acid 
added  and  the  remainder  multiplied  by  0.0289  gm- 

The  Periodid  Method  of  Gordin  and  Prescott  may  be  employed 
for  the  assay  of  belladonna  (see  page  518). 

Other  methods  recommended  are: 

Frank  X.  Moerk  (A.  J.  Ph.,  1899,  105-120  and  320-326). 

E.  R.  Squibb  (A.  J.  Ph.,  1900,  4-6). 

La  Wall  and  Pursel  (A.  J.  Ph.,  1899,  394). 

E.  Schmidt  (Apoth.  Ztg.,  1900,  13,  14). 

W.A  .  Puckner  (Ph.  Review,  1898,  180-183  and  3°3~3°8;  also 
1902,  457-463). 

Fromme,  (Geschaftsbericht,  Caesar  and  Loretz,  1905,  87). 

Farr  and  Wright  (Ph.  Jour.,  xx,  546). 

Beckurts  (Apoth.  Ztg.,  xvin,  67). 

Thorns  (Bericht.  d.  Ph.  Gesell.,  xv,  85). 

Pearson  and  Roberts  (A.  J.  Ph.  (1908),  LXXX,  368). 

Assay  of  Hyoscyamus  Leaves.  Hyoscyamus  leaves  may  be 
assayed  by  the  method  of  Keller,  or  by  some  modification  of  it. 

Keller's  Method  is  as  follows:  To  20  gms.  of  the  leaves  dried  over 
sulphuric  acid,  reduced  to  No.  40  powder  and  contained  in  a  short - 
necked  flask,  are  added  100  cc.  of  chloroform-ether  mixture  (chloro- 
form 20  cc.,  ether  U.  S.  P.  80  cc.),  and  after  standing  for  a  few  minutes, 
10  cc.  of  ammonia-water  (10  per  cent),  then  the  flask  is  well  corked 
and  shaken  thoroughly  and  frequently  for  one  hour.  At  the  end  of 
this  time  the  mixture  is  transferred  to  a  small  narrow  percolator 
(improvised  by  drawing  out  a  piece  of  glass  tubing  or  a  large  test-tube 
about  25  mm.  in  diameter)  provided  with  a  plug  of  cotton  at  the 
outlet  and  the  percolate  received  in  a  separator.  When  nearly  all 
the  liquid  has  passed  through,  the  drug  is  packed  down  rather  firmly 
by  means  of  a  glass  rod.  When  the  liquid  no  longer  drops  from  the 


530  A  MANUAL  OF   VOLUMETRIC  ANALYSIS 

percolator,  25  cc.  of  the  same  chloroform-ether  mixture  is  poured 
upon  the  drug  remaining  in  the  flask,  and  with  it  the  emainder  of 
the  drug  transferred  to  the  percolator. 

When  this  liquid  also  has  passed,  a  further  25  cc.  of  the  same 
menstruum  is  used  to  complete  the  extraction.  The  united  chloro- 
form-ether solution,  containing  the  alkaloid  from  20  gms.  of  the  drug, 
is  now  extracted  successively  with  10,  10,  10,  and  10  cc.  of  i  per  cent 
hydrochloric  acid.  This  acid  solution,  which  has  been  received  in  a 
second  separator,  is  rendered  alkaline  with  5  cc.  of  ammonia-water 
(10  per  cent)  and  shaken  with  20,  10,  and  10  cc.  of  chloroform.  The 
chloroform  as  it  is  drawn  off  is  passed  through  a  small  pellet  of  cotton 
(to  retain  any  water  accidentally  carried  over)  into  a  shallow  beaker 
and  evaporated  at  a  temperature  of  25°  to  35°  C.  When  all  has 
evaporated  the  residue  is  dissolved  in  5  cc.  of  ether  and  again  brought 
to  dryness.  In  the  residue  the  alkaloid  is  determined  by  adding  ether 
2  or  3  cc.,  cochineal  test  solution  3  to  5  drops,  and  decinormal  acid 
in  slight  but  distinct  excess,  and  after  complete  solution  has  taken 
place,  and  the  ether  which  was  added  to  assist  in  the  solution  of  the 
alkaloid  has  evaporated,  determining  the  excess  of  acid  with  deci- 
normal alkali. 

The  number  of  cc.  of  decinormal  acid  combined  with  the  alkaloid, 
multiplied  by  the  factor  0.02868  gm.,  indicates  the  weight  of  alkaloid 
calculated  as  hyoscyamine  in  20  gms.  of  the  drug. 

N 
If  —  acid  is  used  the  factor  is  0.01434  gm.    Haematoxylin  may  be 

employed  as  indicator. 

The  U.  S.  P.  recommends  the  employment  of  the  same  method 
as  that  used  for  belladonna,  with  the  exception  that  25  gms.  of  hyos- 
cyamus  are  to  be  taken,  the  quantity  of  chloroform-ether  mixture, 
which  is  added  at  first,  increased  from  50  to  100  cc.,  and  the  product 
at  the  end  of  the  assay  multiplied  by  4  instead  of  10.  Not  less  than 
0.08  per  cent  of  mydriatic  alkaloids  should  be  present. 

LITERATURE 

W.  A.  Puckner.    Ph.  Rev.,  1898  (Vol.  xvi),  180  and  (Vol.  xxni). 

H.  Beckurts.     Apoth.  Ztg.,  xvm,  67. 

W.  A.  Puckner.     Proc.  A.  Ph.  A.,  1899,  297. 

H.  Beckurts.     Ph.  Centralh.,  1894,  566. 

K.  Schwickerath.     Ph.  Rundschau,  1893  (Vol.  xi),  282 

E.  Schmidt.     Apoth.  Ztg.,  1900,  13,  14. 

Scopola  and  Stramonium  are  both  assayed  by  the  same  methods 
recommended  for  belladonna  and  hyoscyamus.  The  former  should 


ASSAY   OF  CINCHONA  531 

contain  not  less  than  0.5  per  cent  and  the  latter  not  less  than  0.35 
per  cent  of  mydriatic  alkaloids. 

ASSAY    OF    CINCHONA 

The  U.  S.  P.  (1890)  Method,  (a)  For  Total  Alkaloids.  20  gms. 
of  the  cinchona  in  very  fine  powder  (No.  80  or  finer  and  completely 
dried  at  100°  C.)  are  introduced  into  a  bottle  provided  with  an  ac- 
curately fitting  glass  stopper,  and  to  this  is  added  200  cc.  of  a 
previously  prepared  mixture  of  19  volumes  of  alcohol,  5  volumes  of 
chloroform,  and  i  volume  of  ammonia -water;  the  bottle  is  stoppered, 
and  thoroughly  and  frequently  shaken  during  four  hours.  The  liquid 
is  then  passed  through  a  plug  of  cotton  in  a  funnel  into  another 
bottle,  being  careful  that  there  occurs  no  loss  by  evaporation. 

100  cc.  of  the  clear  filtrate  (representing  10  gms.  of  cinchona) 
are  transferred  to  a  beaker  and  evaporated  to  dryness.  The  crude 
alkaloids  thus  obtained  are  dissolved  in  10  cc.  of  water  and  4  cc.  of 
normal  sulphuric  acid -with  the  aid  of  gentle  heat.  The  cooled  solu- 
tion is  then  filtered  into  a  separator,  and  the  beaker  and  filter  washed 
with  water  until  the  washings  no  longer  have  an  alkaline  reaction, 
using  as  little  water  as  possible. 

Now  add  5  cc.  of  potassium  hydroxid  V.  S.,-  or  sufficient  to  render 
the  liquid  alkaline.  The  alkaloids  are  thereby  reliberated,  and  may 
be  shaken  out  by  chloroform.  20  cc.  of  chloroform  are  first  added, 
and  the  extraction  repeated,  using  10  cc.  at  a  time,  until  a  drop  of 
the  last  chloroform  extraction  leaves  no  residue  when  evaporated  on 
a  watch-glass. 

The  chloroformic  extracts  are  then  mixed,  evaporated  in  a  tared 
beaker,  the  residue  dried  at  100°  C.  (212°  F.)  and  weighed. 

The  weight  multiplied  by  10  will  give  the  precentage  of  total 
alkaloids  in  the  specimen  tested. 

The  volumetric  method  cannot  very  well  be  employed  here,  as  the  alka- 
loids exist  in  varying  proportions  and  are  very  numerous,  thus  making  it 
difficult  to  find  a  factor  which  will  answer  for  all  cases. 

However,  some  experimenters  dissolve  the  weighed  alkaloids  in  10  or 
15  gms.  of  alcohol,  adding  water  until  slight  permanent  turbidity  appears, 

N 
and  then  titrate  with  —  hydrochloric  acid,  using  haematoxylin  as  indicator. 

N 
i  cc.  of  the  —  acid  is  assumed  to  be  equivalent  to  0.0315  gm.  of  calisaya 

alkaloids  and  of  0.0304  gm.  of  succirubra  alkaloids. 


over 


b)  For  Quinine.     Transfer  50  cc.  of  the  clear  filtrate  remaining 
from  the  preceding  process  (and  representing  5  gms.  of  cinchona) 


532  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

to  a  beaker,  evaporate  it  to  dryness,  and  proceed  as  directed  in  the 
assay  for  total  alkaloids,  using,  however,  only  half  the  amounts  of 
volumetric  acid  and  alkali  there  directed. 

Add  the  united  chloroformic  extracts  containing  the  alkaloids  in 
solution,  gradually  and  in  small  portions  at  a  time,  to  about  5  gms. 
of  powdered  glass  contained  in  a  porcelain  capsule  placed  over  a 
•vater-bath,  so  that  when  the  contents  of  the  capsule  are  dry,  all  or 
nearly  all  of  the  dry  alkaloids  shall  be  in  intimate  admixture  with 
the  powdered  glass,  and  the  chloroform  completely  expelled.  Now 
moisten  the  residue  with  ether,  and  having  placed  a  funnel  containing 
a  filter  (7  cm.  in  diameter)  and  well  wetted  with  ether  over  a  small 
graduated  tube  A,  transfer  to  the  filter  the  ether-moistened  residue 
from  the  capsule.  Rinse  the  latter,  several  times  if  necessary,  with 
fresh  ether,  so  as  to  transfer  the  whole  of  the  residue  to  the  filter; 
then  percolate  with  ether,  drop  by  drop,  until  exactly  10  cc.  of  per- 
colate are  obtained.  Then  collect  another  10  cc.  by  similar  slow 
percolation  with  ether  in  a  second  graduated  tube  B.  Transfer  the 
contents  of  the  tubes  to  two  small  tared  capsules,  properly  marked, 
A  and  B,  and  evaporate  to  a  constant  weight  at  100°  C.  (212°  F.) 
and  weigh  them.  (The  residue  in  A  will  contain  practically  all  of 
the  quinine,  together  with  a  portion  of  the  alkaloids  less  soluble  in 
ether;  the  residue  in  B  will  consist  almost  entirely  of  these  alkaloids.) 

From  the  amount  of  residue  obtained  in  A  deduct  that  contained 
in  B.  This  will  give  approximately  the  amount  of  quinine  present  in 
the  5  gms.  of  sample.  Multiply  this  by  20,  and  the  percentage  of 
quinine  containing  one  molecule  of  water  is  obtained. 

Cinchona  calisaya  should  contain  not  less  than  5  per  cent  of  total 
alkaloids,  and  at  least  2.5  per  cent  of  quinine. 

The  assay  method  of  the  1900  U.  S.  P.  is  a  gravimetric  one,  and 
is  based  on  Keller's  method.  It  is  directed  to  macerate  15  gms.  of 
the  drug  in  No.  80  powder  or  finer,  with  125  cc.  of  ether,  25  cc.  of 
chloroform,  and  10  cc.  of  ammonia-water  during  five  hours.  Fjom 
100  cc.  of  the  decanted  liquid  the  alkaloids  are  abstracted  with  dilute 
acid.  In  one  half  of  the  acid  extraction  the  total  alkaloids  are  deter- 
mined, in  the  other  half  the  ether-soluble  alkaloids  are  determined. 
This  method  is  criticized  by  A.  B.  Lyons  (Proc.  A.  Ph.  A.,  1906,  440) 
and  by  J.  M.  Francis  (same  vol.,  page  453).  The  ether-soluble  alka- 
loids include  quinine,  quinidine,  and  cinchonidine. 

Gordin's  Modified  Alkalimetric  Method.*  4  gms.  of  the  very 
finely  powdered  bark  are  digested  with  100  cc.  of  modified  Prollius 
fluid,  shaking  frequently  during  four  hours.  50  cc.  of  the  liquid 


*  Proc.  A.  Ph.  A.,  1900,  125. 


ASSAY    OF  CINCHONA  533 

(representing  2  gms.  of  the  bark)  are  then  drawn  off,  evaporated, 
and  the  residue  taken  up  with  strongly  acidulated  water  and  filtered. 
The  filtered  liquid  is  then  made  strongly  alkaline  with  potassium 
hydroxid,  and  shaken  out  three  times  with  a  mixture  of  3  parts  of 
ether  and  i  of  chloroform,  using  25  cc.  each  time.  The  united  ethereal 
liquids  are  then  shaken  up  with  about  half  a  gram  of  calcined  mag- 
nesia. This  completely  removes  the  small  quantity  of  water  together 
with  traces  of  the  alkali  present. 

N 
The  ethereal  liquid  is  filtered  into  a  flask,  40  cc.  of  —  H2SO4  V.  S. 

4° 

added,  and  after  shaking  well  the  ethereal  liquid  is  completely  dis- 
tilled off.  The  acid  liquid  is  then  poured  into  a  ico-cc.  measuring 
flask,  the  distilling  flask  washed  twice  with  5  cc.  of  water,  and  Mayer's 
reagent  added  in  small  quantities  at  a  time  till  the  reagent  is  in  con- 
siderable excess.  The  flask  is  then  filled  up  to  the  loo-cc.  mark  with 
water  and  shaken  until  the  supernatant  liquid  is  clear,  the  liquid 

N 
is  filtered  and  50  cc.  of  the  clear  filtrate,  titrated  with  —  KOH,  using 

phenolphthalein  as  the  indicator. 

For  total  alkaloids,  the  mean  factor  of  quinine  and  cinchonidine  was 

N 

taken,  which  for  —  acid  V.  S.  is  0.0077  gm- 
40 

C.  Kippenberger's  Method.  9  gms.  of  the  finely-powdered  bark 
are  extracted  with  90  cc.  of  alcohol  containing  9  cc.  of  diluted  hydro- 
chloric (G.  Ph.)  and  60  cc.  of  the  filtrate  are  evaporated  to  dryness. 
The  residue  is  dissolved  in  60  cc.  of  water  containing  6  cc.  diluted 
hydrochloric  acid  (G.  Ph.),  and  50  cc.  of  this  filtrate  (=5  gms.  of  the 
bark)  are  treated  with  30  to  40  cc.  of  the  iodin  solution,  modified 
as  under  Alcoholic  Extract  of  Cinchona,  and  then  further  treated  as 
under  Assay  of  Aconite  Leaves. 

In  the  alkalimetric  estimation  of  cinchona,  haematoxylin,  Brazil- 
wood, or  cochineal  may  be  used  as  indicator.  To  these  indicators, 
compounds  like  the  normal  quinine  sulphate  react  netitral.  To  methyl- 
orange,  on  the  other  hand,  the  acid  sulphate  of  quinine  reacts  neutral 
and  the  normal  sulphate  alkaline.  Hence  if  this  latter  indicator  is 
used,  the  titration  equivalent  for  quinine  is  one-half  what  it  is  with 
the  other  indicators. 

The  German  Pharmacopoeia  Method.  Place  12  gms.  of  the 
drug  (in  fine  powder,  and  dried  at  100°  C.)  into  a  25o-cc.  flask,  add 
30  gms.  of  chloroform  and  90  gms.  of  ether,  stopper  the  bottle  and 
shake  it  briskly. 

Add  10  cc.  of  soda  solution  (G.  Ph.  15  per  cent)  and  shake 
frequently  for  three  hours;  then  add  10  cc.  of  water,  or  as  much  more 
as  may  be  necessary  to  cause  the  powder  to  agglutinate  into  lumps 


534  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

upon  shaking  and  the  supernatant  ethereal  liquid  to  become  quite 
clear.  After  standing  for  one  hour,  decant  100  gms.  of  the  clear 
chloroform-ether  solution  (representing  10  gms.  of  the  drug),  and 
pass  it  through  a  small,  dry,  well  covered  filter  into  a  flask  and  distil 
off  about  half  of  it.  Introduce  the  rest  into  a  separating  funnel, 
rinsing  the  flask  three  times  with  a  mixture  of  chloroform  and  ether 
(1:3),  using  5  cc.  each  time  and  adding  the  rinsings  to  the  contents  of 
the  separator. 

N 
Then  add  25  cc.  of  —  hydrochloric  acid,  shake  thoroughly,  and 

set  aside  until  the  liquids  have  separated.  If  necessary,  add  a  little 
more  ether  to  insure  more  complete  separation.  Draw  off  the  aqueous 
layer,  pass  it  through  a  filter  moistened  with  water,  into  a  loo-cc.  flask, 
wash  the  chloroform-ether  mixture  in  the  separator  with  three  por- 
tions of  water  (10  cc.  each),  passing  the  washings  through  the  same 
filter  into  the  flask,  and  further  dilute  with  water  to  100  cc.  Remove 
50  cc.  of  this  solution,  add  a  few  drops  of  a  freshly  prepared  alcoholic 

N 
solution  of  haematoxylin,  and  titrate  with  —  KOH  until,  upon  shaking, 

a  violet-blue  color  is  obtained.  Not  more  than  4.3  cc.  should  be 
required. 

Florence's  Method.*  (I)  The  Simple  Rapid  Method.  This 
method  is  recommended  where  extreme  accuracy  is  not  required. 
12  gms.  of  the  finely-powdered  bark  are  shaken  with  120  gms.  of  the 
pure  alcohol-free  ether,  10  cc.  of  10  per  cent  sodium  hydroxid  are 
added,  and  the  mixture  is  frequently  shaken  during  one  hour.  Then 
10  gms.  of  water  are  added,  so  that  the  powdered  bark  may  separate 
as  a  coherent  mass,  whereupon  the  ether  extraction  is  decanted  and 
shaken  with  20  to  30  cc.  of  lime  water  to  remove  resinous  constituents. 
Now  decant  100  gms.  (or  an  aliquot  portion)  of  the  ether  extraction 

N 
into  a  glass-stoppered  vessel,  add  30  cc.  of  water,  allow  —  ethereal. 

solution  of  oxalic  acid  (freshly  prepared  by  dissolving  0.63  gm.  crys- 
tallized oxalic  acid  in  100  cc.  of  pure  ether)  to  flow  into  the  ethereal 
solution  of  alkaloid  from  a  burette,  as  long  as  a  precipitate  is  pro- 
duced or  until  a  drop  of  the  aqueous  layer  reacts  neutral  with  litmus. 
The  number  of  cc.  of  oxalic  acid  solution  consumed,  multiplied  by 
0.035,  gives  the  amount  of  total  alkaloid  in  10  gms.  of  bark  (=  100  gms. 
of  ether  extraction). 

Under  the  conditions  of  this  experiment,  all  of  the  cinchona  alka- 
loids in  the  ether  extraction  form  white  precipitates  which,  with  the 

*  Bull,  des  scienc.  pharmacolog.,  1906,  365  (Pharm.  Centralh.,  1907,  48,  405. 


ASSAY    OF    CINCHONA  535 

exception  of  the  quinine  oxalate  formed,  are  dissolved  in  the  water 
provided  for  this  purpose.  The 

Determination  of  Quinine  is  therefore  simply  effected  by  collecting 
the  precipitate  on  a  filter,  washing  it  with  water  until  lime  water  ceases 
to  produce  turbidity,  then  drying  and  weighing  it.  The  quantity  of 
quinine  oxalate  so  ascertained,  multiplied  by  0.878,  gives  the  amount 
of  quinine  in  10  gms.  of  the  sample  (i  gm.  quinine  oxalate= 0.878  gm. 
quinine). 

(II)  The  Exact  Method.  In  the  exact  method  the  drug  is  treated 
in  an  extraction  apparatus  with  a  mixture  of  ether,  four  parts,  and 
chloroform,  one  part,  until  a  portion  of  the  percolate  is  not  rendered 
turbid  by  addition  of  ethereal  oxalic  acid  solution.  The  ethereal 
liquid  is  then  extracted  in  a  separatory  funnel  with  three  portions  of 
lime  water,  the  latter  extracted  twice  with  a  little  ether,  the  ethereal 
liquids  united,  brought  to  dryness  and  the  residue  weighed  as  total 
alkaloids.  To  determine  the  quinine  the  total  alkaloids  are  dissolved 
in  ether,  or  in  ether  to  which  one  fifth  its  volume  of  chloroform  has 
been  added,  and  30  cc.  of  an  aqueous  saturated  solution  of  quinine 
oxalate  and  then  the  quinine  precipitated  with  the  ethereal  oxalic  acid 
solution,  as  in  the  short  method.  The  ether  is  decanted  to  a  tared 
filter,  then  the  precipitate  transferred  to  the  filter  and  washed  with  a 
saturated  solution  of  quinine  oxalate  until  the  washings  are  rendered 
no  more  turbid  on  the  addition  of  lime  water  than  a  saturated  solution 
of  quinine  oxalate  when  treated  in  the  same  way.  The  precipitate  is 
allowed  to  drain,  the  filter  and  precipitate  pressed  between  filter-paper 
to  absorb  most  of  the  retained  wash  fluid  and  weighed.  It  is  then 
dried,  finally  at  100°  C.  and  again  weighed.  Since  i  cc.  water  dis- 
solved 0.00069  gm.  quinine  oxalate  there  is  subtracted  from  the  last 
weight  0.00069  gm.  for  every  gram  difference  between  the  first  and 
second  weight  and  also  the  weight  of  the  filter — the  remainder  is  the 
weight  of  quinine  oxalate.  The  quinine  oxalate  solution  is  prepared  by 
treating  quinine  sulphate  with  sodium  hydroxid  and  ether,  precipitating 
the  ether  solution  of  quinine  with  an  ether  solution  of  oxalic  acid, 
collecting  the  precipitate,  washing  it  with  ether  and  drying  it. 

Fromme's  Method.  G.  Fromme*  recommends  the  following 
method  for  assaying  cinchona  bark:  2.5  gms.  of  the  air-dry  powder  (fine 
or  coarse)  are  heated  for  ten  minutes  on  a  steam  bath  with  a  mixture  of 
2  cc.  of  pure  25  per  cent  hydrochloric  acid  and  20  cc.  of  distilled  water. 
After  cooling,  50  gms.  of  ether  and  25  gms.  of  chloroform  are  added,  the 
mixture  is  vigorously  shaken,  then  supersaturated  with  5  cc.  of  15  per 
cent  sodium  hydroxid  solution,  vigorously  and  continuously  shaken 

*  Ph.  Ztg.,  I,  No.  73  (1905),  770;   from  Ber.  of  Caesar  and  Loretz. 


536  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

during  ten  minutes,  and  1.5  gms.  of  powdered  tragacanth  having  been 
added,  the  mixture  is  once  more  well  shaken.  The  now  clear  chloro- 
form-ether solution  is  filtered  into  a  thoroughly  .cleansed  2oo-gm. 
flask  until  60  gms.  of  filtrate  (corresponding  to  2  gms.  of  the  bark) 
are  collected,  and  in  this  the  alkaloid  is  determined  either  gravimetrically 
or  volumetrically  as  follows: 

a.  Gravimetrically.     The  filtrate  (60  gms.,  2  gms.  bark)  is  shaken 
out  with  20,  10,  10  cc.  of  i  per  cent  hydrochloric  acid — if  necessary, 

N 
with  a  fourth  portion,  consisting  of  10  cc.  of  —  hydrochloric  acid  if  a 

reaction  is  obtained  with  Mayer's  reagent  in  the  last  portion  of  acid 
liquid.  The  united  acid  liquids  are  mixed  with  15  cc.  of  chloroform, 
ammonia -water  is  added  in  moderate  excess,  and  the  mixture  is  vigor- 
ously shaken.  The  chloroform  solution  is  filtered  through  a  double 
filter  into  a  small,  tared  Erlenmeyer  flask,  and  the  aqueous  liquid 
shaken  out  twice  niore  than  10  and  10  cc.  of  chloroform,  the  solution 
being  filtered  through  the  same  double  filter  into  the  flask.  The 
chloroform  is  distilled  off  and  the  residue  heated  to  100°  C.  to  constant 
weight;  the  ascertained  weight,  multiplied  by  50,  giving  the  percen- 
tage of  alkaloid. 

b.  Volumetrically.     The   filtrate    (60  gms.  =  2   gms.   bark)   is   sub- 
jected to  evaporation  (or  distillation);  the  residual  alkaloid  is  dissolved 
in  10  cc.  of  alcohol,  10  cc.  of  ether  and  30  cc.  of  water  are  added, 

N 
followed  by  a  few  drops  of  haematoxylin  solution,  and  titrated  with  — 

hydrochloric  acid,  shaking  after  each  addition,  and  adding  towards 
the  end  of  the  titration  10  and  30  cc.  more  of  water.  The  titration  is 
ended  as  soon  as  the  liquid  assumes  a  lemon-yellow  color.  Each  cc. 

N 
of  the  -     acid  combines  with  0.0309  gm.  of  cinchona  alkaloids;    by 

10  .  N 

multiplying  the  number  of  cc.  of  —  acid  consumed  by  0.0309,  the 

10 

amount  of  alkaloid  in  2  gms.  of  bark  is  ascertained,  and  this,  multiplied 
by  50,  gives  the  percentage.  Of  the  two  methods  the  gravimetric 
method  is  regarded  as  the  more  reliable. 

A.  Panchaud  *  has  determined  that  cinchona  alkaloids  readily 
decompose  chloroform  according  to  CHCl3+O=COCl2-hHCl.  If 
cinchona  alkaloids  are  dissolved  in  chloroform  in  the  evening  and 
titrated  the  next  morning,  from  20  to  100  per  cent  of  the  alkaloid  will 
be  found  to  have  been  neutralized  by  the  hydrochloric  acid  produced 
in  the  decomposition  of  the  chloroform.  Since  the  decomposition  of 

*  Schweiz.  Wochenschr.  f.  Pharm.,  44,  580  (Chem.  Centralbl.,  1906,  2,  1212. 


ASSAY    OF    COCA    LEAVES  537 

0.0229  gm.  chloroform  will  produce  sufficient  hydrochloric  acid  to 
neutralize  0.120  gm.  alkaloids,  the  error  liable  to  be  introduced  thereby 
in  the  volumetric  estimation  of  cinchona  alkaloids  is  obvious.  Pan- 
chaud  therefore  cautions  that  any  solution  of  cinchona  alkaloids  which 
contains  chloroform  must  be  evaporated  at  once. 

A.  Limmer,  at  the  University  of  Strasburg,  has  studied  this  ques- 
tion and  has  not  confirmed  Panchaud's  results.  While  he  found  that 
a  number  of  alkaloids  do  decompose  chloroform  with  formation  of 
chlorid,  the  amount  so  decomposed  is  slight.  There  is,  nevertheless, 
some  danger  of  error  from  this  decomposition  in  the  estimation  of 
alkaloids,  and  it  should  be  borne  in  mind. 

LITERATURE 

De  Vrij.     Ph.  Jour,  and  Trans.,  1875,  461. 

De  Vrij.     The  Hague,  July  5,  1880. 

J.  Muter.     Analyst,  1880,  223. 

A.  Petit.     Chem.  and  Drug.,  1884. 

G.  Shimoyama.     Arch.  d.  Ph.  (1885),  81-209. 

E.  Landrin.     Compt.  rend.,  108,  750. 

W.  Haubensak.     Schweiz.  Wochenschr.  (1891),  147. 

C.  Kursteiner.     Schweiz.  Wochenschr.  (1892),  xxx,  473. 

W.  Lenz.  Ph.  Ztg.,  Sept.  24  (1898),  683,  in  which  chloral  hydrate  is 
used  as  a  solvent. 

E.  R.  Squibb.  A.  J.  Ph.  (1899),  312,  in  which  acetic  acid  is  used  as  a 
menstruum. 

Raymond  High.     Am.  Drug.  (1899),  354. 

Sidney  C.  Gadd.     Ph.  Jour.  (4),  21,  134. 

J.  M.  Francis.     Bull.  Ph.  (1005),  364. 

Vigeron.     J.  Ph.  Chim.  (6),  21,  180. 

P.  W.  Roberts.    Proc.  Chem.  Soc.,  21,  242. 

W.  Duncan.     Ph.  Jour.  (4),  20,  437. 

ASSAY  OF  COCA  LEAVES 

The  principal  active  alkaloid  of  this  drug  is  cocaine,  which  may 
be  readily  extracted  by  means  of  the  ordinary  solvents,  especially 
ether,  petroleum-ether  (benzin),  and  also  kerosene.  This  alkaloid  is 
very  unstable  and  is  accompanied  by  other  kindred  alkaloids  which 
may,  however,  be  converted  into  ordinary  cocaine.  In  assaying  coca 
and  its  preparations,  it  is  not  absolutely  necessary  to  determine  the 
quantity  of  cocaine.  It  is  usually  considered  sufficiently  accurate  to 
estimate  the  ether-soluble  (or  benzin-soluble)  alkaloids. 

For  the  extraction  Keller's  process  may  be  employed  with  very 
satisfactory  results.  Quite  as  good  results  may  be  obtained  by  the 


538  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

use  of  petroleum-benzin,  with  a  boiling-point  of  about  80°  C.  The 
kerosene  process  of  Dr.  Squibb  is  also  a  good  one. 

The  U.  S.  P.  VIII  Process.  Place  10  gms.  of  the  coca  leaves  in  an 
Erlenmeyer  flask,  add  50  cc.  of  a  mixture  of  chloroform  i  volume,  and 
ether  four  volumes,  and  insert  the  stopper  securely.  Allow  the  flask  to 
stand  ten  minutes,  then  add  2  cc.  of  ammonia-water  mixed  with  3  cc. 
of  distilled  water,  and  shake  the  flask  well,  at  frequent  intervals,  during 
one  hour.  Then  transfer  as  much  as  possible  of  the  contents  of  the 
flask  to  a  small  percolator  which  has  been  provided  with  a  pledget 
of  cotton  packed  firmly  in  the  neck,  and  inserted  in  a  separator  con- 
taining 6  cc.  of  normal  sulphuric  acid  V.  S.,  diluted  with  20  cc.  of 
distilled  water.  When  the  liquid  has  passed  through  the  cotton,  pack 
the  coca  firmly  in  the  percolator  with  the  aid  of  a  glass  rod,  and,  having 
rinsed  the  flask  with  10  cc.  of  chloroform-ether  mixture,  transfer  the 
remaining  contents  of  the  flask  to  the  percolator  by  the  aid  of  several 
small  portions  (5  cc.)  of  a  chloroform-ether  mixture,  using  the  same 
proportions  as  before,  and  continue  the  percolation  with  successive 
small  portions  of  the  same  liquid  (in  all  50  cc.).  Next,  shake  the  sepa- 
rator well  for  one  minute,  after  securely  inserting  the  stopper,  and 
when  the  liquids  have  completely  separated,  draw  off  the  acid  liquid 
into  another  separator.  Add  to  the  chloroform-ether  mixture  10  cc. 
of  a  sulphuric  acid  mixture,  using  the  same  proportions  as  before, 
agitate  well,  and  again  draw  off  the  acid  liquid.  Repeat  this  opera- 
tion once  more,  drawing  off  the  acid  solution  as  before  into  the  second 
separator,  introduce  a  small  piece  of  red  litmus  paper,  add  ammonia- 
water  until  the  liquid  is  distinctly  alkaline,  and  shake  out  with  three 
successive  portions  of  ether  (25,  20,  and  15  cc.).  Collect  the  ether 
solutions  in  a  beaker,  place  it  on  a  water-bath  filled  with  warm  water, 
and  allow  the  ether  to  evaporate  entirely.  Dissolve  the  residue  in 
3  cc.  of  ether,  and  let  this  also  evaporate  completely.  To  the  alka- 
loidal  residue  add  4  cc.  of  tenth-normal  sulphuric  acid  V.  S.  and  five 
drops  of  haematoxylin  or  iodeosin  T.  S.,  then  titrate  the  excess  of 
acid  with  fiftieth-normal  potassium  hydroxid  V.  S.  Divide  the  number 
of  cc.  of  fiftieth-normal  potassium  hydroxid  V.  S.  used  by  5,  subtract 
this  number  from  4  (the  4  cc.  of  tenth-normal  sulphuric  acid  V.  S. 
taken),  and  multiply  the  remainder  by  0.03  and  this  product  by  10, 
to  obtain  the  percentage  of  ether-soluble  alkaloids  contained  in  the 
coca. 

Among  other  methods  are  Kebler's  modification  of  Keller's  method. 
(See  page  517.) 

Lyons1  Process  No.  I  (Lyons*  "Assay  of  Drugs  ").  Into  a  four- 
ounce  prescription  vial  put  10  gms.  of  the  drug  in  moderately  fine 
powder.  Pour  in  100  cc.  of  Prollius  fluid,  cork  securely  and  shake 
very  frequently  during  four  or  five  hours.  Then  draw  off  50  cc.  of 


ASSAY    OF    COCA    LEAVES  539 

the  clear  liquid  (representing  5  gms.  of  the  drug)  and  evaporate  it  to 
dryness.  The  residue  is  taken  up  with  acid-free  ether,  drained  off, 
and  again  evaporated  to  dryness.  Once  more  10  cc.  of  pure  ether 

N 
are  added  together  with  5  cc.  of  —  acid  V.  S.,  the  ether  evaporated 

off,  and  after  adding  an  indicator,  the  excess  of  acid  is  retitrated  with 

N 

—  alkali.    The  quantity  of  alkali  used  deducted  from  the  5  cc.  gives 

20  N 

the  quantity  of  —  acid  V.  S.  which  combined  with  the  alkaloid.    This, 

multiplied  by  the  factor  and  then  by  2,  gives  the  quantity  of  alkaloid 

N 
present  in  the  10  gms.  taken.    The  factor  for  cocaine  when  —  V.  S. 

is  used  is  0.01565  gm. 

The  Squibb  Kerosene  Process  as  Modified  by  Lamar.*  This 
process,  which,  according  to  several  experimenters,  is  quite  satisfactory, 
is  as  follows : 

Place  25  gms.  of  the  powdered  leaf  into  a  covered  jar  of  about 
450  cc.  capacity,  add  25  cc.  of  an  approximatelv  2  per  cent  ammonia 
solution  (Squibb  uses  7  per  cent  solution  of  crystallized  sodium  carbo- 
nate), and  mix  well  together  by  means  of  a  stout  glass  rod  of  such 
a  length  that,  while  in  the  jar,  will  allow  the  cover  to  rest  in  its  normal 
position.  Permit  this  to  macerate  for  half  an  hour,  covered,  stirring 
from  time  to  time.  At  the  end  of  this  time,  the  odor  of  ammonia 
being  still  perceptible,  add  gradually,  with  stirring,  75  cc.  of  kerosene 
oil.  Then  cover  the  jar  and  set  aside  for  an  hour  or  more,  stirring 
frequently,  after  which  transfer  to  a  cylindrical  percolator  of  500  cc. 
capacity  and  percolate  with  the  oil  at  the  rate  of  six  or  eight  drops  per 
minute,  collecting  about  450  cc.  of  percolate.  Transfer  this  percolate 
to  a  separatory  funnel  of  700  to  750  cc.  capacity  (Squibb 's  pattern) 

N 

and  add  25  cc.  of  —  hydrochloric  acid  and  shake  continually  for  ten 
10 

minutes.  Allow  the  separator  to  rest,  and  when  the  liquids  have 
separated  draw  off  the  acid  liquid  into  another  separator  of  265  to 
285  cc.  capacity.  Then  shake  the  oil  in  the  separator  with  two  further 

N 
portions  of  25  cc.  each  of  —  hydrochloric  acid,  mixing  the  three  acid 

solutions  so  collected.  To  the  united  acid  solutions,  now  add  20  cc. 
of  ether  and  shake  well.  When  the  liquids  have  separated,  the  acid 
solution  is  drawn  off  into  another  separator  and  15  cc.  of  ether  added 


*  See  Squibbs'  Ephemeris,   III,   1104;    and  William   R.   Lamar,   A.   J.   Ph. 
(1901)*!  25. 


540  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

to  it,  the  mixture  shaken,  and  the  separated  acid  solution  again  drawn 
off  into  a  third  separator,  thus  removing  the  last  traces  of  kerosene 
oil  and  coloring  matter.  The  ether  remaining  in  the  first  separator 
is  shaken  successively  with  two  portions  of  water,  of  5  cc.  each,  after 
separation  has  taken  place;  these  in  their  turn  are  added  to  the  second 
ether  washing,  and  after  separating,  drawn  off  into  the  third  separator, 
containing  the  major  portion  of  the  acid  solution.  To  this  is  then 
added  a  sufficient  quantity  of  10  per  cent  ammonia-water  diluted  with 
four  times  its  volume  of  water  to  render  the  liquid  slightly  alkaline. 
Now  extract  the  alkaloids  with  three  successive  portions  of  ether, 
using  40,  30,  and  30  cc.,  taking  care  in  each  instance  to  allow  the 
ether  to  separate  completely,  drawing  off  the  aqueous  liquid  carefully 
into  another  separator  and  pouring  the  ethereal  solution  of  the  alka- 
loids out  through  the  upper  opening  of  the  separator  into  a  tared 
beaker  of  160  cc.  capacity. 

Rinse  the  separator  with  10  cc.  of  ether,  pouring  it  out  at  the  top, 
into  the  separator  containing  the  aqueous  portion.  Now  rinse  the 
rim  of  the  separator  and  its  cork  stopper  with  ether  in  such  a  manner 
as  to  cause  the  ether  to  flow  into  the  separator  containing  the  aqueous 
portion,  and  finally  add  more  ether  to  make  in  all  30  cc.  for  the  second 
extraction.  Shake  the  separator  with  its  contents  actively  for  a  few 
minutes,  then  allow  the  liquids  to  separate,  drawing  off  the  aqueous 
portion  into  the  separator  previously  emptied,  the  ethereal  layer  being 
added  to  that  already  in  the  beaker.  This  operation  is  repeated  a 
third  time.  The  beaker  containing  the  ethereal  solution  of  the  alka- 
loids is  set  in  a  warm  place  (30-35°  C.),  and  as  soon  as  the  ether 
has  evaporated,  it  is  dried  at  a  temperature  of  60°  C.  until  of  a  constant 
weight,  this  usually  requiring  about  three  hours.  The  weight  obtained, 
multiplied  by  4,  expresses  the  percentage  of  alkaloids  in  the 
leaf. 

The  alkaloids  so  obtained  are  almost  colorless,  possessing  only  a 
faint  cream  tint,  and  are  beautifully  crystalline  in  appearance.  If 
it  is  desirable,  as  a  check  upon  the  weight,  they  may  be  titrated,  using 

N 

an  excess  of  —  sulphuric  acid  (about  25  cc.)  and  a  few  cc.  of  ether 
20 

to  facilitate  the  solution,  and  after  the  ether  has  been  entirely  dis- 

N 
sipated,  the  excess  of  acid  is  determined  by  means  of  —  potassium 

hydroxid,  using  cochineal  tincture  as  indicator. 

The   factor  for  the  pure  alkaloids  as  determined   by  numerous 

N 
assays  is  0.01514  gm.  as  the  equivalent  of  i  cc.  of  —  sulphuric  acid. 

The  extremes  are  0.01493  and  0.0155  gm.  * 


ASSAY    OF    COCA    LEAVES  541 

Lager's  Process.*  Having  determined  the  amount  of  moisture 
in  a  small  amount  of  the  powdered  leaves,  a  quantity  of  the  same 
powder  equivalent  to  25  gms.  of  the  dried  leaves  is  intimately  mixed 
in  a  mortar  with  5  gms.  of  magnesia  and  15  cc.  of  distilled  water. 
The  mixture  is  introduced  into  a  i -liter  wide-mouthed,  glass -stoppered 
flask,  and  treated  with  625  cc.  of  ether  (sp.gr.  0.721),  saturated  with 
water.  The  flask  is  then  stoppered,  tied  down  with  a  piece  of  cloth, 
well  shaken  up,  and  set  aside  for  twelve  hours,  with  frequent  agitation. 
The  whole  is  then  shaken  up,  transferred  to  a  filter,  the  filtrate  col- 
lected in  a  5OO-CC.  graduated  flask,  the  funnel  being  covered  with  a 
glass  plate  during  filtration.  The  500  cc.  of  filtrate  thus  collected, 
equivalent  to  20  gms.  of  dry  powder,  is  distilled  in  several  portions 
from  a  dry  250-0:,  flask  by  plunging  the  latter  in  warm  water.  The 

N 

green  residue  is  dissolved  in  20  cc.  of  neutral  ether,  10  cc.  of  —  hydro- 
chloric acid,  and  20  cc.  of  water  are  added,  the  flask  closed  with  a 
rubber  stopper,  and  agitated.  The  whole  contents  are  then  trans- 
ferred to  a  separator,  and  the  acid  liquid,  after  separation,  withdrawn 
into  a  conical  flask.  The  ether  layer  is  then  twice  shaken  out  with 
25  cc.  of  distilled  water,  these  washings  being  added  to  the  acid  liquid 
in  the  flask.  This  acid  solution  is  filtered  through  a  moistened  double 
filter  into  a  wide-mouthed  glass -stoppered  5oo-cc.  flask,  and  the  filter 
thoroughly  washed  through  into  the  same.  Sufficient  distilled  water  is 
added  to  make  up  the  volume  to  150  cc.,  when  sufficient  neutral  ether 
to  give  a  layer  i  cm.  deep  is  added.  Five  or  six  drops  of  0.2  per  cent 
iodeosin  solution  are  then  added,  and  the  amount  of  free  acid  titrated 

N 
back  in  the  usual  manner  with  —  potassium  hydroxid  solution.     The 

number  of  cc.  of  acid  thus  found  to  be  combined  with  the  coca  alka- 
loids, multiplied  by  0.1535,  gives  the  percentage  of  these  in  the 
powder. 

LITERATURE  ON  THE  ASSAY  OF  COCA 

Koehler.    A.  J.  Ph.,  1888,  238. 

Van  der  Marck.     Ph.  Ztg.  (1889),  39,  282. 

Schwickerath.     Ph.  Runds.,  1893,  282- 

A.  Gunn.    J.  Ph.  u.  Chem.,  1893,  99>  X52' 

K.  De  Jong.     Ph.  Ztg.,  No.  37  (1905),  991. 

E.  R.  Squibb.     Ephemeris,  Vol.  in,  noi. 

Bloementhal.     Ph.  Ztg.,  No.  47  (1005),  497. 

H.  Matthes  and  O.  Ramstedt.     Ph.  Ztg.  (1906),  LI,  1031. 

*  E.  Leger,  J.  Ph.  Chim.  (6),  19,  334. 


542  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 


ASSAY  OF  COLCHICUM 

Owing  to  the  ready  decomposition  of  the  alkaloid  colchicine  (the 
active  principle  of  the  root  and  seeds  of  Colchicum  aulumnale)  by 
both  acids  and  alkalies,  its  extraction  by  immiscible  solvents,  such  as 
Prollius'  fluid,  etc.,  is  not  advisable.  This,  together  with  the  presence 
of  oily  matter  in  the  drug,  makes  the  assay  of  colchicum  a  matter  of 
considerable  difficulty.  The  drug  is  best  exhausted  by  hot  extraction 
with  alcohol,  and  the  following  method,  which  depends  upon  the 
saponification  of  the  alkaloid  and  retitration  with  acid,  may  be  used. 

Method  of  Gordin  and  Prescott  (Proc.  A.  Ph.  A.,  1900,  133). 
25  gms.  of  the  powdered  drug  are  exhausted  in  a  Soxhlet  extraction 
apparatus  with  strong  alcohol  for  two  hours.  The  alcohol  is  then 
distilled  off  completely,  the  oily  residue  taken  up  with  about  10  cc. 
of  hot  water,  and  the  mixture  poured  into  a  separating  funnel.  The 
extraction  apparatus  is  washed  several  times  with  hot  water  and  the 
washings  added  to  the  mixture  in  the  separating  funnel;  203  cc.  of 
petroleum  ether  are  now  poured  carefully  into  the  separating  funnel 
and  the  latter  inclined  in  different  directions  without  shaking.  The 
petroleum  ether  then  takes  up  the  oily  drops  adhering  to  the  sides  of 
the  vessel.  In  fifteen  or  twenty  minutes  the  oil  will  be  found  floating 
on  the  surface  of  the  aqueous  liquid.  The  lower  aqueous  liquid  is 
run  off  into  a  loo-cc,  measuring-flask. 

In  order  to  completely  remove  any  colchicine  which  may  still  be  in 
the  oil,  about  10  cc.  of  water  are  added  to  the  contents  of  the  separator, 
the  latter  shaken  vigorously,  and  its  contents  drawn  off  into  an  evaporating- 
dish.  The  petroleum  ether  is  then  evaporated  off  by  heating  on  a  water- 
bath.  This  will  leave  a  clear  aqueous  liquid  with  an  oily  layer  on  its  sur- 
face. This  is  returned  to  the  separator  and  treated  as  before  with  2  or  3  cc. 
of  petroleum  ether,  and  the  aqueous  liquid  drawn  off  into  the  measuring- 
flask.  The  washing  of  the  oil  with  water  may  be  repeated  until  no  trace 
of  colchicine  is  left  in  the  oil,  as  shown  by  acidulating  the  aqueous  liquid 
drawn  off  and  treating  with  Mayer's  or  Wagner's  reagent. 

The  turbid  liquid  in  the  flask  is  now  diluted  to  100  cc.,  i  or  2  gms. 
of  purified  talcum  added,  the  flask  thoroughly  shaken,  and  the  liquid 
filtered  through  a  dry  filter. 

80  cc.  of  the  clear  filtrate  (representing  20  gms.  of  the  drug)  are 
measured  off  into  a  separator  and  shaken  out  three  times  with  chloro- 
form, using  about  30  cc.  each  time.  The  chloroformic  solutions  of 
the  alkaloid  are  filtered  into  a  flask  and  the  chloroform  distilled  off. 
The  residue  of  colchicine  left  in  the  flask  is  treated  with  10  cc.  of  water, 
and  while  the  flask  is  kept  on  a  boiling  water-bath  air  is  drawn  over 
its  surface  by  means  of  a  pump  for  about  half  an  hour.  The  colchicine 


ASSAY    OF    COLCHICUM  543 

can  now  be  estimated  either  gravimetrically  or  by  saponification  or 
by  both  methods  in  succession,  using  one  method  as  a  check  upon 

N 
the  other.     For  the  saponification  method  40  cc.  —  KOH  V.  S.  are 

added  to  the  solution  of  the  colchicine,  the  flask  connected  with  a 
reversed  condenser,  and  the  liquid  boiled  briskly  on  a  wire  gauze  for 
two  hours,  then  cooled,  the  liquid  diluted  to  about  100  cc.,  and  the 

N 

excess  of  KOH  titrated  back  with  —  HC1  V.  S.,  using  phenolphthalein 

40 

N 
as  indicator.     The  number  of  cc.  of  —  HC1  deducted  from  40  gives 

N  4° 

the  quantity  of  --  KOH,  which  was  consumed  in  the  saponification. 
40 

This  divided  by  20  gives  the  percentage  of  colchicine  in  the  drug. 

N 

Assuming  that  12.2  cc.  of  the  —  KOH  were  consumed,  then  the 

40 

20  gms.  contained  0.61  per  cent  of  colchicine. 

N 

The  —  factor  is  about  0.0099275  gm. 
40 

Panchaud's  Method.*  Panchaud  has  determined  the  solubility 
of  colchicine  in  mixtures  of  chloroform,  ether,  and  petroleum-ether. 
He  finds  that  petroleum-ether  having  a  boiling-point  of  50°  to  60°  C. 
must  be  used  in  the  assay  and  the  ether  must  be  completely  dehydrated 
over  metallic  sodium.  For  the  estimation  of  colchicine  he  directs  that 
15  gms.  coarsely  powdered'  colchicum  seeds  be  treated  in  a  flask  with 
150  gms.  chloroform,  the  mixture  shaken  frequently  during  thirty 
minutes,  then  6  cc.  10  per  cent  ammonia-water  added  and  the  mixture 
shaken  thoroughly.  After  occasional  shaking  during  one  half  hour, 
100  gms.  are  to  be  filtered  off  through  a  plain  filter  of  20  cm.  diameter 
into  a  200-cc.  Erlenmeyer  flask,  the  funnel  being  kept  covered.  The 
solution  is  distilled  to  complete  dryness  and  the  residue  dissolved  in 
i  gm.  dry  chloroform,  i  gm.  dry  ether  added,  and  then  30  gms.  dry 
petroleum-ether.  The  liquid  and  precipitate  is  transferred  to  a  plain 
filter  of  8  cm.  diameter,  using  further  petroleum-ether  to  complete 
the  transfer.  The  funnel  containing  the  precipitate  is  placed  on  an 
empty  flask  and  the  precipitate  dissolved  with  warm  chloroform,  care 
being  taken  that  it  is  completely  dissolved  by  washing  the  edge  of 
the  filter  with  chloroform.  The  chloroformic  solution  is  distilled  and 
the  residue  dissolved  in  15  drops  of  chloroform,  2  gms.  absolute  ether 
added  and,  after  solution,  30  cc.  dry  petroleum-ether.  The  liquid 


*Schweiz.  Wochenschr.  f.  Ch.  u.  Ph.,   1906,   564;    Ph.  Centrh.,  1907(48), 

75- 


544  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

and  precipitate  is  poured  on  a  tared,  plain  filter  of  8  cm.  diameter. 
Floccules  adhering  to  the  flask  are  dissolved  in  five  drops  of  chloroform, 
i  gm.  ether  added  and  then  10  gms.  dry  petroleum-ether;  the  mixture 
transferred  to  the  first  filter  and  the  precipitate  washed  with  a  little 
petroleum-ether.  The  weight  of  the  precipitate,  plus  0.0022  gm. 
(correction  for  solubility  of  colchicine  in  the  quantity  of  solvent  used), 
multiplied  by  10,  furnishes  the  colchicine  content  of  the  drug. 


ASSAY  OF  CONIUM 

The  active  principle  of  this  drug  is  the  volatile  alkaloid  coniine. 
Because  of  the  volatile  nature  of  this  alkaloid,  specimens  of  the  drug 
are  frequently  deficient  in  alkaloid  and  sometimes  entirely  devoid 
of  coniine.  The  volatile  character  of  this  alkaloid  makes  it  further- 
more necessary  to  exercise  great  care  in  its  estimation.  The  following 
method  of  A.  B.  Lyons,  and  which  is  also  the  pharmacopceial  methcd, 
is  a  practicable  one: 

Place  the  conium  in  a  2oo-cc.  Erlenmeyer  flask,  add  100  cc.  of  a 
mixture  of  ether  98  parts,  alcohol  8  parts,  and  ammonia-water  3  parts 
(by  volume),  insert  the  stopper  securely,  and  shake  the  flask  at  intervals 
during  four  hours.  After  the  powder  has  settled,  decant  50  cc.  of 
the  clear  liquid  into  a  beaker  (representing  5  gms.  of  conium),  and 
add  sufficient  normal  sulphuric  acid  V.  S.  to  produce  a  distinctly  acid 
reaction.  Evaporate  the  ether  at  a  gentle  heat  by  the  aid  of  a  water- 
bath;  then  add  15  cc.  of  alcohol,  and  set  the  beaker  aside  in  a  cool 
place  for  two  hours  to  allow  the  ammonium  sulphate  to  deposit. 
Filter  the  liquid;  wash  the  residue  and  filter  with  a  little  alcohol,  and 
add  the  washings  to  the  filtrate;  neutralize  any  excessive  amount  of 
acid  with  sodium  carbonate  T.  S.,  being  careful  to  retain  a  slight 
acidity.  Concentrate  the  liquid  to  3  cc.  by  the  aid  of  a  gentle  heat 
on  a  water-bath,  add  3  cc.  of  distilled  water  and  two  drops  of  normal 
sulphuric  acid  V.  S.  Add  15  cc.  of  ether  to  remove  traces  of  fatty 
matter,  pour  off  the  ether  solution  and  repeat  the  washing  with  15  cc. 
of  ether.  Then  transfer  the  acid  liquid  to  a  separator,  introduce  a 
small  piece  of  red  litmus  paper,  and  add  sufficient  sodium  carbonate 
T.  S.  to  render  the  liquid  slightly  alkaline;  then  shake  out  with  succes- 
sive portions  of  15,  15,  and  10  cc.  of  ether.  The  assay  is  then  finished 
gravimetrically,  according  to  the  U.  S.  P.,  by  following  (I)  or,  volu- 
metrically,  by  following  directions  under  (II). 

(I)  To  the  combined  ether  solutions,  in  a  tared  beaker,  add,  drop 
by  drop,  sufficient  hydrochloric  acid  solution  (5  per  cent)  to  insure 
an  excess  of  acid,  and  then  evaporate  the  ether  by  a  gentle  heat  on  a 
water-bath.  Remove  the  excess  of  hydrochloric  acid  by  adding  to 


ASSAY    OF   HYDRASTIS  CAN  ADEN  SIS  545 

the  residue  3  cc.  of  alcohol  and  heating  gently  to  evaporate  the  liquid, 
repeat  this  operation  once,  and  dry  the  residue  (coniine  hydrochlorid) 
at  a  temperature  not  exceeding  60°  C.  (140°  F.)  until  the  weight, 
after  cooling  in  a  desiccator,  remains  constant.  The  weight  of  the 
residue  multiplied  by  0.777,  and  this  product  by  20,  gives  the  per- 
centage of  coniine  contained  in  the  conium. 

(II)  Treat  the  ethereal  solution  with  exsiccated  calcium  sulphate 
(neutral)  to  remove  minute  droplets  of  alkaline  water,  and  then  titrate 

N         N 

with  standard  acid  —  or  —  sulphuric  acid.     lodeosin   is   the  indi- 
20         10 

N 
cat  or.     Each  cc.  of  —  acid  represents  0.01262  gm.  of  coniine. 

A.  B.  Lyons,  in  his  "Handbook  of  Assaying  of  Drugs  and  Galeni- 
cals," gives  an  alternative  process  in  which  the  drug  is  moistened 
with  an  equal  weight  of  5  per  cent  crystallized  sodium  carbonate 
solution,  and  then  macerated  for  four  hours  with  petroleum-benzin, 
or  percolated  with  this  menstruum.  Then,  in  an  aliquot  portion  of 
this  percolate,  the  alkaloid  is  extracted  by  shaking  out  with  acidulated 
water,  the  assay  being  then  completed  as  directed  in  the  preced- 
ing process.  Other  methods  are  proposed  by  Schwickerath  (Ph. 
Record,  1893,  282)  and  R.  A.  Cripps  (Ph.  Jour.  Trans.  (3),  18, 
pp.  12,  54,  820  and  888);  J.  v.  Braun  (Berichte  d.  Chem.  Ges.,  38, 
3108);  W.  A.  H.  Naylor  (Brit.  Col.  Drug.,  48,  77);  T.  Maben 
(Brit.  Col.  Drug.,  48,  83). 


ASSAY  OF  HYDRASTIS  CANADENSIS 

This  drug  contains  two  principle  alkaloids,  namely,  hydrastine 
(C2iH2iNO6)  and  berberine  (C2oH17NO4).  The  former,  a  colorless 
crystalline  alkaloid,  is  the  characteristic  alkaloid  of  the  drug  and  is 
recognized  as  the  true  active  principle  of  hydrastis  canadensis.  The 
other  is  a  deep  yellow  alkaloid  and  is  the  one  to  which  the  drug  owes 
its  bitterness  and  its  color.  This  alkaloid  is  found  in  many  other 
plants  and  is  therefore  not  characteristic  of  hydrastis.  Hydrastine  is 
freely  soluble  in  ether  and  is  also  soluble  in  petroleum-benzin  and  the 
usual  alkaloidal  solvents.  It  is  very  feebly  alkaline.  Berberine,  on 
the  other  hand,  is  practically  insoluble  in  ether,  but  is  soluble  in  chloro- 
form and  forms  crystallizable  salts  which  are  mostly  insoluble  in  water, 
particularly  in  the  presence  of  acids.  In  all  assays  of  this  drug  its 
therapeutic  value  is  based  upon  the  quantity  of  hydrastine  present. 
The  most  satisfactory  assay  methods  for  this  drug  are  gravimetric. 
Hydrastine  being  so  feebly  alkaline,  alkalimetric  methods  are  not  satis- 
factory as  a  rule.  Lyons,  in  his  "  Assaying  of  Drugs  and  Galenicals," 


546  A     MANUAL    OF    VOLUMETRIC    ANALYSIS 

1899,  page  177,  asserts  that  he  finds  no  difficulty  in  the  titration,  using 
hsematoxylin,  cochineal,  or  methyl-orange.  A  residual  titration  may 
be  made,  indeed,  in  a  sufficiently  concentrated  aqueous  solution  with- 
out using  any  indicator,  owing  to  the  sparing  solubility  of  the  alkaloid. 
The  U.  S.  P.  Method.  Introduce  15  gms.  of  the  hydrastis  (in 
No.  60  powder)  into  an  Erlenmeyer  flask  of  250  cc.  capacity,  add 
150  cc.  of  ether,  shake  the  flask  during  ten  minutes,  and  add  5  cc. 
of  ammonia-water,  again  shaking  the  flask  at  intervals  during  half  an 
hour.  Then  add  15  cc.  of  distilled  water  to  the  mixture  in  the  flask 
and  shake  it  until  the  drug  collects  in  masses,  and  at  once  pour  off, 
into  a  measuring-cylinder,  100  cc.  of  the  supernatant  ether  solution 
and  transfer  it  to  a  separator.  Add  15  cc.  of  normal  sulphuric  acid 
V.  S.  to  the  separator,  and  shake  it  moderately  during  one  minute. 
Allow  the  liquids  to  separate,  and  draw  off  the  lower  acid  liquid  into 
a  second  separator.  Again  shake  out  the  ether  solution  with  5  cc. 
of  normal  sulphuric  acid  V.  S.  and  5  cc.  of  distilled  water,  and  shake 
the  separator  for  one  minute.  After  the  liquids  have  separated,  draw 
off  the  acid  solution  as  before  into  the  second  separator.  Repeat  the 
same  process  with  5  cc.  of  distilled  water,  drawing  this  also  into  the 
second  separator.  Introduce  a  small  piece  of  red  litmus  paper  into  the 
second  separator,  add  enough  ammonia-water  to  render  the  liquid 
alkaline,  and  then  25  cc.  of  ether,  and  shake  the  separator  moderately 
during  one  minute,  and  when  the  liquids  have  separated  draw  off  the 
lower  alkaline  liquid  into  another  separator,  and  the  ether  solution 
into  a  tared  beaker.  Again  shake  out  the  alkaline  liquid,  using  20  cc. 
of  ether,  shake  the  separator  for  one  minute,  and  when  the  liquids 
have  separated,  draw  off  the  alkaline  liquid  into  the  other  separator, 
and  the  ether  solution  into  the  tared  beaker.  Finally,  again  shake 
out  the  alkaline  liquid,  using  15  cc.  of  ether,  proceeding  as  before, 
and  adding  the  ether  solution  to  the  liquid  in  the  tared  beaker. 
Evaporate  the  ether  carefully  with  the  aid  of  a  water-bath,  and 
dry  the  alkaloidal  residue  in  the  beaker  to  a  constant  weight  at  100°  C. 
(212°  F.).  The  weight  found,  multiplied  by  10,  will  give  the  percentage 
of  hydrastine  in  the  hydrastis. 

Puckner  *  gives  the  following  modification  of  the  foregoing: 
The  modified  assay  process  is  carried  out  as  follows:  To  5  gms. 
drug  in  No.  60  powder  and  contained  in  an  Erlenmeyer  flask,  50  cc. 
ether  is  added,  and  the  mixture  rotated  occasionally  during  ten 
minutes.  Then  ammonia-water,  2  cc.,  is  added,  the  mixture  shaken 
thoroughly  and  frequently  during  one-half  hour,  and  then  transferred 
to  a  percolator.  The  percolate  is  received  in  a  separator  and  the 

*  Ph.  Rev.,  1908,  136. 


ASSAY   OF    HYDRAST1S    CANADENSIS  547 

drug  extracted  with  a  further  quantity  of  ether,  50  cc.  The  ethereal 
liquid  is  extracted  successively  with  a  mixture  of  dilute  hydrochloric 
acid,  2  cc.,  and  water,  18  cc.  with  a  mixture  of  dilute  hydrochloric 
acid,  5  drops,  and  water,  10  cc.,  and  finally  with  water,  10  cc.  The 
united  extractions  are  rendered  alkaline  with  ammonia-water,  using 
litmus  test  solution  as  an  indicator  and  extracted  with  three  portions 
of  ether,  20  cc.  each,  using  three  separators.  The  united  ethereal 
solutions  are  evaporated  at  a'  moderate  temperature,  and  the  residue 
dried  at  98°  to  100°  until  it  ceases  to  lose  in  weight. 

The  Method  of  Gordin  and  Prescott.*  This  method,  which 
estimates  both  hydrastine  and  berberine,  is  based  upon  the  following 
plan:  The  alkaloids  of  the  powdered  root  are  first  set  free  by  the 
action  of  a  mixture  of  ammonia  and  ether  (stronger  ammonia-water 
5  cc.,  alcohol  5  cc.,  ether  30  cc.).  After  drying  the  powder  is  extracted 
with  absolute  ether,  and  the  ethereal  extract,  after  evaporation  of  the 
ether  and  taking  up  of  the  residue  with  acidulated  water,  is  used  for 
the  estimation  of  hydrastine.  Through  the  powdered  root  left  in  the 
extraction  apparatus,  air  is  passed  till  it  is  dry,  and  then  the  powder 
is  extracted  with  alcohol  to  exhaustion.  The  alcoholic  extract,  after 
dilution  with  water,  evaporation  of  the  alcohol  and  taking  up  the 
residue  with  dilute  acetic  acid,  is  used  for  the  estimation  of  berberine. 
The  berberine  is  first  precipitated  as  berberine  acetone,  the  latter 
washed,  decomposed  by  the  aid  of  acid,  and  the  purified  berberine 
estimated  by  standard  solution  of  potassium  iodid,  silver  nitrate,  and 
ammonium  thiocyanate. 

The  Assay.  10  gms.  of  the  finely  powdered  hydrastis  are  rubbed 
up  to  a  paste  with  a  few  cc.  of  the  above-mentioned  ammonia-ether 
mixture  in  an  eight-ounce  ointment  jar,  and  a  few  cc.  more  of  the 
same  mixture  are  then  added  so  as  to  have  the  powder  well  covered 
with  the  liquid.  The  small  pestle  is  then  left  inside, 'and  the  jar  well 
covered  and  set  aside  over  night.  The  jar  is  then  opened,  put  into  a 
good  current  of  air  till  the  odor  of  ammonia  has  disappeared,  and 
then  in  a  vacuum  over  sulphuric  acid  for  about  five  or  six  hours;  the 
powder  is  then  inclosed  in  filter -paper,  placed  in  a  Soxhlet  extraction 
apparatus,  the  jar  rinsed  out  several  times  with  powdered  glass  or 
with  coarsely  powdered  barium  nitrate,  the  rinsings  added  to  the 
Soxhlet,  the  latter  connected  with  an  Erlenmeyer  flask  containing 
about  40  or  50  cc.  of  absolute  ether,  and  the  extraction  conducted  in 
the  usual  way,  till  a  few  drops,  after  evaporation  of  the  ether  and 
acidulation,  give  no  reaction  with  Mayer's  or  with  Wagner's  solution. 

The   Erlenmeyer  is  then  detached  from  the   Soxhlet,   the  ether 

*  J.  A.  C.  S.,  1899,  735. 


548  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

poured  out  into  a  flat  evaporating-dish,  the  Erlenmeyer  washed  out 
several  times  with  water  containing  about  2  per  cent  sulphuric  acid, 
the  washings  added  to  the  contents  of  the  evaporating-dish,  and  the 
latter  put  into  a  draught  at  about  30°  C.  till  the  ether  has  disappeared. 

The  contents  of  the  dish  are  poured  into  a  loo-cc.  flask,  the  dish 
washed,  the  washings  added  into  the  flask  and  the  latter  filled  up 
to  the  loo-cc.  mark.  The  solution  containing  hydrastine  sulphate, 
of  which  every  10  cc.  represent  i  gm.  of  the  root,  is  used  for  the  esti- 
mation of  hydrastine. 

For  the  iodometric  estimation  20  cc.  of  the  filtered  solution  (repre- 
senting 2  gms.  of  the  drug)  are  run  from  a  burette  into  a  loo-cc.  flask 
containing  20  or  30  cc.  of  a  standardized  solution  of  iodin  of  any 
known  strength  (that  in  the  neighborhood  of  one  per  cent  is  the  best) 
and  the  analysis  carried  out  exactly  as  described  on  page  507.  From 
the  amount  of  iodin  consumed  the  amount  of  hydrastine  is  deduced 
by  using  the  factor  of  the  hydrastine  hexaiodid,  i.e.,  0.60403  gm.  of 
hydrastine  for  one  of  iodin  consumed. 

For  the  estimation  of  berberine  a  current  of  dry  air  is  passed  through 
the  Soxhlet  till  all  the  ether  is  removed,  the  Soxhlet  connected  with 
an  Erlenmeyer  containing  40  or  50  cc.  of  alcohol,  and  the  extraction 
continued  until  the  alcohol  comes  out  colorless.  The  alcoholic  extract 
containing  the  berberine,  and  considerable  quantities  of  extractive 
matter,  is  poured  out  into  an  evaporating-dish,  the  Erlenmeyer  washed 
out  with  hot  water  and  a  little  dilute  acetic  acid,  the  washings  added 
to  the  evaporating-dish  and  the  latter  kept  on  a  water-bath,  adding 
water  from  time  to  time  till  all  the  alcohol  has  disappeared.  A  little 
more  diluted  acetic  acid  is  now  added,  the  dish  covered,  and  when 
completely  cold  its  contents  are  filtered  into  an  Erlenmeyer  having 
the  capacity  of  about  300  or  400  cc.* 

Six  to  eight  cc.  of  acetone  are  added  to  the  contents  of  the  Erlen- 
meyer, to  which  the  washings  of  the  dish  and  the  filter  have  been 
added,  and  then  a  10  per  cent  solution  of  sodium  hydroxid  is  added, 
drop  by  drop,  till  the  precipitate  first  formed  ceases  to  disappear  on 
shaking,  and  the  liquid  acquires  a  strongly  alkaline  reaction.  The 
Erlenmeyer  is  then  stoppered  and  shaken  in  a  circular  direction  for 
about  ten  or  fifteen  minutes,  and  then  set  aside  in  a  cool  place  for 

*  In  the  remaining  procedure,  the  simplest  way  would  be  to  precipitate  the 
berberine  with  hydrochloric  or  nitric  acid,  but  in  this  case  a  considerable 
amount  of  extractive  matter  contaminates  the  precipitate  and  too  high  a  yield 
would  result  though  the  error  in  this  respect  might  be  compensated  to  some 
extent  by  the  solubility  of  the  hydrochlorid  or  nitrate  in  water.  But  the  best  way 
is  to  purify  the  berberine  by  converting  it  into  berberine  acetone,  regenerate  the 
alkaloid  by  means  of  sulphuric  acid  and  then  estimate  it  volumetrically  by 
standard  potassium  iodid. 


ASSAY    OF    HYDRASTIS    CAN  ADEN  SIS  549 

two  or  three  hours.  The  berberine  acetone  separates  out  in  crystals, 
some  of  which  adhere  to  the  sides  of  the  vessel.  The  supernatant 
liquid  is  then  poured  on  a  small  filter,  the  precipitate  washed  once 
or  twice  by  decantation  and  then  on  the  filter  till  the  washings  are 
colorless.  The  filter  is  then  pierced  and,  by  means  of  the  wash-bottle, 
the  precipitate  is  returned  to  the  same  Erlenmeyer  in  which  the  pre- 
cipitation took  place.  In  this  way  all  loss  is  avoided.  To  the 
precipitate  about  4  or  5  cc.  of  a  5  per  cent  solution  of  sulphuric  acid 
is  now  added,  and  then  water  enough  to  make  about  100  or  200  cc. 
The  Erlenmeyer  is  now  put  into  hot  water  when  the  precipitate  will 
completely  dissolve  in  the  course  of  a  few  minutes.  The  solution  is 
now  poured  out  into  a  long-necked  flask,  washing  the  Erlenmeyer 
several  times,  the  flask  put  on  an  asbestos  plate  and  kept  very  gently 
boiling. for  about  an  hour  an  a  half  or  two  hours,  adding  hot  water  from 
time  to  time  if  necessary.  The  flask  is  now  cooled  and  its  contents 

N 
poured  out  into  a  liter  flask  containing  100  cc.  of  —  potassium  iodid 

solution.  The  flask  is  washed  several  times,  the  washings  added  to 
the  measuring-flask  and  the  latter  filled  up  to  one  liter  and  set  aside 
over  night.  500  cc.  are  now  filtered  off  into  another  liter  flask,  50  cc. 

of  —  silver  nitrate  and  nitric  acid  added  to  the  flask,  which  is  filled 
20 

up  to  one  liter,  well  shaken,  filtered,  and  500  cc.  of  the  liquid  titrated 

N 
back  with  —  ammonium  sulphocyanate,  using  ferric  alum  as  indicator. 

40 

Twice  the  number  of  cc.  of  sulphocyanate  solution  used  is  equal  to  the 
number  of  cc.  of  the  potassium  iodid  solution  consumed  by  the 
berberine,  representing  10  gms.  of  the  hydrastis  root.  By  multiplying 

N 
the  number  of  cc.  of  --  potassium  iodid  consumed  by  0.167125,  the 

per  cent  of  anhydrous  berberine  in  the  root  is  obtained,  as  i  cc.  of  the 
potassium  iodid  solution  is  equal  to  0.0167125  gm.  of  berberine. 

O.  Schreiber*  has  subjected  ten  samples  of  hydrastis  root,  as 
found  in  the  European  markets,  to  alkaloidal  assay  by  the  following 
method:  The  amount  of  moisture  having  been  determined  in  10  gms. 
of  the  powdered  sample  by  drying  to  constant  weight,  the  dried 
powder  was  moistened  with  a  mixture  of  ammonia,  5  cc.,  alcohol, 
5  cc.,  and  ether,  30  cc.,  and  dried.  It  was  then  extracted  in  a  Soxhlet 
with  ether;  the  ether  extract  shaken  out  with  15  gms.  of  5  per  cent 
hydrochloric  acid  in  a  graduated  cylinder.  The  ethereal  layer  was 
decanted,  the  acid  extract  washed  with  more  ether  to  remove  resinous 

*  Ph.  Post.,  1901,  36,  321. 


550  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

matter,  and  the  ether  decanted.  The  volume  of  ether  over  the  acid 
liquor  was  then  adjusted  to  exactly  50  cc.,  10  cc.  of  ammonia  added, 
and  the  whole  well  shaken  until  all  of  the  precipitated  alkaloid 
was  dissolved  in  the  ethereal  layer.  After  separation,  40  cc.  of  this 
was  decanted  (=£  of  the  whole),  into  a  tared  capsule,  about  half  the 
ether  evaporated  off  with  a  gentle  heat,  the  rest  allowed  to  evaporate 
spontaneously.  In  this  manner  almost  colorless  crystals  of  hydrastine 
were  obtained,  which  were  finally  dried  to  constant  weight  on  the 
water-bath.  The  poorest  sample  examined  was  thus  shown  to  contain 
2.85  per  cent,  and  the  best  4.16  per  cent  of  alkaloid. 


ASSAY  OF  IPECAC 

In  the  U.  S.  P.  VIII  ipecac  is  defined  as  "The  dried  root,  to 
which  may  be  attached  a  portion  of  the  stem  not  exceeding  7  cm.  in 
length,  of  Cephaelis  Ipecacuanha  (Brotero),  A.  Richard  (Fam.  Rubi- 
aca),  known  commercially  as  Rio,  Brazilian,  or  Para  ipecac,  or  the 
corresponding  portion  of  Cephaelis  Acuminata,  Karsten,  known  com- 
mercially as  Carthagena  ipecac,  yielding  when  assayed  by  the  process 
given  below,*  not  less  than  2  per  cent  of  'ipecac  alkaloids.'  '  Ipecac 
contains,  according  to  Paul  and  Cownley,  three  distinct  alkaloids, 
namely,  emetine,  cephaeline,  and  psychotrine.  There  is  a  marked  dif- 
ference between  Brazilian  and  Carthagena  ipecac  in  the  relative  propor- 
tions of  the  contained  alkaloids.  In  the  former  the  proportion  of  emetine 
in  the  total  alkaloidal  content  is  about  70  per  cent,  wrhile  in  the 
latter  it  is  about  40  per  cent.  The  table  by  Paul  and  Cownley 
on  page  551  shows  the  relative  proportion  in  the  two  varieties  of 
root. 

The  two  first  alkaloids  differ  considerably  in  their  physiological 
and  therapeutic  action.  Emetine  being  regarded  as  the  expectorant 
and  cephaeline  as  the  emetic  principle  of  the  drug.  The  two  varieties 
of  the  drug  cannot  therefore  be  considered  as  therapeutically  identical. 
Because  of  this  difference  in  the  proportion  of  the  contained  alkaloids 
it  is  always  advisable  to  specify  the  variety  of  ipecac  assayed.  The 
third  alkaloid  (psychotrine)  is  present  in  such  small  quantity  that  it 
may  be  ignored  in  the  assay  of  the  root.  It  is  desirable  not  only  to 
insure  complete  extraction  of  the  alkaloids  but  their  separation.  This 
latter  is,  however,  not  absolutely  necessary  since  it  is  considered 
sufficiently  accurate  for  the  purpose  of  the  pharmacist  to  express  the 
result  as  mixed  alkaloids,  as  in  the  U.  S.  P.  assay. 

*  The  U.  S.  P.  process. 


ASSAY    OF    IPECAC 


551 


Brazilian. 

Columbian. 

Root. 

Stems. 

Emetine                                          

1-45 
0.52 
0.04 

1.18 

°-59 
0.03 

0.89 
I    25 
0.06 

Cephaeline    .          

Psvchotrine                                

TABLE  SHOWING  PER  CENT  OF  EACH  ALKALOID  IN  THE  TOTAL 
ALKALOIDAL  CONTENT 


Brazilian. 

Columbian. 

Root. 

Stems. 

Emetine             

72.14 
25-87 
i-99 

65.6 
32-8 
1.6 

40.5 
56.8 

2,7 

Cephaeline                                           

Psvchotrine  

In  this  assay,  which  is  a  typical  Keller  method,  the  two  principal 
alkaloids  are  assumed  to  be  present  in  almost  equal  proportions,  and 
the  factor  for  "ipecac  alkaloids  "  is  found  by  taking  the  mean  of  their 
molecular  weights.  Thus 

Emetine,  Ci5H2iNO2 245.34 

Cephaeline,  C^HigNC^ 231 .43 

2)476.77 
238.385 

The  formulae  here  given  tor  emetine  and  cephaeline  depend  upon 
the  assumption  that  these  alkaloids  are  non-acid  bases.  The  formula 
of  Kunz,  CaoHio^Os,  for  emetin  is  obviously  incorrect,  because  he 
operated  upon  a  mixture  of  the  alkaloids.  Kunz  considered  emetin 
to  be  a  diacid  base,  with  a  molecular  weight  of  504.5.  Therefore  in 
assaying  ipecac,  if  the  alkaloidal  value  is  expressed  in  terms  of  emetin- 
Kunz,  it  will  not  differ  very  greatly  from  an  assay  in  which  the  result 

N 

is  expressed  as  "ipecac  alkaloids."    The  —  factor  being  in  the  former 

10 

case  0.0252  gm.  and  in  the  latter  0.0238  gm.  However,  an  assay 
calculated  in  either  of  these  ways,  although  it  is  sufficiently  accurate 
for  the  purpose  of  the  pharmacist,  does  not  give  the  relative  pro- 
portions of  the  two  principal  alkaloids;  and  since  these  alkaloids 


552  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

differ  decidedly  in  their  therapeutic  action,  it  is  a  matter  of  some 
importance  to  know  which  is  present  in  the  greater  quantity.  The 
advisability  of  specifying  the  variety  of  ipecac  assayed  is  therefore 
apparent. 

The  U.  S.  P.  Method.  "Introduce  15  gms.  of  the  ipecac  (in 
No.  80  powder)  into  an  Erlenmeyer  flask  of  250  cc.  capacity,  add 
115  cc.  of  ether  and  35  cc.  of  chloroform,  shake  the  flask  during  five 
minutes,  and  then  add  3  cc.  of  ammonia-water  and  again  shake  the 
flask  at  intervals  during  half  an  hour.  Now  add  10  cc.  of  distilled 
water,  shake  the  liquid  until  the  powder  collects  in  masses,  and  pour 
off  100  cc.  of  the  clear  ethereal  solution  into  a  measuring-cylinder. 
Transfer  the  latter  to  a  separator,  add  10  cc.  of  normal  sulphuric 
acid  and  ip  cc.  of  distilled  water.  Shake  the  separator  moderately 
during  two  minutes,  and  when  the  liquids  have  separated,  draw  off 
the  lower  acid  solution  into  a  second  separator.  Repeat  the  shaking 
out  of  the  ether  solution  with  3  cc.  of  normal  sulphuric  acid  and 
5  cc.  of  distilled  water,  drawing  the  acid  solution  into  the  second 
separator.  Repeat  the  shaking  out  again,  using  10  cc.  of  distilled 
water,  and  add  the  aqueous  solution  to  the  second  separator.  Reject 
the  ether  in  the  first  separator,  introduce  a  small  piece  of  red  litmus 
paper  into  the  second  separator,  add  enough  ammonia-water  to  render 
the  liquid  alkaline,  and  25  cc.  of  ether,  and  then  shake  the  separator 
vigorously  during  one  minute ;  draw  off  the  alkaline  aqueous  liquid  into 
another  separator,  and  transfer  the  ether  solution  to  a  flask.  Add 
20  cc.  of  ether  to  the  alkaline  liquid  in  the  separator,  shake  it  for  one 
minute,  and,  having  allowed  the  liquids  to  separate,  draw  off  the  alka- 
line liquid  into  the  other  separator,  and  transfer  the  ether  solution  to 
the  flask.  Again  shake  out  the  alkaline  liquid  with  10  cc.  of  ether, 
and,  when  the  fluids  have  separated,  reject  the  alkaline  liquid  and 
add  the  ether  solution  to  the  liquid  in  the  flask.  Distil  the  ether  from 
the  flask  with  the  aid  of  a  water-bath,  and  dissolve  the  alkaloidal 
residue  in  12  cc.  of  tenth-normal  sulphuric  acid,  warming  it  gently 
on  a  water-bath  if  necessary.  Then  add  five  drops  of  haematoxylin 
T.  S.  and  titrate  with  fiftieth -normal  potassium  hydroxid.  Divide 
the  number  of  cc.  of  fiftieth-normal  potassium  hydroxid  used  by  5, 
subtract  the  quotient  from  12  (the  12  cc.  of  tenth-normal  sulphuric 
acid  taken),  and  multiply  the  remainder  by  0.0238,  and  this  product 
by  10,  which  will  give  the  percentage  of  alkaloids  in  the  ipecac." 

The  German  Pharmacopoea  employs  the  same  method  for  ipecac 
as  for  aconite. 

Gordin's  Method  (A.  J.  Ph.,  1906,  461).  Put  5  gms.  of  ipecac 
(No.  60  powder)  into  the  shaking  tube  (Fig.  89);  add  2.5  cc.  of  a 
10  per  cent  solution  of  sodium  carbonate  and  25  cc.  of  a  mixture  of 
three  volumes  of  ether  and  one  volume  of  chloroform.  After  shaking 


ASSAY  OF  IPECAC  553 

for  one  hour,  percolate  to  exhaustion.  Shake  out  the  percolate  three 
times  with  small  quantities  of  very  dilute  sulphuric  acid,  add  excess 
of  sodium  hydroxid,  and  shake  out  three  times  with  ether-chloroform. 
Distil  the  ethereal  solution  to  about  one-half,  dilute  with  ether  to 
about  the  original  volume,  and  finish  as  directed  for  aconite  root 
(page  525). 

G.  Fromme's  Method.*  This  simple  method  was  designed  to 
overcome  the  difficulty  in  titrating  the  ipecac  alkaloids,  occasioned 
by  their  discoloring  the  solutions. 

Six  grams  of  the  drug  in  fine  powder,  120  gms.  ether,  and  5  cc. 
ammonia- water  10  per  cent,  are  shaken  during  half  an  hour,  put  aside 
to  settle,  and  then  100  gms.  decanted  through  a  pledget  of  cotton. 
The  ether  is  distilled  off,  the  residue  dissolved  in  5  cc.  of  absolute 
alcohol,  20  cc.  of  ether,  and  10  cc.  of  water.  Three  drops  of  haema- 

N 

toxylin  solution  are  then  added  and  the  —  acid  run  in;   toward  the 

10 

end  of  the  titration  30  cc.  of  water  are  added  gradually  and  the  addi- 
tion of  the  standard  acid  continued  until  the  color  change  is  complete. 
Separate  Estimation  of  Emetine  and  Cephaeline  (Paterson).f 
The  following  method  is  recommended  as  being  accurate,  quick,  and 
easy  and  well  adapted  to  small  quantities  (say  10  gms.)  of  the  root. 
Agitate  10  gms.  of  the  powdered  drug  with  10  cc.  of  ammonia 
water  (or  10  cc.  of  sodium  carbonate  solution  1:3)  and  120  cc.  of  a 
menstruum,  composed  of  i  part  of  chloroform,  i  part  of  amyl  alcohol, 
and  3  parts  of  ether,  in  a  stoppered  bottle  during  one  hour.  Then 
add  from  10  to  15  cc.  of  water  in  order  to  aggregate  the  powder;  decant 
100  cc.  of  the  ethereal  liquid,  evaporate  it  to  one  half,  and  shake  it 

N 

out  with  15  cc.  (or  an  excess)  of  —  hydrochloric  acid,  followed  by 

10 

three  portions  of  5  cc.  each  of  water.  To  the  aqueous  solution  of  the 
alkaloids  so  obtained  new  add  an  excess  (about  2  cc.)  of  normal  potas- 
sium hydroxid,  and  shake  it  out  with  ether  in  four  portions  of  15, 
10,  10,  and  5  cc.  respectively,  reserving  both  the  ethereal  and  aqueous 
portions.  Having  mixed  the  ethereal  solution,  shake  out  three  times 

N 
with  10,  5,  and  5  cc.  of  —  potassium  hydroxid,  mix  the   latter  and 

20 

shake  out  with  10  cc.  of  ether;  then  evaporate  the  ethereal  solution,  and 

N 
weigh  the  residue  as  emetine  (or  titrate  it;    i  cc.  —  acid=o.o2453  gm. 

emetine).     Finally,  mix  all  the  aqueous  solutions,  acidify  with  hydro- 


*  Ph.  Ztg.,  XLIX  (Sept.  17,  1904),  791. 

•j*  Ph.  Jour.,  July  18  and  25,  1903,  73-75  and  101,  102. 


554 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


chloric  acid.  Make  alkaline  with  ammonia,  and  shake  out  with  four 
portions  of  20,  10,  10,  and  5  cc.  of  ether-chloroform  (1:6);  evaporate 
and  weigh  as  cephaeline.  Instead  of  weighing  the  cephaeline  it  may 
also  be  titrated — the  factor  being  0.02314 — using  methyl-orange  as 
indicator. 

Paul  and  Cownley's  Method.*  Mix  50  gms.  of  the  powdered 
ipecac  with  10  gms.  of  freshly -slaked  lime,  moistened  with  water, 
and  extract  by  percolation  with  amylic  alcohol.  Extract  the  alka- 
loids from  the  percolate  by  shaking  out  with  dilute  sulphuric  acid, 
make  the  solution  alkaline  with  ammonia-water  and  shake  out  with 
ether.  The  psychotrine  will  remain  in  the  aqueous  solution  from 
which  it  can  be  removed  by  chloroform.  The  ethereal  solution  is 
evaporated  and  the  residue  titrated  with  seminormal  hydrochloric 
acid.  The  hydrochloric  acid  solution  is  mixed  with  sodium  hydroxid 
solution  in  excess,  and  shaken  out  with  ether  to  remove  emetine.  Some 
cephaeline,  however,  accompanies  it,  so  that  it  is  necessary  to  redissolve 
the  emetine  in  dilute  acid  and  repeat  the  treatment  with  soda  and  ether 
until  the  residual  alkaline  solution  is  no  longer  rendered  cloudy  upon 
the  addition  of  ammonium  chlorid.  The  purified  emetine  is  finally 
determined  by  titration  with  standard  acid.  The  cephaeline  is  obtained 
from  the  alkaline  residues  containing  it,  by  acidifying,  adding  ammonia 
and  shaking  out  with  ether.  It  is  finally  titrated  with  standard  acid. 
The  total  number  of  cc.  of  seminormal  hydrochloric  acid  used  in 
titrating  the  separated  bases  should  be  equal  to  the  number  required 
before  their  separation.  When  the  separation  has  been  satisfacotrily 
made,  the  emetine  hydrochlorid  should  be  readily  obtained  in  crystal- 
line form  on  evaporation  of  the  solution;  and  the  solution  of  cephaeline 
hydrochlorid  should  give  the  characteristic  crystals  of  cephaeline  when 
shaken  out  with  ether  and  ammonia. 

The  following  table  is  interesting  in  that  it  demonstrates  that  the 
two  standard  methods,  Keller's  and  Paul  and  Cownley's,  give  very 
closely  agreeing  results : 


Keller. 
Total 

Paul  and 

Cownley. 

Alkaloids. 

Emetine. 

Cephaeline. 

Rio  root  .  . 

2  846 

2   026 

o  84° 

Rio  root  .     

2    207 

I    3^< 

o  084 

7  oho  re  root 

2    51  1 

I     ^  3O 

Carthagena  root  ,  . 

2    87^ 

1  -oJv 

T     ^44 

I    ^80 

*  Ph.  Jour.,  1896  (April  25),  321. 


ASSAY   OF  IPECAC  555 

G.  Frerichs  and  N.  de  Fuentes  Tapis*  propose  the  following 
method,  in  which  the  small  quantity  of  psychotrine  (believed  to  be 
medicinally  inert)  is  ignored: 

Agitate  6  gms.  of  the  finely  powdered  root  for  one  hour  with  a 
mixture  of  60  gms.  ether  and  5  cc.  ammonia  solution,  or  in  place  of 
the  latter,  5  cc.  of  sodium  carbonate  solution  (1:3),  then  adding  10  cc. 
water  and  evaporating  50  gms.  of  the  ether  solution  (=5  gms.  of  the 
drug)  to  one-half,  after  filtering,  shaking  that  liquid  out  with  10  cc. 
decinormal  hydrochloric  acid  and  washing  it  twice  with  10  cc.  water. 
The  acid  liquor  diluted  to  100  cc.  is  then  titrated  with  decinormal 
potassium  hydroxid  in  the  presence  of  a  layer  of  ether,  using  iodeosin 
as  an  indicator,  i  cc.  of  decinormal  hydrochloric  acid  is  taken  as 
being  equal  to  0.0241  gm.  emetine  and  cephaeline,  corresponding  to  the 
mean  of  248  and  234.  Instead  of  titrating  the  alkaloid  as  described, 
the  acid  solution  may  be  shaken  out  with  ether  and  ammonia,  the 
ether  residue  being  dried  at  100°  C.  and  weighed;  or  the  dried  residue 
may  be  titrated;  but  as  a  slight  decomposition  always  occurs  on  drying 
these  sensitive  bases,  giving  rise  to  a  strongly  colored  solution,  the 
titration  is  not  in  that  case  very  distinct. 

LITERATURE 

1885.  A.  B.  Lyons.    A.  J.  Ph.,  page  531. 

1889.  Cripps  and  Whitley.     Ph.  Jour.  Trans.,  page  721. 

1890.  Arndt.    Apoth.  Ztg.,  page  781. 

1891.  Kottmeyer.     Ph.  Post.,  pages  913-933. 

1895.  R.  A.  Cripps.     Ph.  Jour.,  page  159. 

1896.  Paul  and  Cownley.     Ph.  Jour.  (Apr.  25),  page  321. 
1896.  Lyman  F.  Kebler.     A.  J.  Ph.,  page  196. 

1899.  Geschaftsbericht,  Caesar  and  Loretz  (Sept.). 

1900.  J.  V.  S.  Stanislaus.     Proc.  Indiana  Ph.  A.,  pages  61-64. 
1000.  Geschafetsbericht,  Caesar  and  Loretz  (Sept.). 

1900.  La  Wall  and  Pursel.     Proc.  Penn.  Ph.  A.,  page  160. 
1000.  F.  C.  Bird.     Ph.  Jour.,  pages  175-178,  334~335,  414-416. 

1901.  Paul  and  Cownley.    A.  J.  Ph.,  pages  57-66  and  107-116. 

1902.  Paul  and  Cownley.     Ph.  Jour.,  pages  256  and  317. 

1902.  Frerichs  and  N.  de  Fuentes  Tapis.     Arch.   d.   Ph.,   240   (July  25), 

page  390;  (Sept.  10),  page  401. 

1903.  Hammond  and  Sayre.    Drug.  Circular,  No.  47,  page  227. 

1903.  Paterson.     Ph.  Jour.,  pages  73-75  and  101-102. 

1904.  G.  Fromme.     Ph.  Ztg.  (Sept.  17),  page  791. 
1906.  H.  M.  Gordin.     A.  J.  Ph.,  page  461. 

*  Arch.  d.  Pharm.,  240  (Sept.  10,  1902),  401, 


556  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


ASSAY  OF  NUX  VOMICA 

The  alkaloids  of  nux  vomica  are  strychnine  and  brucine.  The 
proportion  of  strychnine  in  the  total  alkaloidal  content  is  from  40  to 
upward  of  50  per  cent.  The  same  alkaloids  are  present  in  ignatia, 
but  in  this  the  proportion  of  strychnine  is  somewhat  larger.  These 
two  alkaloids  are  somewhat  similar  in  their  medicinal  properties, 
but  the  toxicity  of  strychnine  is  much  greater  than  that  of  brucine. 

In  the  assay  methods  of  nux  vomica,  the  object  is  either  to  deter- 
mine the  total  alkaloidal  content,  or  to  separate  and  determine  the 
quantity  of  strychnine.  A  determination  of  the  total  alkaloids  gives 
a  fairly  good  estimate  of  the  value  of  a  sample  of  the  drug,  but  it 
must  be  admitted  that  the  most  satisfactory  method  is  one  which 
includes  a  determination  of  the  quantity  of  strychnine. 

Methods  in  which  the  "Total  Alkaloids"  is  Determined. 
Nux  vomica  may  be  assayed  by  the  Keller  method,  page  515,  and 
by  the  various  modifications  of  it,  among  them,  Puckner's,  page 
516;  Lyons',  page  516;  and  Kebler's,  page  517,  besides  those  described 
below. 

N 

The  —  factor  for  total  alkaloids  is  0.036152  gm.     This  factor  is 
10 

obtained  by  assuming  that  the  two  alkaloids  strychnine  and  brucine 
are  present  in  equal  proportions,  and  taking  the  mean  of  their  molecular 
weights. 

Strychnine 331-73 

Brucine 39* -31 


Haematoxylin  or  Brazil-wood  solutions  may  be  used  as  indicators. 

The  German  Pharmacopoeia  Method.  15  gms.  of  nux  vomica 
(in  medium  fine  powder,  dried  at  100°  C.)  are  introduced  into  a  flask, 
100  gms.  of  ether  and  50  gms.  of  chloroform  are  added  and  the  flask 
thoroughly  shaken.  Then  10  cc.  of  a  mixture  of  two  parts  of  sodium 
hydroxid  solution  (Ph.  G.  15  per  cent)  and  one  part  of  water 
are  added,  and  the  mixture  shaken  frequently  during  three  hours. 
Then  15  cc.  or  a  sufficient  quantity  of  water  are  added,  to  cause  the 
powder  to  agglutinate  into  a  lump  upon  shaking,  and  the  supernatant 
chloroform-ether  solution  to  separate  clear.  After  standing  one  hour 
100  gms.  of  the  chloroform-ether  solution  (representing  10  gms.  of 
the  drug)  are  filtered  through  a  dry,  well  covered  filter  into  a  flask. 

About  one  half  of  this  is  distilled  off,  and  the  rest  introduced  into 


ASSAY  OF  NUX   VOMICA  557 

a  separating  funnel,  rinsing  the  flask  three  times  with  a  mixture  of 
chloroform  and  ether  (1:3),  using  50:.  eacl:  time,  and  adding  the 
rinsings  to  the  contents  of  the  separator.  The  mixed  solutions  are 

N 

then  shaken  with  10  cc.  of  —  hydrochloric  acid,  and  set  aside  until 

10 

the  liquids  have  separated.  If  necessary,  add  a  little  more  ether  to 
insure  the  complete  separation  of  the  chloroform-ether  solution.  The 
lower  aqueous  solution  is  now  drawn  off  and  filtered  through  a  small 
filter,  moistened  with  water,  into  a  loo-cc.  flask.  Wash  the  chloroform- 
ether  solution  by  shaking  out  with  three  portions  of  water  (10  cc.  each), 
passing  the  washings  through  the  same  filter,  and  further  wash  with 
water  also  passed  through  the  same  filter  to  make  100  cc.  Measure 
off  50  cc.  of  .this  solution  into  a  white  glass  flask  of  200  cc.  capacity 
and  add  50  cc.  of  water,  and  sufficient  ether  to  form  a  layer  i  cm.  in 
depth.  Then  add  five  drops  of  iodeosin  solution  and  titrate  with 

KOH,  shaking  after  each  addition  of  the  standard  alkali  solution 

100 

until  the  lower  layer  assumes  a  pale  red  color.  Not  more  than  15.6  cc. 
should  be  required. 

N 
Each  cc.  of  —  HC1  0.0036152  gm.  of  total  alkaloids. 

Puckner's  Method  (Proc.  A.  Ph.  A.,  1903,  197).  In  this  method 
the  taking  of  an  aliquot  part  is  avoided,  the  drug  being  exhausted 
by  maceration  and  percolation.  By  eliminating  the  aliquot  part  the 
use  of  dry-measuring  vessels  and  their  subsequent  cleansing  is  avoided, 
the  loss  of  volatile  solvent  by  evaporation  is  of  no  consequence,  and 
the  time  of  maceration  may  be  reduced  to  an  hour  or  half  an  hour, 
while  in  the  methods  where  aliquot  parts  are  taken  three,  six,  or  even 
twelve  hours'  maceration,  with  frequent,  or  even  continuous,  agitation 
is  directed. 

To  5  gms.  of  powdered  nux  vomica,  add  2  cc.  of  ammonia-water 
and  50  cc.  of  a  mixture,  made  by  mixing  7  cc.  of  alcohol,  23  cc.  of 
chloroform  and  70  cc.  of  ether.  Shake  frequently  during  one  hour. 
Then  transfer  the  whole  to  a  small  percolator,  and  receive  the  percolate 
in  a  separator.  When  the  menstruum  has  all  passed  through,  the 
drug  is  packed  down  and  exhausted  by  percolating  with  80  cc.  of  the 
same  mixture.  The  alkaloids  are  then  extracted  from  the  percolate 
by  shaking  out  with  10,  10,  and  10  cc.  of  normal  sulphuric  acid.  To 
the  combined  acid  extractions,  a  drop  of  cochineal  T.  S.  is  added, 
then  an  excess  of  ammonia-water,  and  the  alkaloids  abstracted  with 
10,  10,  and  10  cc.  of  chloroform.  After  the  evaporation  of  the  chloro- 

N 
form,  the  alkaloidal  residue  is  titrated  with  --  acid,  cochineal  being 

used  as  indicator. 


558  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

The  Modified  Alkalimetric  Method  (Gordin's).  8  gins,  of  the 
drug  in  No.  50  powder  are  extracted  for  about  two  and  a  half  hours 
in  a  Soxhlet  apparatus  with  alcohol,  the  alcoholic  extract  reduced 
by  evaporation  to  about  10  cc.,  and  then  diluted  with  acidulated  water 
to  50  cc.  This  liquid  is  then  filtered  through  a  little  talcum  powder 
and  25  cc.  of  the  clear  filtrate  (representing  4  gms.  of  the  drug)  made 
strongly  alkaline  with  potassium  hydroxid  solution,  shaken  out  three 
times  with  a  mixture  of  ether  and  chloroform  (3  and  i),  using  25  cc. 
each  time.  The  united  ethereal  liquids  are  then  shaken  up  with 
0.5  gm.  of  calcined  magnesia.  This  completely  removes  the  small 
quantity  of  water  together  with  traces  of  alkali  contained  in  the  ethereal 

N 

liquid.    The  ether-chloroform  is  filtered  into  a  flask,  40  cc.  of   — 

40 

sulphuric  acid  added,  the  flask  shaken  well,  and  the  ethereal  liquid 
removed  by  distillation. 

The  acid  liquid  is  then  poured  into  a  loo-cc.  measuring- flask. 
The  distilling  flask  is  washed  two  or  three  times  with  5  cc.  of  water 
and  the  washings  added  to  the  contents  of  the  measuring-flask.  Mayer's 
reagent  is  then  added  in  small  quantities  at  a  time,  until  the  reagent 
is  in  considerable  excess.  The  flask  is  then  filled  up  to  the  loo-cc. 
mark,  shaken  t.ill  the  supernatant  liquid  is  clear,  the  liquid  filtered, 

N 
and  50  cc.  of  the  clear  filtrate  removed  and  carefully  titrated  with  — 

potassium  hydroxid  solution,  using  phenlophthalein  as  the  indicator 

N 

and  the  —  factor  for  total  alkaloids  0.0091  gm. 
40 

Kippenberger's  Method  (Apoth.  Ztg.,  1898,  664-674).  15  gms. 
of  the  drug  in  fine  powder  (No.  50)  are  extracted  with  a  mixture 
of  140  cc.  alcohol,  7  cc.  water,  and  3  cc.  diluted  hydrochloric  acid. 
100  cc.  of  the  filtered  extract  are  evaporated  to  dryness  on  a  water- 
bath,  the  residue  dissolved  in  60  cc.  of  water  containing  2  cc.  of  dilute 
hydrochloric  acid,  the  solution  filtered,  and  45  cc.  of  the  filtrate  (repre- 
senting 7.5  gms.  of  the  drug)  are  introduced  into  a  flask,  and  treated 
with  20  cc.  of  iodin  solution  (iodin  20  gms.,  potassium  iodid  60  gms., 
water  to  make  1000  cc.). 

The  mixture  is  allowed  to  stand  about  ten  minutes  or  until  the 
alkaloids  are  completely  precipitated  as  periodid.  The  precipitate  is 
collected  on  a  small  plaited  filter  and  washed  two  or  three  times  with 
water  to  which  a  few  drops  of  the  iodin  solution  have  been  added. 

The  precipitate  is  then  dissolved  in  acetone,  about  20  cc.  being 
required  for  complete  solution,  an  excess  of  potassium  hydroxid  solu- 
tion is  added,  and  then  an  excess  of  hydrochloric  acid.  The  mixture 
is  diluted  with  water  and  shaken  out  with  petroleum  ether  to  remove 


ASSAY   OF  NUX    VOMICA  559 

the  acetone,  the  last  traces  of  which  are  removed  by  heating  on  a  water- 
bath. 

The  liquid  is  now  supersaturated  with  potassium  hydroxid  solu- 
tion and  shaken  out  with  chloroform.  The  chloroformic  solution  of 
the  pure  alkaloid  so  obtained  is  cautiously  evaporated  and  the  amount 
of  alkaloid  determined  either  by  direct  weighing  or  by  titration. 

Methods  in  which  Strychnine,  is  Determined.  The  determination 
of  strychnine  in  admixture  with  brucine,  as  in  nux  vomica  and  ignatia, 
is  not  a  difficult  matter.  This  may  be  made  (i)  By  oxidizing  the 
brucine  sulphate  or  picrate  by  means  of  dilute  nitric  acid  into  com- 
pounds having  no  basic  character,  while  the  strychnine  remains  un- 
changed, or  at  least  only  slightly  altered  under  the  same  conditions 
Gerock,*  Keller,  j  Nagelvoort,t  and  Gordin  §  have  elaborated  pro- 
cesses based  upon  this  reaction  and  a  modification  of  Gordin's 
procedure  is  now  official  in  the  U.  S.  P.  VIII.  (2)  By  precipitating 
the  strychnine  as  ferrocyanid,  as  designed  by  Beckurts  and  Hoist  [ 
and  Dunstan  and  Short. If 

In  Gerock 's  method,  the  alkaloids  are  precipitated  by  picric  acid, 
and  after  treating  the  combined  picrates  with  dilute  nitric  acid  (sp.gr. 
1.056),  the  unchanged  strychnine  is  collected  and  weighed. 

Keller's  Method.  The  alkaloidal  residue  from  an  extraction 
(about  0.3  gm.)  is  dissolved  by  the  aid  of  water-bath  heat,  in  10  cc, 
of  dilute  sulphuric  acid  (10  per  cent).  To  this  liquid,  when  cold, 
i  cc.  of  nitric  acid  is  added  (sp.gr.  1.42),  the  mixture  well  mixed  and 
set  aside  for  one  and  a  half  to  two  hours.  The  liquid  is  then  treated 
with  10  cc.  of  ammonia-water,  and  the  strychnine  shaken  out  with  a 
mixture  consisting  of  equal  parts  of  chloroform  and  ether,  successive 
portions  being  taken,  using  about  80  cc.  in  all.  Then  put  into  a 
flask  40  cc.  of  the  filtered  chloroform-ether  solution,  distil  off  the 
solvent,  dry  at  95°  to  100°  C.  and  weigh.  The  distillation  should  be 
discontinued  wrhen  crystallization  of  the  strychnine  begins  and  the 
remainder  of  the  solvent  driven  off  by  a  current  of  air. 

The  U.  S.  P.  Method.  In  this  method  the  brucine  is  destroyed 
according  to  the  conditions  worked  out  by  H.  M.  Gordin,  and  the 
strychnine  isolated  and  titrated.  The  directions  are,  however,  some- 
what at  variance  with  those  of  Gordin,  which,  according  to  several 
critics,  spoils  the  method. 


*  Arch.  d.  Ph.,  1889,  158  and  A.  J.  Ph.,  1889,  180. 

f  Zeit.  Oest.  Apoth.  Ver.,  1893,  587;   Proc.  A.  Ph.  A.,  1894,  531. 

$  Proc.  A.  Ph.  A.,  1893    164. 

§  Proc.  A.  Ph.  A.,  1902,  336. 

||  Arch.  d.  Ph.  (3),  XXV,  313. 

^[  Brit.  Pharmacopoeia. 


560  A   MAN  UAL  OF    VOLUMETRIC  ANALYSIS 

The  Process.  Introduce  20  gms.  of  the  mix  vomica  into  a  250-00. 
Erlenmeyer  flask  and  add  to  it  200  cc.  of  a  mixture  of  137.5  cc.  of 
ether,  44  cc.  of  chloroform,  13.5  cc.  of  alcohol,  and  5  cc.  of  ammonia- 
water;  insert  the  stopper  securely  and  macerate  with  frequent  shaking 
during  one  hour  and  allow  it  to  stand  in  a  cool  place  for  twelve  hours. 
Decant  into  a  measuring  cylinder  100  cc.  of  the  liquid  (representing 
10  gms.  of  nux  vomica),  and  pour  this  into  a  separator,  preferably  of 
a  globular  shape.  Rinse  the  cylinder  with  a  little  chloroform,  add 
this  to  the  separator,  and  then  add  15  cc.  of  normal  sulphuric  acid 
V.  S.;  shake  the  mixture  moderately  during  one  minute,  being  careful 
to  avoid  emulsification;  when  the  liquids  have  separated  completely, 
draw  off  the  acid  liquid  into  a  beaker.  Repeat  the  shaking  out  with 
successive  portions  of  5  and  3  cc.  of  normal  sulphuric  acid  V.  S.; 
collect  the  acid  solutions  and  pour  them  into  a  separator.  If  a  drop 
of  the  last  acid  solution  yields  a  precipitate  with  mercuric  potassium 
iodid  T.  S.,  repeat  the  shaking  out  of  the  ether  solution  with  5  cc.  of 
normal  sulphuric  acid  V.  S.  To  the  combined  acid  solutions  in  the 
separator  add  a  small  piece  of  red  litmus  paper,  25  cc.  of  chloroform 
and  then  sufficient  ammonia-water  to  render  the  liquid  alkaline,  and 
shake  the  separator  thoroughly.  When  the  liquids  have  separated 
draw  off  the  chloroform  into  a  flask  of  100  cc.  capacity,  and  repeat 
the  shaking  out  of  the  alkaline  liquid  with  two  successive  portions  of 
15  cc.  each  of  chloroform,  adding  the  latter  to  that  already  in  the 
flask.  Evaporate  the  combined  chloroformic  solutions  in  the  flask 
until  the  alkaloidal  residue  is  dry,  then  dissolve  it  in  15  cc.  of  sulphuric 
acid  (3  per  cent)  warming  it  on  a  water-bath.  When  the  solution  has 
cooled,  add  3  cc.  of  a  cooled  mixture  of  equal  volumes  of  nitric  acid 
(sp.gr,  1.40)  and  distilled  water,  and  after  rotating  the  liquid  a  few 
times,  set  it  aside  for  exactly  ten  minutes,  shaking  it  gently  three  times 
during  this  interval.  Transfer  the  resulting  red  liquid  to  a  separator 
containing  25  cc.  of  an  aqueous  solution  of  sodium  hydroxid  (i  :  10) 
and  wash  the  flask  three  times  with  very  small  amounts  of  distilled 
water,  and  add  the  washings  to  the  separator.  If  the  liquid  is  not 
turbid  add  2  cc.  more  of  the  solution  of  sodium  hydroxid.  Now  add 
20  cc.  of  chloroform  to  the  separator,  and  shake  it  well  by  a  rotating 
motion  for  a  few  minutes;  allow  the  liquids  to  separate,  and  draw 
off  the  chloroform  through  a  small  filter  wetted  with  chloroform,  into 
a  flask.  Repeat  this  twice,  using  10  cc.  of  chloroform  each  time, 
and  draw  off  both  portions  into  the  flask,  using  the  same  filter.  Finally, 
wash  the  filter  and  funnel  with  5  cc.  of  chloroform,  and  then 
evaporate  all  the  chloroform  by  means  of  a  water-bath  very  care- 
fully, to  avoid  decrepitation.  To  the  alkaloidal  residue  add  6  cc. 
of  tenth-normal  sulphuric  acid  V.  S.,  five  drops  of  iodeosin  T.  S., 
about  80  cc.  of  distilled  water,  and  20  cc.  of  ether.  When  all  the 


ASSAY  OF  NUX    VOMICA  561 

alkaloid  is  dissolved,  titrate  the  excess  of  acid  with  fiftieth-normal 
potassium  hydroxid  V.  S.  until  the  aqueous  liquid  just  turns  pink. 
Divide  the  number  of  cc.  of  fiftieth-normal  potassium  hydroxid  V.  S. 
used  by  5,  subtract  this  number  from  6  (the  6  cc.  of  tenth-normal 
sulphuric  acid  V.  S.  taken),  multiply  the  remainder  by  0.0332,  and 
this  product  by  10,  which  will  give  the  percentage  of  strychnine  in 
the  nux  vomica. 

This  method,  although  essentially  that  of  Gordin,  differs  in  some 
important  respects;  for  instance,  nitric  acid  (sp.gr.  1.40)  is  used, 
whereas  Gordin  directs  to  use  a  nitric  acid  of  sp.gr.  1.42.  This  means 
a  difference  of  4.5  per  cent  in  the  strength  of  the  acid,  which,  according 
to  Gordin,  Smith,  and  Webster  and  Pursel,  is  sufficient  a  difference 
to  destroy  the  value  of  the  method.  Another  change  from  the  original 
method  is  the  omission  of  the  amyl  alcohol  at  the  end  of  the  evapo- 
ration of  the  alkaloidal  solution.  The  use  of  this  is  intended  to 
obviate  the  necessity  of  evaporation  by  heating  and  thus  prevent  loss 
of  strychnine. 

Webster  and  Pursel  point  out  that  the  lack  of  uniformity  in  results 
with  this  method  is  due  to  the  absence  or  varying  proportions,  if 
present,  of  the  lower  oxids  of  nitrogen  in  the  nitric  acid,  and  conclude 
that  the  most  suitable  reagent  for  nitrating  the  brucine  would  be  nitric 
acid  containing  a  fair  proportion  of  the  lower  oxids  of  nitrogen  in 
solution.  This  may  be  secured  either  by  substituting  fuming  nitric 
acid  (commercial  nitrous  acid  sp.gr.  1.42)  for  the  U.  S.  P.  nitric  acid, 
or  better,  by  adding  to  the  latter  a  certain  quantity  of  sodium  nitrite. 

These  modifications,  added  to  the  text  of  the  U.  S.  P.,  are  as  follows: 
Dissolve  the  alkaloidal  residue  in  15  cc.  of  3  per  cent  sulphuric  acid. 
To  this  solution  add  3  cc.  of  a  mixture  of  equal  parts  of  nitric  acid 
(sp.gr.  1.40)  and  distilled  water;  then  add  i  cc.  of  a  $  per  cent  solution 
of  sodium  nitrite  in  water,  and  after  rotating  the  liquid  a  few  times, 
set  aside  exactly  30  minutes,  stirring  it  gently  three  times  during  the 
interval.  The  solution  is  then  made  alkaline  and  shaken  out  in  the 
usual  way.  This  modification  they  affirm  is  accurate  over  a  wide 
range  of  temperature. 

Gordin's  Method.*  The  following  description  in  the  author's 
own  words  is  from  his  paper  read  before  the  A.  Ph.  A.  (1902):  The 
mixed  alkaloids,  for  example,  the  residue  of  total  alkaloids  obtained 
in  the  assay  of  nux  vomica  from  8  to  10  gms.  of  the  drug,  are  dissolved 
in  15  cc.  of  3  per  cent  sulphuric  acid  by  the  aid  of  water- bath  heat, 
the  solution  is  cooled  to  ordinary  temperature,  and  3  cc.  of  a  pre- 
viously prepared  and  cooled  mixture  of  equal  parts  of  strong  nitric 

*  Proc.  A.  Ph.  A.  (1902),  336. 


562  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

acid  (sp.gr.  1.42)  and  water  added  to  the  alkaloidal  solution.  The 
liquid  is  set  aside  for  exactly  ten  minutes,  shaking  it  gently  three  or 
four  times  during  this  time.  The  red  liquid  is  now  transferred  to  a 
separator  containing  20  or  25  cc.  of  10  per  cent  sodium  hydroxid 
solution,*  and  the  vessel  in  which  the  digestion  of  the  alkaloids  had 
taken  place  is  washed  three  or  four  times  with  very  small  amounts  of 
water.  The  liquid  in  the  separator  will  now  be  very  turbid,  from 
separation  of  strychnine.  If  this  is  not  the  case,  there  is  not  enough 
alkali,  and  a  further  addition  of  i  or  2  cc.  alkali  must  be  made.  After 
the  addition  of  sufficient  alkali,  the  liquid  is  shaken  out  three  times 
with  chloroform,  vising  20  cc.  for  the  first  shaking  out,  and  10  cc. 
each  time  for  the  two  subsequent  ones.  The  chloroformic  solution 
is  filtered  through  a  small  plain  double  filter,  arranged  so  that  there 
are  four  folds  of  paper  on  each  side,  into  a  light,  tared  flask,  taking 
care  to  wash  the  stem  of  the  separator  with  a  little  chloroform;  the 
filter  and  stem  of  the  funnel  are  also  washed  a  few  times  with  small 
amounts  of  chloroform,  and  to  the  perfectly  colorless  solution  of 
strychnine  thus  obtained,  are  added  2  or  3  cc.  of  pure  amyl  alcohol 
which  distils  between  128°  and  132°  C.,  and  leaves  no  residue  on 
evaporation. 

The  chloroform  is  now  distilled  off  completely  and  the  small  amount 
of  amyl  alcohol  left  behind  removed  by  keeping  the  vessel  on  the 
water-bath  and  blowing  air  over  its  opening,  but  so  as  not  to  blow 
out  some  alkaloid  by  the  air  current.  The  strychnin  obtained  in 
this  method  is  very  pure  and  may  be  weighed,  or  titrated  with  standard 
acid,  using  haematoxylin  as  indicator. 

The  Methods  Depending  upon  the  Precipitation  of  Strych- 
nine by  Means  of  Potassium  Ferrocyanid  are  those  of  Beckurts 
and  Hoist  and  Dunstan  and  Short.  The  former  consists  in  dissolving  the 
alkaloidal  residue  in  water  strongly  acidulated  with  hydrochloric  acid, 
and  titrating  with  standard  solution  of  potassium  ferrocyanid  which  pre- 
cipitates the  strychnine  but  leaves  the  brucine  in  solution.  For  deter- 
mining the  end-point  a  drop  of  the  solution  is  taken  out  on  a  glass 
rod  and  brought  in  contact  with  paper  moistened  with  a  weak  solution 
of  ferric  chlorid.  This  is  repeated  frequently  until  a  blue  color  is 
produced,  indicating  that  all  of  the  strychnine  has  been  precipitated. 
Because  of  the  tediousness  and  inconvenience  of  the  method  of  finding 
the  end-point,  this  method  is  not  in  great  favor;  it  is,  however,  capable 
of  very  exact  results.  The  method  of  Dunstan  and  Short  is  the 

*  It  is  best  to  place  the  alkali  in  the  separator  while  the  alkaloids  are  being 
digested  with  the  acids,  so  that  after  the  lapse  of  ten  minutes,  when  the  acid 
liquor  is  poured  into  the  separator,  the  action  of  nitric  acid  upon  strychnine  is 
quickly  arrested. 


ASSAY    OF    OPIUM  563 

official  method  of  the  British  pharmacopoeia.  In  this,  the  precipi- 
tated strychnine  ferrocyanid  is  collected  on  a  filter  and  washed  with 
acidulated  water.  It  is  then  decomposed  by  ammonia  and  the  strych- 
nine shaken  out  with  chloroform. 

LITERATURE 

Schweissinger.     Am.  Drug.,  1885,  230. 

Gerock.     Arch.  d.  Ph.,  1889,  158,  and  A.  J.  Ph.  (1889),  180. 

Nagelvoort.     Proc.  A.  Ph.  A.,  1893,  *65- 

Keller.  Zeit.  Oest.  Ap.  Ver.,  1893,  587,  and  Schweitz.  Wochenschr., 
1893,  33,  452;  also  Proc.  A.  Ph.  A.,  1894,  531. 

Gomberg.     J.  A.  C.  S.,  1896,  339. 

Prescott  and  Gordin.  J.  A.  C.  S.,  1898,  722;  and  Proc.  A.  Ph.  A.,  1899, 
278. 

Lenton.     Ph.  Jour.  (Vol.  21),  864. 

Reynolds  and  Sutcliffe.     Ph.  Jour.  (76),  555. 

Beckurts  and  Hoist.     Arch.  d.  Ph.  (3),  xxv,  313. 

C.  E.  Smith.     A.  J.  Ph.,  1896,  189. 

Bird.     Ph.  Jour.,  1900,  Sept.  8th,  286. 

Farr  and  Wright.     Proc.  A.  Ph.  A.,  1901,  883. 

Gordin.     A.  J.  Ph.,  1901,  211;    and  Proc.  A.  Ph.  A.,  1902,  336. 

F.  J.  Smith.     A.  J.  Ph.,  1903,  253. 

Puckner.     Proc.  A.  Ph.  A.,  1903,  197. 

Howard.     Analyst,  1905  (xxx),  261. 

Farr  and  Wright.  Trans.  Brit.  Ph.  Conf.  (Yearbook),  226-238;  and 
Chem.  and  Drug.,  July  28,  1906. 

Webster  and  Pursel.     A.  J   Ph.,  1907,  1-7. 

Gordin.     A.  J.  Ph.,  1907,  61. 

ASSAY  OF  OPIUM 

Opium  contains  numerous  alkaloids  of  which  morphine  is  the 
most  abundant  and  by  far  the  most  important.  It  exists  in  the  drug 
in  the  form  of  a  salt  of  meconic  acid,  in  which  form  is  is  readily 
soluble  in  water.  Narcotine  and  most  of  the  other  alkaloids  being 
relatively  insoluble  in  water  are  left  behind  in  the  aqueous  extrac- 
tion. Any  narcotine  which  dissolves  may  be  separated  by  precipi- 
tating the  morphine  with  ammonia  in  the  presence  of  ether.  The 
latter  holds  narcotine  in  solution.  The  separation  may  be  also  effected 
by  treatment  with  lime;  this  dissolves  morphine  but  renders  narcotine 
insoluble.  Lead  acetate*  and  lead  subacetate  f  are  also  employed 
for  the  purpose  of  separating  morphine  from  its  accompanying  im- 
purities. 

*  Dieterich.  f  Parker. 


564  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Morphine  (the  alkaloid)  is  nearly  insoluble  in  cold  water.  In  fes 
crystalline  form  it  is  very  sparingly  soluble  in  ether,  but  if  ether  is 
present  at  the  moment  that  the  alkaloid  is  set  free  by  an  alkali,  it 
dissolves  much  more  readily  but  most  of  it  will  in  a  short  time  crystal- 
lize out  of  the  ethereal  solution. 

The  U.  S.  P.  Method  1890.  This  is  the  method  proposed  by 
Dr.  Squibb.*  It  is  described  in  the  U.  S.  P.  as  follows:  Opium,  in 
any  condition  to  be  valued,  10  gms.;  ammonia-water,  3.5  cc.;  alcohol, 
ether,  water,  each  a  sufficient  quantity.  Introduce  the  opium  (which, 
if  fresh,  should  be  in  very  small  pieces,  and  if  dry,  in  very  fine  powder) 
into  a  bottle  having  a  capacity  of  300  cc.;  add  100  cc.  of  water;  cork 
well.  Agitate  the  bottle  frequently  during  twelve  hours;  then  pour 
the  whole  as  evenly  as  possible  upon  a  wetted  filter  having  a  diametei 
of  12  cm.,  and  when  the  liquid  has  drained  off  wash  the  residue  with 
water  carefully  dropped  upon  the  edges  of  the  filter  and  contents 
until  150  cc.  of  filtrate  are  obtained.  Then  carefully  transfer  the 
moist  opium  back  to  the  bottle  by  means  of  a  spatula,  add  50  cc.  of 
water,  agitate  thoroughly  and  repeatedly  during  fifteen  minutes,  and 
return  the  whole  to  the  filter. 

When  the  liquid  has  drained  off,  wash  the  residue  as  before  until 
the  second  filtrate  measures  150  cc.,  and  finally  collect  about  20  cc. 
more  of  a  third  filtrate. 

Evaporate  in  a  tared  capsule;  first,  the  second  filtrate  to  a  small 
volume,  then  add  the  first  filtrate,  rinsing  the  vessel  with  the  third 
filtrate,  and  continue  the  evaporation  until  the  residue  weighs  14  gms. 
Pour  the  liquid  into  a  tared  flask,  rinse  the  capsule,  and  add  the 
rinsings  until  the  entire  solution  weighs  20  gms.  Then  add  12.2  cc. 
of  alcohol;  shake  well;  add  25  cc.  of  ether;  shake  again.  Now  add 
the  ammonia-water,  cork  well,  shake  for  ten  minutes,  and  set  aside 
for  at  least  six  hours  or  overnight,  so  that  the  crystals  may  form. 

At  the  expiration  of  this  time  decant  the  ethereal  layer  upon  a 
double,  plain,  rapidly  acting  filter  previously  wet  with  ether;  add 
10  cc.  of  ether  to  the  contents  of  the  flask,  rotate,  and  again  decant. 
Repeat  this  operation  with  another  10  cc.  of  ether.  Then  pour  the 
liquid  in  the  bottle  upon  the  filter,  in  small  portions  at  a  time,  so  as  to 
transfer  the  greater  portion  of  the  crystals  to  the  filter,  and  wash  the 
remaining  crystals  on  to  the  filter  with  the  aid  of  a  small  quantity 
of  water,  using  not  more  than  10  cc.  Then  wash  the  crystals,  first 
with  a  few  drops  of  water,  then  with  an  alcoholic  solution  of  morphine, 
and  finally  with  ether  to  displace  the  alcohol.  Dry  the  crystals  to  a 
constant  weight  and  weigh  on  a  tared  watch-glass. 

*  See  Ephemeris   III,  1150,  1161. 


ASSAY  OF  OPIUM  565 

The  weight  of  the  crystals  obtained,  when  multiplied  by  10,  repre- 
sents the  percentage  of  crystallized  morphine  present  in  the  sample  of 
gum.  Opium  should  contain  9  per  cent;  the  powdered  not  less  than 
12  per  cent  nor  more  than  12.5  per  cent. 

N 
Instead  of  weighing,  the  crystals  may  be  dissolved  in  —  sulphuric 

N  I0 

acid,  and  the  solution  retitrated  with  —  potassium  hydroxid  solution, 

50 

N 
using  haematoxylin  as  the  indicator  and  the  —  factor  for  morphine 

0.006015  gm-  ^° 

The  result  of  this  titration  is  not  entirely  satisfactory  in  that  the 
impurity  mixed  with  the  morphine  crystals  containing  as  it  does  cal- 
cium compounds  (meconate  and  sulphate)  is  capable  of  neutralizing 
strong  acids.  These  impurities  are,  however,  in  a  great  part  insoluble 
in  limewater,  whereas  morphine  is  soluble,  hence,  in  the  8th  Revision 
of  the  U.  S.  P.,  an  attempt  is  made  to  separate  the  alkaloid  by  treat- 
ing the  weighed  precipitate  with  lime  water.  The  insoluble  residue 
containing  the  impurities  is  then  separated  by  filtration,  dried  and 
weighed;  the  latter  weight  deducted  from  the  former  gives  the  soluble 
portion,  which  is  computed  as  morphine. 

The  lime  water  filtrate  so  obtained,  is  however,  colored,  and  hence 
indicates  that  some  of  the  impurity  is  soluble.  This  soluble  portion 
may  amount  to  4  or  5  per  cent. 

It  is  also  possible,  as  pointed  out  by  Parker  and  others,  that  the 
lime  water  reacts  with  a  portion  of  the  impurities  (calcium-ammonium- 
meconate),  changing  it  to  calcium  meconate,  so  that  the  separated 
residue  has  neither  the  same  weight  nor  composition  as  the  impurities 
originally  weighed  with  the  morphine;  therefore  this  method,  though 
an  improvement  over  the  old,  is,  nevertheless,  far  from  perfect. 

Other  solvents,  such  as  potassium  hydroxid,  ethyl,  and  methyl 
alcohol,  which  have  been  tried,  with  a  view  to  a  separation  of  the 
morphine  from  its  impurities,  are  without  any  better  results. 

The  U.  S.  P.  Method  (8th  Decennial  Revision).  Opium,  in 
any  condition  to  be  valued,  10  gms.,  ammonia-water,  3.5  cc.,  alcohol, 
ether,  distilled  water,  limewater,  each  a  sufficient  quantity.  Introduce 
the  opium  (which,  if  fresh,  should  be  in  very  small  pieces,  and  if  dry 
in  very  fine  powder)  into  an  Erlenmeyer  flask  having  a  capacity  of 
about  300  cc.,  add  100  cc.  of  distilled  water,  stopper  the  flask,  and 
agitate  it  every  ten  minutes  (or  continuously  in  a  mechanical  shaker) 
during  three  hours.  Then  pour  the  contents  as  evenly  as  possible 
upon  a  wetted  filter  having  a  diameter  of  12  cm.,  and,  when  the  liquid 
has  drained  off,  wash  the  residue  with  distilled  water,  carefully  dropped 
upon  the  edges  of  the  filter  and  its  contents,  until  150  cc.  of  filtrate 


566  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

have  been  obtained.  Then  carefully  transfer  the  moist  opium  back 
to  the  flask  by  means  of  a  spatula,  add  50  cc.  of  distilled  water,  agitate 
it  thoroughly  and  repeatedly  during  fifteen  minutes,  and  return  the 
whole  to  the  filter.  When  the  liquid  has  drained  off,  wash  the  residue 
as  before,  until  the  second  filtrate  measures  150  cc.,  and  finally  collect 
about  20  cc.  more  of  a  third  filtrate.  Evaporate  carefully  in  a  tared 
dish;  first,  the  second  filtrate  to  a  small  volume,  then  add  the  first 
filtrate,  rinsing  the  vessels  with  the  third  filtrate,  and  continue  the 
evaporation  until  the  residue  weighs  14  gms.  Rotate  the  concentrated 
solution  about  in  the  dish  until  the  rings  of  extract  are  redissolved, 
pour  the  liquid  into  a  tared  Erlenmeyer  flask  having  a  capacity  of 
about  100  cc.,  and  rinse  the  dish  with  a  few  drops  of  water  at  a  time 
until  the  entire  solution,  after  the  rinsings  have  been  added  to  the 
flask,  weighs  20  gms.  Then  add  10  gms.  (or  12.2  cc.)  of  alcohol, 
shake  the  flask  well,  add  25  cc.  of  ether,  and  repeat  the  shaking.  Now 
add  the  ammonia-water  from  a  graduated  pipette  or  burette,  stopper 
the  flask  with  a  sound  cork,  shake  it  thoroughly  during  ten  minutes, 
and  then  set  it  aside,  in  a  moderately  cool  place,  for  at  least  six  hours, 
or  over  night. 

Remove  the  stopper  carefully,  and  should  any  crystals  adhere  to 
it,  brush  them  into  the  flask.  Place  in  a  small  funnel  two  rapidly 
acting  filters,  of  a  diameter  of  7  cm.,  plainly  folded,  one  within  the 
other  (the  triple  fold  of  the  inner  filter  being  laid  against  the  single 
side  of  the  outer  filter),  wet  them  well  with  ether,  and  decant  the 
ethereal  solution  as  completely  as  possible  upon  the  inner  filter.  Add 
10  cc.  of  ether  to  the  contents  of  the  flask,  rotate  it,  and  again  decant 
the  ethereal  layer  upon  the  inner  filter.  Repeat  this  operation  with 
another  portion  of  10  cc.  of  ether.  Then  pour  the  liquid  in  the  flask 
into  the  filter,  in  portions,  in  such  a  way  as  to  transfer  the  greater 
portion  of  the  crystals  to  the  filter  and,  when  the  liquid  has  passed 
through,  transfer  the  remaining  crystals  to  the  filter  by  washing  the 
flask  with  several  portions  of  water,  using  not  more  than  15  cc.  in 
all.  Use  a  feather  or  rubbed-tipped  glass  rod  to  remove  the  crystals 
that  adhere  to  the  flask.  Allow  the  double  filter  to  drain,  then  apply 
water  to  the  crystals,  drop  by  drop,  until  they  are  practically  free  from 
mother-liquor,  and  afterwards  wash  them,  drop  by  drop,  from  a 
pipette,  with  alcohol  previously  saturated  with  powdered  morphine. 
When  this  has  passed  through,  displace  the  remaining  alcohol  by 
ether,  using  about  10  cc.  or  more,  if  necessary.  Allow  the  filter  to 
dry  in  a  moderately  warm  place,  at  a  temperaure  not  exceeding  60°  C. 
(140°  F.)  until  its  weight  remains  constant,  then  carefully  transfer 
the  crystals  to  a  tared  watch-glass  and  weigh  them. 

Place  the  crystals   (which  are  not  quite  pure)  in  an  Erlenmeyer 
flask,  add  1'me  water  (10  cc.  for  each  o.i  gm.  of  morphine)  and  shake 


ASSAY  OF  OPIUM  567 

the  flask  at  intervals  during  half  an  hour.  Pass  the  liquid  through 
two  counterpoised  rapidly  acting,  plainly  folded  filters,  one  within 
the  other  (the  triple  fold  of  the  inner  filter  being  laid  against  the  single 
fold  of  the  outer  filter),  rinse  the  flask  with  more  limewater  and  pass 
the  washings  through  the  filter  until  the  filtrate,  after  acidulating,  no 
longer  yields  a  precipitate  with  mercuric  potassium  iodid  T.  S.  Press 
the  fillers  until  nearly  dry  between  bibulous  paper  and  dry  them  to 
a  constant  weight,  then  weigh  the  contents,  using  the  outer  filter  as  a 
counterpoise.  Deduct  the  weight  of  the  insoluble  matter  on  the 
filter  from  the  weight  of  the  impure  morphine  previously  found.  The 
difference,  multiplied  by  10,  represents  the  precentage  of  crystallized 
morphine  contained  in  the  opium. 

Lyons  objects  to  the  direction  that  the  precipitate  be  washed  with 
morphinated  alcohol,  as  moderate  changes  in  temperature  affect  the 
solubility,  causing  the  solution  to  deposit  or  dissolve  morphine,  and 
also  morphine  is  apt  to  be  deposited  by  evaporation.  He  prefers  liberal 
washing  with  morphinated  water,  and  drying  the  filter  between  folds 
of  absorbent  paper. 

Lamar's  Modification  of  the  U.  S.  P.  VIII  Method.  This  is 
given  in  Bulletin  No.  105,  A.  O.  A.  C.,  U.  S.  Dept.  of  Agriculture,  in 
the  following  words: 

Proceed  as  directed  by  the  Pharmacopoeia  to  precipitation  of  mor- 
phine. To  the  20  gms.  of  aqueous  extract  add  60  gms.  of  alcohol, 
stopple  flask,  shake  well  for  one  minute  and  set  aside  for  thirty  minutes, 
during  which  time  the  precipitated  material  should  have  completely 
subsided.  Decant  the  clear  supernatant  liquid  into  a  tared  250-0:. 
evaporating-dish,  transfer  the  precipitate  to  a  y-cm.  filter  previously 
moistened  with  a  mixture  of  alcohol  (three  parts)  and  water  (one  part.) 
The  last  portions  of  the  residue  are  transferred  to  the  filter  by  using 
small  portions  of  the  above  hydro-alcoholic  solution.  The  filtrate  is 
to  be  collected  in  the  tared  evaporating-dish.  Continue  washing  the 
residue  and  filter  by  dropping  the  alcoholic  solution  on  the  filter  and 
the  residue  until  the  filtrate  is  no  longer  bitter.  Add  35  cc.  of  water 
to  the  contents  of  the  evaporating-dish  and  evaporate  on  water-bath 
to  14  gms.,  then  proceed  as  directed  by  the  Pharmacopoeia. 

Mallinckrodt  suggests  a  reassay  of  the  precipitated  impure  mor- 
phine crystals  by  a  modification  of  the  old  lime  method  as 
follows : 

Place  1.2  gms.  mixed  crude  morphine  in  an  8o-cc.  Erlenmeyer  flask, 
add  0.5  gm.  freshly  slacked  lime  and  20  cc.  water,  cork,  and  shake 
occasionally  for  one  half  hour.  Filter  into  a  similar  tared  flask  with 
gentle  suction  (reinforcing  the  point  of  the  filter  with  a  platinum 
or  hardened  paper  cone),  wash  the  flask  and  residue  with  lime  water 
until  the  total  filtrate  and  washings  amount  to  35  gms.  Add  3  cc. 


568  -1    MANUAL    OF    VOLUMETRIC    ANALYSIS 

alcohol,  20  cc.  ether,  rotate,  add  0.5  gm.  ammonium  chlorid,  cork, 
and  shake  vigorously. 

Let  stand  two  hours,  then  filter,  dry,  and  weigh  the  precipitated 
morphin  according  to  the  directions  of  the  Pharmacopoeia. 

This  method  is  not  expeditious,  and  its  results  need  correction  by 
factors  not  yet  well  determined.  Mallinckrodt  thinks  that  a  cor- 
rection of  20  to  30  mg.  should  be  added  to  the  weight  of  the  reassay 
morphine  for  solubility  in  the  mother-liquor,  which  is  equivalent  to 
raising  all  results  about  0.21  per  cent. 

An  aqueous  extraction  of  opium  contains  sulphate  and  meconate 
of  morphine  and  other  alkaloids,  meconic  acid,  calcium  salts,  extrac- 
tive, resinous  matters,  etc.  Barium  chlorid  has  been  used  for  removing 
the  sulphuric  acid,  ammonium  oxalate  for  removing  the  calcium  salts, 
and  alcohol  for  removing  the  extractives  and  other  impurities.  Lead 
acetate  has  also  been  suggested  for  removing  impurities,  and  recently 
Parker  has  devised  a  method  in  which  lead  subacetate  is  employed 
for  this  purpose  and  oxalic  acid  to  remove  the  excess  of  lead.  Any 
small  amount  of  lead  still  remaining  in  solution  is  finally  removed  by 
treatment  with  hydric  sulphid.  The  method  is  as  follows :  * 

Parker's  Method.  Introduce  10  gms.  of  the  opium  into  a 
3oo-cc.  flask,  add  100  cc.  of  water,  cork  and  shake  for  two  and  one  half 
hours;  add  25  cc.  of  lead  subacetate  solution,  cork  and  shake  for 
one  half  hour.  Filter  through  a  wetted  filter  12  cm.  in  diameter, 
and  wash  the  residue  carefully  with  water  until  the  total  filtrate  amounts 
to  about  175  cc.  Return  the  residue  to  the  flask,  add  50  cc.  of  water, 
cork,  shake  about  ten  minutes  and  return  the  whole  to  the  filter, 
washing  the  residue  until  the  second  filtrate  amounts  to  about  150  cc. 
Combine  the  two  filtrates  in  a  beaker,  and  from  a  burette  add  normal 
oxalic  acid  solution,  at  first  in  portions  of  about  5  cc.  at  a  time,  stirring 
and  allowing  to  settle  after  each  addition,  and  then  more  slowly,  until 
the  point  where  precipitation  just  ceases  is  reached  (about  26  cc.); 
then  add  5  cc.  more,  or  i  cc.  for  each  3  per  cent  of  morphine  if  the 
approximate  amount  is  known.  Filter  the  solution,  wash  the  pre- 
cipitate with  water,  and  evaporate  the  filtrate  in  flat -bottomed  dishes 
to  a  volume  of  about  20  cc.,  uniting  the  whole  in  one  dish  when  the 
volume  is  sufficiently  reduced,  and  rinsing  carefully  after  with  water. 
Treat  the  concentrated  solution  in  the  dish  (facilitated  by  slightly 
tilting  the  latter)  with  hydrogen  sulphid,  and  filter  through  a  5-cm. 
paper,  washing  the  dish  and  filter  after  into  a  small  evaporator,  with 
a  minimum  amount  of  hot  water.  Evaporate  to  small  volume  (some- 
what less  than  that  finally  required),  transfer  to  a  tared  loo-cc.  flask, 

*  From  Proc.  A.  Ph.  A.,  1907,  495. 


ASSAY    OF    OPIUM  569 

rinsing  after  with  a  minimum  amount  of  hot  water,  and  add  water 
to  bring  the  weight  to  10,  15,  or  20  gms.  as  may  be  desired.  Add 
5,  7.5  or  10  gms.  of  alcohol,  as  the  case  may  be,  rotate,  add  25  cc. 
of  ether,  rotate  again  and  add  2  cc.  of  ammonia -water  (10  per  cent), 
or  a  moderate  excess.  (Cork  the  flask,  shake,  and  suspend  a  strip 
of  dry,  neutral  litmus  paper  under  the  cork;  it  should  turn  blue  in 
about  one  minute.  Cork,  shake  vigorously  for  ten  minutes  and  set 
aside  for  twelve  hours  or  over  night  in  a  cool  place.) 

Filter  through  double  counterpoised  niters  and  wash,  dry,  and 
weigh  as  directed  by  the  Pharmacopoeia,  except  that  a  saturated 
solution  of  morphine  in  water  is  used  instead  of  pure  water  for  washing 
the  precipitate.  The  precipitate,  or  a  weighed  portion  of  it,  is  dis- 
solved in  a  known  amount  of  decinormal  acid,  and  after  addition  of 
cochineal  indicator,  is  titrated  back  with  fiftieth-normal  potassium 
hydroxid  solution. 

Method  of  Gordin  and  Prescott  (Proc.  A.  Ph.  A.,  1900,  126). 
Materials  and  Utensils  for  the  Assay.  Opium  in  very  fine  powder, 
powdered  sodium  chlorid,  an  ethereo-ammoniacal  mixture  composed 
of  stronger  ammonia-water  and  alcohol,  each  5  cc.,  chloroform  10  cc., 

N 
and  ether  20  cc.;   benzene  boiling  at  about  80°  C.;    —  sulphuric  acid 

N  4°  N 

and  —  potassium  hvdroxid  solution;    phenolphthalein   solution;   — 

40  N  10 

iodin  solution  (Wagner's  reagent);    —  sodium  thiosulphate  solution; 

10 

a  small  Dunstan  and  Short  extraction  apparatus  (Proc.  A.  Ph.  A.,  xxi, 
33);  a  screw-top  ointment  jar  having  a  concave  bottom  within,  and 
a  small  pestle,  just  long  enough  to  rest  half  upright  within  the  jar 
when  it  is  closed. 

Directions  for  the  Assay.  Weigh  out  2  gms.  of  the  opium  into  the 
ointment  jar,  rub  it  by  means  of  the  pestle  with  a  few  cc.  of  the  ethereo- 
ammoniacal  solution  to  a  smooth  paste,  taking  care  not  to  smear 
the  sides  of  the  jar  unnecessarily,  then  add  about  2  cc.  more  of  the 
same  mixture,  so  as  to  have  the  opium  well  covered  with  the  liquid; 
screw  down  the  top,  leaving  the  pestle  inside,  and  set  the  jar  aside 
for  five  or  six  hours.  After  that  time  the  jar  is  opened,  about  10  gms. 
of  sodium  chlorid  well  mixed  with  the  opium,  and  the  open  jar  placed 
in  a  good  current  of  air,  stirring  frequently  with  the  pestle  to  prevent 
lumping.  In  about  an  hour,  when  the  powder  is  nearly  dry,  the  jar 
is  placed  in  a  vacuum  desiccator  containing  besides  sulphuric  acid 
a  vessel  of  paraffin  and  left  there  over  night.  The  jar  is  then  taken 
out,  any  lumps  in  the  powder  carefully  crushed,  and  the  mixture 
transferred  to  glazed  paper  and  then  to  the  inner  tube  of  the  extraction 
apparatus,  in  the  bottom  of  which  a  plug  of  cotton  has  been  placed. 


570  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  jar  is  rubbed  out  several  times  with  small  quantities  of  sodium 
chlorid,  the  rinsings  added  to  the  tube,  and  having  placed  a  plug  of 
cotton  and  a  piece  of  glass  on  top  of  the  powder,  the  tube  is  connected 
with  the  glass  stop-cock  by  means  of  rubber  tubing,  and  regulating  the 
flow  with  the  stop-cock,  the  powder  is  extracted  with  benzene  by 
percolating  very  slowly  until,  upon  evaporating  four  or  five  drops  of 
the  percolate  on  a  watch-glass  and  dissolving  the  residue  in  ten  or 
twelve  drops  of  slightly  acidulated  water,  no  turbidity  appears  upon 
the  addition  of  a  few  drops  of  Wagner's  reagent.  A  current  of  dry 
air  is  then  passed  through  the  tube  until  the  powder  is  dry,  the  tube 
placed  into  the  outer  jacket  of  the  apparatus,  and  the  latter  connected 
with  a  small  round-bottomed  flask  containing  40  to  50  cc.  of  chloro- 
form, and  the  powder  extracted  until  exhaustion  is  complete.  This 
will  take  about  two  and  one-half  hours.  Care  should  be  taken  that 
only  a  small  portion  of  the  bottom  of  the  flask  be  heated,  and  that  a 
kyer  of  the  solvent  be  constantly  on  top  of  the  powder.  The  chlo- 

N 

reform  is  then  distilled  off  and  the  residue  dissolved  in  60  cc.  —  sul- 

40 

phuric  acid  by  the  aid  of  gentle  heat. 

The  solution  is  poured  into  a  loo-cc.  graduated  cylinder,  the  latter 
filled  up  to  the  mark  of  100  cc.,  and  the  liquid  filtered,  using  a  little 
talcum  powder  if  necessary.  50  cc.  of  the  clear  filtrate  (representing 
i  gm.  of  the  opium)  are  put  into  a  loo-cc.  measuring-flask  and  Wagner's 
reagent  added  in  small  quantities  at  a  time,  shaking  well  after  each 
addition  till  the  supernatant  liquid  is  very  dark  red.  The  flask  is 
then  filled  up  to  100  cc.  and  shaken  till  the  supernatant  liquid  is 
perfectly  transparent  and  dark  red.  The  liquid  is  filtered  and  in 
50  cc.  of  the  clear  filtrate,  after  decolorizing  it  with  a  few  drops  of 

N 
the   thiosulphate  solution,   titrate  the  free  acid  with  —  potassium 

hydroxid,  using  phenolphthalein  as  indicator.     Figure  out  the  number 

N 

of  cc.  of  the  —  acid  consumed  by  i  gm.  of  the  opium  and  multiply 
40 

it  by  0.71 ;  the  result  is  the  percentage  of  morphine  in  the  drug. 

In  order  to  obtain  very  exact  results  the  acid  should  be  standardized 
with  a  small  quantity  of  anhydrous  morphine. 

The  German  Pharmacopoeial  Method.  6  gms.  of  medium  fine 
powdered  opium  are  mixed  with  6  gms.  of  water,  the  mixture  is  rinsed 
with  water  into  a  dry,  weighed  flask;  then  by  the  addition  of  more 
water,  the  contents  of  the  flask  is  made  to  weigh  54  gms.  After  the 
mixture  has  been  allowed  to  stand  (with  frequent  shaking)  for  several 
hours,  the  mass  is  pressed  through  a  dry  piece  of  linen.  Of  the  liquid 
so  obtained,  42  gms.  are  filtered  through  a  dry  folded  filter  (10  cm.  in 


ASSAY   OF   OPIUM  571 

diameter)  into  a  dry  flask.  Add  to  this  filtrate  2  gms.  of  sodium 
salicylate  solution  (1:2)  and  shake  well.  Then  filter  off  36  gms.  of 
the  clear  solution  (representing  4  gms.  of  opium)  through  a  dry  folded 
filter  (10  cm.  in  diameter)  into  a  flask.  Mix  the  filtrate  (rotating  the 
flask)  with  10  gms.  of  ether  and  add  5  gms.  of  a  mixture  consisting 
of  17  gms.  of  ammonia-water  and  83  gms.  of  water.  The  flask  is  then 
stoppered  and  strongly  shaken  for  ten  minutes  and  set  aside  for  twenty- 
four  hours.  At  the  end  of  this  time  the  ether  layer  is  poured  off  as 
completely  as  possible  onto  a  plain  filter  (8  cm.  in  diameter).  To 
the  aqueous  solution  remaining  in  the  flask  is  added  another  10  gms. 
of  ether,  the  mixture  is  rotated  a  few  moments  and  the  ether  again 
poured  onto  the  filter.  The  aqueous  solution  is  then  poured  onto 
the  filter,  paying  no  attention  to  the  crystals  adhering  to  the  walls  of 
the  flask,  and  the  filter  as  well  as  the  flask  rinsed  three  times  with 
ether-saturated  water,  using  5  gms.  each  time.  After  the  flask  is 
thoroughly  drained  and  the  filtrate  entirely  passed  through,  the  mor- 

N 

phine  crystals  are  dried,  and  -dissolved  in  25  cc.  of  —  hydrochloric 

10 

acid.  The  solution  is  poured  into  a  loo-cc.  flask,  the  filter  and  flask 
are  then  carefully  washed  and  the  solution  diluted  to  100  cc.  50  cc. 
of  this  solution  are  then  introduced  into  a  2oo-cc.  bottle  of  white  glass. 
50  cc.  of  water  are  added,  and  enough  ether  to  make  a  layer  i  cm. 
in  depth.  Then,  after  the  addition  of  five  drops  of  iodeosin  solution, 

N 
there   is  delivered    from   a  burette   —  potassium  hydroxid,  shaking 

thoroughly  after  each  addition  until  the  lower  aqueous  layer  assumes 
a  pale  red  color.  To  obtain  this  color  not  more  than  5.4  nor  less 
than  4.1  cc.  of  the  alkali  should  be  required.  This  method,  compared 
with  others,  has  been  found  to  give  results  which  are  i  to  1.2  per  cent 
low. 

A.  B.  Stevens'  Method.*  Take  4  gms.  of  opium  in  fine  powder 
and  triturate  in  a  mortar  with  2  gms.  of  fresh  burnt  lime  (not  air- 
slaked)  and  10  cc.  of  water  until  a  uniform  mixture  results.  Add 
19  cc.  of  water  and  stir  frequently  for  half  an  hour.  Filter  through  a 
dry  filter,  about  10  cm.  in  diameter.  Transfer  exactly  15  cc.  to  a 
6o-cc.  bottle.  To  this  add  4  cc.  of  alcohol  and  10  cc.  of  ether  and 
shake  the  mixture.  Then  add  0.5  gm.  of  ammonium  chlorid.  Shake 
well  and  frequently  during  half  an  hour.  Set  aside  in  a  cool  place 
for  twelve  hours.  Remove  the  stopper  carefully  and  preserve,  with 
any  adhering  crystals,  for  further  use.  Pour  the  ethereal  layer  into 
a  small  funnel,  the  neck  of  which  has  been  previously  closed  with  a 

*  Pharm.  Archiv.,  March,  1902,  41. 


572  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

piece  of  absorbent  cotton.  Rinse  the  bottle  with  10  cc.  of  ether,  and 
when  this  has  passed  through,  pour  the  contents  of  the  bottle  :nto 
the  funnel.  Without  trying  to  remove  all  the  crystals  from  the  bottle, 
\vash  the  bottle  and  contents  of  the  funnel  with  morphinated  water 
until  the  washings  are  colorless.  When  the  crystals  have  drained, 
place  the  funnel  in  the  bottle  containing  adhering  crystals,  and  with 
a  small  glass  rod  drawn  out  to  a  curved  point,  lift  the  cotton  and  rinse 
the  crystals  into  the  bottle  writh  12  cc.  of  decinormal  sulphuric  acid, 
using  the  cotton  on  the  end  of  the  rod  to  detach  any  adhering  crystals. 
Place  the  cotton  in  the  bottle,  replace  the  cork  and  agitate  until  the 
crystals  are  all  dissolved.  Rinse  the  cork  and  funnel  with  water  and 

N 

titrate  the  excess  of  acid  with  —  potassium  hydroxid.     The  number 

40 

of  cc.  of  decinormal  acid  consumed  by  the  morphine,  multiplied  by 
1.5038,  will  give  the  percentage  of  morphine  obtained.  To  this  add 
1. 1 2  for  the  morphine  remaining  in  solution. 

Prof.  L.  E.  Sayre,  who  has  had  considerable  experience  with  this 
method,  states  that  it  leaves  little  chance  for  error,  is  easy  of  manipu- 
lation, economical  in  point  of  time,  and  will  give  trustworthy  and 
concordant  results.  He  believes  that  the  process  may  be  much  short- 
ened by  allowing  the  mixture  of  opium  solution  and  ammonium  chlorid 
to  stand  four  hours,  instead  of  twelve,  in  a  cool  place. 

Dr.  Ph.  Ascher  *  gives  the  following  modification  of  Stevens' 
method,  which  he  found  to  give  results  within  a  few  milligrams  of  the 
quantity  of  morphine  originally  taken.  The  full  modified  process, 
showing  the  modifications  in  italics,  is  as  follows: 

Place  4  gms.  of  dried  or  powdered  opium  in  a  loo-cc.  tared  porce- 
lain evaporating-dish,  add  5  cc.  of  KOH  solution,  5  per  cent  or  its 
equivalent  of  a  stronger  solution,  mix  thoroughly  with  a  rubber-tipped 
glass  rod  and  evaporate  on  water -bath  or  drying  closet,  until  of  constant 
weight,  then  add  2  gms.  of  dry,  freshly-slaked  lime  and  10  cc.  of 
water  and  triturate  continually  for  fifteen  minutes  until  a  perfectly 
smooth  mixture  results.  Finally,  add  19  cc.  of  water,  triturating 
frequently  during  half  an  hour  and  filter  through  a  dry  filter  about 
10  cm.  in  diameter.  Transfer  exactly  15  cc.  to  a  loo-cc.  Erlenmeyer 
flask  and  add  to  this  4  cc.  of  alcohol  and  10  cc.  of  concentrated  ether 
and  shake  the  mixture.  Then  add  0.5  gm.  ammonium  chlorid.  Shake 
well  and  frequently  during  half  an  hour.  Set  aside  in  a  cool  place 
for  twelve  hours. 

Remove  the  stopper  carefully  and  preserve  with  any  adhering 
crystals  for  future  use.  Pour  the  ethereal  layer  into  a  small  funnel, 

*  A.  J.  Ph.,  1906,  262. 


ASSAY    OF    OPIUM  573 

the  neck  of  which  has  been  previously  closed  with  a  piece  of  absorbent 
cotton.  Rinse  the  flask  with  10  cc.  of  ether,  shake  continually  for 
five  minutes  and  pour  as  before  into  the  funnel,  and  when  this  has  passed 
through,  pour  the  contents  of  the  flask  into  the  funnel.  Add  5  cc.  of 
ether  to  the  flask,  rotate  gently  and  pour  into  funnel,  repeating  with  5  cc. 
more  of  ether.  Without  trying  to  remove  all  the  crystals  from  the 
bottle,  wash  the  flask  and  contents  of  the  funnel  with  saturated  solution 
of  morphin,  small  portions  at  a  time,  using  15  cc.  in  all.  When 
the  crystals  have  drained,  place  the  funnel  in  the  bottle  containing 
adhering  crystals,  and  with  a  small  rod 'drawn  out  to  a  curved  point, 
lift  the  cotton  and  rinse  the  crystals  into  the  bottle  with  12  cc.  of 
decinormal  sulphuric  acid,  using  the  cotton  on  the  end  of  the  rod 
to  detach  any  adhering  crystals.  Place  the  cotton  carefully  into  the 
flask,  replace  the  stopper  and  agitate  until  the  crystals  are  all  dis- 
solved. Rinse  the  cork  and  funnel  with  water,  and  titrate  the  excess 
of  acid  with  fortieth-normal  potassium  hydroxid  solution,  using  haema- 
toxylin  as  indicator. 

Divide  the  number  of  cc.  of  potassium  hydroxid  solution  used  by 
4  and  subtract  the  product  from  12  cc.  of  acid  used;  the  remainder 
will  be  the  amount  of  acid  consumed  by  the  morphine,  which  number, 
multiplied  by  1.5046,  and  the  addition  of  0.070  as  the  corrective  factor 
for  loss  of  morphine  during  estimation,  gives  the  percentage  of  morphine 
in  the  sample  under  examination.  The  corrective  factor  0.112,  given 
by  Stevens,  the  author  considers  too  high;  he  finds  0.070  to  be  nearer 
the  difference  between  the  results  obtained  and  the  amount  of  sub- 
stance originally  taken. 

The  use  of  potassium  hydroxid  in  this  process,  with  subsequent 
evaporation  to  dryness,  is  for  the  purpose  of  expelling  ammonia,  the 
presence  of  salts  of  which  in  opium,  Ascher  concludes,  interferes  with 
the  estimation  of  morphine  in  that  complete  solution  of  this  alkaloid 
by  treatment  with  lime  is  prevented. 

I.  Picard  *  suggests  the  following  modification  of  Leger's  method 
for  the  assay  of  opium:  6  gms.  of  opium  in  No.  120  powder,  dried 
at  60°  C.,  are  rubbed  down  with  a  very  little  lime  water  to  make  a 
soft  mass,  which  is  thoroughly,  worked.  The  remainder  of  48  cc.  of 
lime  water  is  then  added  so  as  to  form  a  homogeneous  mixture,  which 
is  carefully  covered  and  set  aside  for  two  hours.  50  cc.  of  5  per  cent 
solution  of  sodium  salicylate  is  then  mixed  in,  and  after  ten  minutes' 
contact  the  mixture  is  thrown  on  a  cloth  and  strained  with  expression. 
The  strained  liquid  is  then  filtered  through  a  small  filter  into  a  small 
flask  graduated  at  36  cc.  and  to  that  volume  of  filtrate  4  cc.  of  pure 

*  Bull.  Soc.  Pharm.  de  Bordeaux,  1906  (XLVI),  250;  and  Ph.  Jour.,  1907,  59. 


574  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

ether  is  added.  The  liquid  is  then  neutralized  with  dilute  solution  of 
ammonia,  added  drop  by  drop,  and  tested  after  each  addition  with 
litmus  paper.  When  neutral,  six  drops  in  excess  are  added,  the  flask 
is  corked,  shaken  for  ten  minutes  and  set  aside  for  twenty-four  hours. 
The  liquid  is  then  passed  through  two  counterpoised  niters,  any 
adhering  crystals  of  morphin  being  washed  off  the  flask  on  to  the 
filter  with  8  cc.  of  water.  The  beak  of  the  funnel  is  then  closed  with 
a  piece  of  India-rubber  tubing  carrying  a  pinch-cock.  The  filter 
and  funnel  are  then  filled  with  distilled  water  to  which  a  few  drops  of 
pure  ether  have  been  added.  After  five,  minutes'  contact  this  water 
is  run  off  and  the  washing  repeated  a  second  time  in  the  same  manner. 
The  precipitate  is  then  drained,  dried  at  100°  C.,  and  weighed.  If 
desired  the  dry  precipitate  may  be  washed  with  20  cc.  of  benzene  and 
again  dried  and  "weighed.  This  may  remove  a  trace  of  narcotine,  but, 
in  the  author's  opinion,  it  is  not  necessary. 

Thomas  Tickle  *  describes  a  new  process  for  the  morphiometric 
assay  of  opium,  the  essential  point  of  which  consists  in  the  employ- 
ment of  metacresol  as  a  solvent  of  the  alkaloid.  In  contact  with  a 
concentrated  solution  of  morphine,  cresol  readily  dissolves  the  liberated 
alkaloid  up  to  a  strength  of  40  per  cent,  whilst  amyl  alcohol  dissolves 
only  5  mg.  per  cc.  In  very  weak  solutions,  however,  the  cresol 
only  takes  up  about  twice  as  much  as  amyl  aclohol,  a  fact  due  to  the 
aqueous  layer  being  saturated  with  cresol,  and,  therefore,  a  stronger 
solvent  than  water  alone.  But  this  intermiscibility  of  solvent  and 
water  is  diminished  by  the  admixture  of  some  other  solvent,  notably 
amyl  alcohol.  The  general  process  outlined  by  the  author  for  the 
isolation  of  morphine  consists  in  liberating  the  alkaloid  contained  in 
100  cc.  of  solution  with  sodium  bicarbonate,  agitating  with  a  mixture 
of  pure  or  recently  distilled  cresol  (two  parts)  and  amyl  alcohol  (one 
part)  in  four  separate  fractions.  The  mixed  fractions,  totaling  30  cc., 
are  next  treated  with  15  cc.  of  ether,  which  has  the  curious  property 
of  annulling  all  tendency  of  the  solvent  to  retain  alkaloid;  then  30  cc. 
of  petroleum  ether  is  added  to  further  facilitate  the  extraction  by 
diluted  acetic  acid.  10  cc.  of  i  per  cent  acetic  acid  are  used  for  the 
first  shaking  out  of  the  morphine,  and  5  cc.  for  succeeding  extractions 
until  exhausted.  The  solution  of  morphine  acetate  thus  obtained  is 
evaporated  to  dryness,  taken  up  with  water  and  placed  in  a  covered 
vessel  side  by  side  with  an  open  beaker  containing  very  dilute  ammonia. 
The  morphine  solution  rapidly  absorbs  ammonia  vapor  and  deposits 
the  alkaloid  in  a  crystalline  state.  By  this  ingenious  procedure  there 
is  no  danger  of  introducing  excess  of  ammonia  in  which  the  alkaloid 

*  Ph.  Jour.,  Feb.  16,  1907,  162-164. 


ASSAY    OF    PHYSOSTIGMA  575 

is  more  soluble  than  in  water.     The  crystals  thus  obtained  are  dried 
at  110°  C.  and  weighed. 

LITERATURE  ON  OPIUM  ASSAYING 

J.  Perger.     J.  Chem.  Soc.,  1884,  701. 

J.  H.  Wainwright.     J.  A.  C.  S.,  1885  (Vol.  7),  48. 

P.  C.  Plugge.     Archiv.  de  Ph.,  1887,  343. 

E.  Dieterich.     Helfenberger  Annalen,  1887,  54. 

D.  B.  Dott.     Ph.  Jour.,  1888,  701. 

E.  R.  Squibb.     Ephemeris,  1889  (Vol.  in),  1150-1161. 
Fliickiger.    A.  J.  Ph.,  1890,  14. 

J  B.  Nagelvoort.     A.  J.  Ph.,  1800,  407. 
Beckurts.     Apoth.  Ztg.,  1891,  526. 
D.  B.  Dott.     Britt.  and  Col.  Drug.,  1894,  372. 
D.  B.  Dott.     Ph.  Jour.,  1894,  847. 
D.  B.  Dott.     Ph.  Jour.,  1895,  497. 
Farr  and  Wright.     Ph.  Jour.,  1897,  202. 

Montemartini  and  Trasciatti.     J.  Chem.  Soc.,  1898,  271  (Vol.  n). 
Gordin  and  Prescott.     J.  A.  C.  S.,  1898,  725. 

Lyons.     "Assaying  of  Drugs  and  Galenicals."     Nelson  Baker    &  Co. 
1899. 

Lamar.     A.  J.  Ph.,  1900,  36. 

Gordin  and  Prescott.     Proc.  A.  Ph.  A.,  1900,  126. 

L.  E.  Sayre.     Drug.  Cir.,  Sept.,  1901,  180. 

A.  B.  Stevens.     Pharm.  Arch.,  March,  1902,  41. 

H.  E.  Matthews.     Trans.  Britt.  Ph.  Conf.,  1903,  570. 

P.  L.  Aslanoglon.     Chem.  News.,  Dec.,  1903,  286. 

C.  E.  Caspari.     Proc.  A.  Ph.  A.,  1904,  386. 
Ph.  Schidrowitz.     Analyst,  March,  1904. 

L.  F.  Kebler.     Proc.  A.  Ph.  A.,  1904,  369-375. 
Leo  Eliel.     Proc.  Ind.  Ph.  A.,  1906. 
Ph.  Ascher.     A.  J.  Ph.,  1906,  262. 
I.  Picard.     Ph.  Jour.,  1907,  59. 

D.  B.  Dott.     Ph.  Jour.,  1907,  78. 
Thomas  Tickle.     Ph.  Jour.,  1907,  162. 

C.  E.  Parker.     Proc.  A.  Ph.  A.,  1907,  490. 

Bulletin  No.  107,  Bureau  of  Chem.  U  S.  Dept.  of  Agric. 

Allen's  "Commercial  Organic  Analysis. 

ASSAY  OF  PHYSOSTIGMA  (Calabar  bean) 

This  drug  contains  three  alkaloids, namely,  physostigmine  (also  called 
eserine),  eseridine,  and  calabarine.  The  first  of  these  is  the  active 
principle,  which  together  with  eseridine  is  readily  removed  from  an 
aqueous  solution  by  treatment  with  ammonia  or  alkali  bicarbonate, 
and  shaking  out  with  ether.  Calabarine  being  insoluble  in  ether  is 
left  behind.  All  three  alkaloids  are  soluble  in  chloroform. 


576  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  U.  S.  P.  VIII  Method.  Introduce  20  gms.  of  physos- 
tigma  (in  No.  60  powder)  into  an  Erlenmeyer  flask  of  about  250  cc. 
capacity,  add  200  cc.  of  ether,  and  shake  the  flask  well  during  ten 
minutes.  Then  add  10  cc.  of  an  aqueous  solution  of  sodium  bicar- 
bonate (i :  20),  and  shake  the  mixture  vigorously  at  intervals  during 
four  hours.  Allow  the  powder  to  settle,  and  decant  100  cc.  of  the 
ether  solution  (representing  10  gms.  of  physostigma)  into  a  measuring 
cylinder;  then  transfer  it  to  a  separator,  introduce  a  small  piece  of 
blue  litmus  paper,  and  add  sufficient  normal  sulphuric  acid  V.  S.  to 
render  the  liquid  acid,  and  then  10  cc.  of  distilled  water.  Shake  the 
liquid  wrell  for  several  minutes,  and  draw  off  the  aqueous  layer  into 
another  separator.  Repeat  the  extraction,  using  2  cc.  of  normal 
sulphuric  acid  V.  S.  and  8  cc.  of  distilled  water,  add  the  acid  aqueous 
layer  to  the  second  separator,  and  finally  again  shake  out  the  ether 
solution,  using  i  cc.  of  normal  sulphuric  acid  V.  S.  and  9  cc.  of  dis- 
tilled water,  adding  this  also  to  the  second  separator.  To  the  com- 
bined acid  liquids  in  the  second  separator,  add  25  cc.  of  ether,  a  small 
piece  of  red  litmus  paper,  and  sufficient  sodium  bicarbonate  solution 
(i :  20)  to  render  it  alkaline.  Shake  the  separator  for  one  minute, 
allow  the  liquids  to  separate,  and  draw  off  the  ether  into  a  beaker. 
Repeat  the  shaking  out  process  with  20  cc.  and  again  with  15  cc.  of 
ether  added  to  the  separator,  shake  each  time  for  one  minute,  allow 
the  liquids  to  separate,  and  draw  off  the  ether  into  the  beaker.  Care- 
fully evaporate  the  ether  from  the  combined  solutions  by  means  of  a 
water-bath,  and  when  dry,  dissolve  the  residue  in  5  cc.  of  tenth-normal 
sulphuric  acid  V.  S.  and  20  cc.  of  ether,  which  must  be  strictly  neutral, 
and  transfer  this  solution  to  a  bottle,  rinsing  with  80  cc.  of  water; 
add  five  drops  of  iodeosin  T.  S.,  and  titrate  the  excess  of  acid  with 
fiftieth-normal  potassium  hydroxid  V.  S.,  until,  after  shaking,  the 
aqueous  liquid  just  acquires  a  pink  color.  Divide  the  number  of  cc. 
of  fiftieth -normal  potassium  hydroxid  V.  S.  used  by  5,  subtract  the 
quotient  from  5  (the  5  cc.  of  tenth-normal  sulphuric  acid  V.  S.  taken), 
and  multiply  the  remainder  by  0.0273,  and  this  product  by  10;  the 
result  will  be  the  percentage  of  alkaloids  soluble  in  ether  contained 
in  the  physostigma.  The  figure  0.0273  represents  the  weight  in  grams 
of  alkaloids  (mainly  physostigmine)  required  to  neutralize  i  cc.  of 
tenth-normal  sulphuric  acid  V.  S. 

H.  Beckurts  *  recommends  the  following  method  for  assaying 
calabar  beans:  20  gms.  of  the  powdered  drug  (sieve  v  of  the  G.  P., 
IV)  are  shaken  frequently  during  three  hours  with  120  gms.  of  ether 
and  10  cc.  of  a  10  per  cent  solution  of  potassium  bicarbonate;  then 

*  Apoth.  Ztg.,  XX,  No.  67  (1905),  670. 


ASSAY   OF  PILOCARPUS  LEAVES  577 

90  gms.  of  the  ether  solution  are  filtered,  one  half  of  the  ether  is 
distilled  off,  and  the  residue  transferred  to  a  separator  by  the  aid  of 
some  of  the  recovered  ether.  After  adding  10  cc.  of  petroleum  benzene 
to  prevent  subsequent  emulsification,  the  ether  solution  is  shaken 

N 

out  with  10  cc.,  and  then  thrice  successively  with  5  cc.  of  —  HC1; 

10 

the  acid  liquids  are  united,  45  gms.  of  ether  and  10  cc.  of  a  10  per 
cent  solution  of  potassium  bicarbonate  are  added,  and  the  mixture, 
after  several  vigorous  shakings,  is  allowed  to  separate.  Then  30  gms. 
of  the  ether  solution  (corresponding  to  10  gms.  of  the  drug)  are  mixed 

N 

with    10  cc. HC1,   20  cc.   of  water,   and   five   drops   of  alcoholic 

100 

iodeosin  solution,  and  the  excess  of  acid  is  determined  by  titration 

N  N 

with NaOH.     Each  cc.  of HC1  corresponds  to  0.00273  2m.  of 

100  100 

physostigmine.  Under  the  conditions  of  this  assay  method,  calabarine, 
being  insoluble  in  ether,  is  excluded,  while  the  alkaloids  extracted  by 
the  ether — physostigmine  and  eseridine — 4o  not  undergo  unfavorable 
change  during  twenty-four  hours.  Caustic  alkalies,  as  wrell  as  carbo- 
nates, cannot  be  used  for  the  shaking  out  of  these  alkaloids  from  their 
acid  solutions  because  they  decompose  the  bases,  the  solution  assuming 
a  red  color.  If  ether  and  then  sodium  or  potassium  bicarbonate  is 
added  to  the  alkaloidal  solution  no  decomposition  occurs.  There  is 
no  troublesome  emulsification  produced  during  the  direct  assay  of 
the  beans,  but  when  applying  the  method  to  the  assay  of  the  extract 
there  is  considerable  annoyance  from  this  source,  while  the  color 
reaction  of  the  indicator,  which  is  quite  sharp  when  operating  with 
the  drug,  is  less  distinct  when  assaying  the  extract,  this  being  probably 
due  to  a  partial  decomposition  of  alkaloid  during  its  preparation. 

LITERATURE 

W.  A.  H.  Naylor.     Britt.  and  Col.  Drugg.,  XLVIII,  77. 
H.  M.  Gordin.     Proc.  A.  Ph.  A.,  1906,  380. 
Lyons.     "Assaying  of  Drugs  and  Galenicals." 

ASSAY  OF  PILOCARPUS  LEAVES  (Jaborandi) 

The  leaves  of  several  species  of  pilocarpus  are  found  in  the  market 
and  sold  as  Jaborandi.  The  alkaloidal  content  of  these  is  variable 
and  often  below  the  official  requirement.  Two  species  of  pilocarpus, 
namely,  P.  Jaborandi  and  P.  Microphyllis,  are  official  in  the  U.  S.  P., 
in  which  the  leaves,  when  assayed  by  the  official  process,  should  yield 
not  less  than  0.5  per  cent  of  the  alkaloids. 


578  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Several  alkaloids  are  present  in  this  drug.  Of  these,  pilocarpine 
is  the  most  important  and,  in  fact,  the  one  upon  which  alone  the  thera- 
peutic value  of  the  drug  depends.  The  others  are  isopilocarpine  and 
pilocarpidine. 

The  U.  S.  P.  and  other  methods  recommended  for  the  assay  of 
this  drug  all  depend  upon  a  determination  of  the  total  alkaloidal 
content.  This  information  is  not  considered  of  great  value,  since 
it  gives  no  indication  of  the  amount  of  the  principal  alkaloid  (pilo- 
carpine) present,  but  until  a  more  satisfactory  method  is  found  we 
must  be  content  with  such  as  we  have. 

The  U.  S.  P.  Method.  This  is  based  upon  the  method  pro- 
posed by  Lyons  and  is  as  follows:  Moisten  10  gms.  of  pilocarpus 
(in  No.  60  powder)  with  2  cc.  of  ammonia-water  and  3  cc.  of  chloro- 
form, and  at  once  pack  it  firmly  in  a  small  cylindrical  percolator, 
which  has  been  provided  with  a  pledget  of  cotton  packed  firmly  in 
the  neck.  Percolate  the  powder  slowly  with  chloroform  containing 
about  2  per  cent  of  ammonia -water,  until  it  is  exhausted,  about  100  cc. 
of  menstruum  usually  being  sufficient.  Pour  into  a  separator  the 
percolate,  and  shake  it  out  with  15  cc.  of  normal  sulphuric  acid  V.  S., 
transferring  the  acid  aqueous  layer  to  another  separator,  and  repeat- 
ing the  shaking  out  of  the  chloroform  solution  with  2  cc.  of  normal 
sulphuric  acid  V.  S.,  mixed  with  8  cc.  of  distilled  water.  Add  the 
acid  layer  to  the  second  separator,  and  again  repeat  the  shaking  out 
with  10  cc.  of  distilled  water,  and  add  the  aqueous  liquid  to  the  second 
separator.  Introduce  into  the  second  separator  a  small  piece  of  red 
litmus  paper,  add  enough  ammonia -water  to  render  the  liquid  alkaline 
and  shake  out  the  liquid  with  20  cc.  of  chloroform,  drawing  off  the 
chloroformic  solution  into  a  beaker.  Repeat  the  shaking  out  with 
two  portions  of  15  and  10  cc.  each  of  chloroform,  and  add  the  chloro- 
formic solutions  to  the  beaker.  Evaporate  the  chloroform  by  means 
of  a  water-bath,  and  dissolve  the  alkaloidal  residue  in  7  cc.  of  tenth- 
normal  sulphuric  acid  V.  S.  Add  five  drops  of  cochineal  T  S.  or 
iodeosin  T.  S.,  and  titrate  the  excess  of  acid  with  fiftieth-normal 
potassium  hydroxid  V.  S.  Divide  the  number  of  cc.  of  fiftieth-normal 
potassium  hydroxid  V.  S.  used  by  5,  subtract  the  quotient  from  7 
(the  7  cc.  of  tenth-normal  sulphuric  acid  V.  S.  taken),  and  multiply 
the  remainder  by  0.02,  and  this  product  by  10;  the  result  will  be  the 
percentage  of  alkaloids  contained  in  the  pilocarpus.  The  figure  0.02 
represents  the  weight  in  grams  of  alkaloids  (mainly  pilocarpine)  required 
to  neutralize  i  cc.  of  tenth-normal  sulphuric  acid  V.  S. 

G.  Fromme  *  recommends  the  following  method  for  the  assay  of 

*  Pharm.  Rev.,  Nov.,  1901,  504;  from  Caesar  and  Loretz,  Geschafts-Bericht 
for  1901 


ASSAY  OF  STROPHANTHUS  579 

jaborandi  leaves:  15  gms.  of  the  leaves,  in  moderately  fine  powder, 
are  macerated  in  150  gms.  of  chloroform  and  15  gms.  of  ammonia- 
water  for  half  an  hour,  under  frequent  agitation;  the  whole  is  then 
transferred  to  a  plain  filter  and  covered  with  a  glass  plate,  the  filtration 
towards  the  end  being  hastened  by  pouring  a  little  water  on  the  dregs. 
When  a  little  more  than  100  gms.  of  filtrate  has  been  obtained,  this 
is  shaken  with  i  cc.  of  water  and  set  aside  until  any  fine  powder  which 
may  have  passed  through  the  filter  has  been  taken  up  by  the  water 
and  the  chloroformic  solution  is  perfectly  clear.  Then  100  gms.  of 
this  solution,  representing  10  gms.  of  the  drug,  are  extracted  succes 
sively  with  30,  20,  and  10  cc.  of  i  per  cent  hydrochloric  acid;  the 
united  acid  extraction  is  shaken  with  20  cc.  of  ether  to  remove  chlo- 
rophyl,  fat,  and  resin,  and,  the  acid  layer  being  drawn  off,  it  is  rendered 
alkaline  by  ammonia  and  shaken  out  successively  with  30,  20,  and 
10  cc.  of  chloroform.  The  chloroform  is  evaporated  and  the  residue 

N 
weighed  or  titrated  as  may  be  most  expedient — i  cc.  of hydrochloric 

acid  being  eaual  to  0.00208  gm.  of  pilocarpine. 

LITERATURE 

H.  A.  D.  Jowett     Britt.  Pharm.  Conf.,  1899. 
Farr  and  Wright.     Britt.  Pharm.  Conf.,  1899. 
Dohme  and  Engelhardt.     Drug.  Cir.,  Feb.,  1900,  28. 
H.  M.  Gordin.     Proc.  A.  Ph.  A.,  1906,  380, 
E.  W.  Mann.     Britt.  and  Col.  Drug.,  XL,  493. 
W.  A.  H.  Naylor.     Britt.  and  Col.  Drug.,  XLVIII,  78. 

ASSAY  OF  STROPHANTHUS 

Strophanthus  contains  the  glucosid  strophanthin,  which  is  its  active 
principle.  This  glucosid  is  soluble  in  water,  less  soluble  in  alcohol, 
and  almost  insoluble  in  chloroform  or  ether.  It, is  decomposed  by 
mineral  acids,  strophanthidin  and  glucose  resulting.  Strophanthidin 
dissolves  in  alcohol  and  in  chloroform,  but  is  almost  insoluble  in  water. 

Barclay's  Method  (Ph.  Jour,  and  Trans.,  Nov.  28, 1896).  Accord- 
ing to  Barclay,  if  an  alcoholic  extract  of  the  drug  be  exhausted  with 
water  and  the  aqueous  extract  thus  obtained  is  in  turn  exhausted 
with  absolute  alcohol,  the  product  consists  almost  wholly  of  stro- 
phanthin. The  method  of  assay  which  he  proposes  depends  upon 
the  conversion  of  the  whole  of  the  strophanthin  into  strophanthidin, 
thereby  avoiding  the  possible  loss  occasioned  by  partial  splitting  up 
of  the  glucosid  during  the  process  of  extraction.  From  the  strophan- 
thidin obtained  it  is  easy  to  determine  the  amount  of  strophanthin  in 
the  drug.  The  seeds  are  extracted  with  alcohol,  most  of  the  alcohol 


580  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

recovered  by  distillation,  water  is  added,  and  the  remainder  of  the 
alcohol  driven  off  by  gentle  heat,  and  to  the  aqueous  mixture  chloro- 
form is  added  to  extract  the  oils  and  fats  present.  The  chloroform 
layer  is  then  removed  and  the  aqueous  liquid  acidified  with  dilute 
sulphuric  acid  and  heated  for  one  hour  on  a  water-bath;  the  resulting 
turbid  solution  is  shaken  out  several  times  in  a  separator  with  portions 
of  chloroform  until  the  chloroform  removes  nothing  further  from  the 
solution.  The  combined  chloroform  solutions  are  evaporated  care- 
fully and  the  residue,  consisting  of  strophanthidin,  carefully  dried  at 
a  temperature  not  exceeding  150°  F.  (65.5°  C.)  and  weighed.  The 
weight  multiplied  by  the  factor  2.74  gives  the  amount  of  strophanthin 
originally  present  in  the  drug.  In  the  above  assay  operate  on  12  gms, 
of  the  drug  finely  powdered.  Macerate  about  twelve  hours,  with 
frequent  shaking,  with  180  cc.  of  alcohol  U.  S.  P.  Pour  off  150  cc. 
of  the  tincture,  representing  10  gms.  of  the  drug,  and  remove  the 
alcohol  by  evaporation  on  a  water-bath  or  by  distillation.  To  the 
residue  add  30  cc.  of  water,  transfer  the  aqueous  mixture  to  a  separator, 
shake  out  with  two  successive  portions  of  chloroform  (10  cc.  each 
time)  to  remove  fatty  substances.  Transfer  the  aqueous  liquid  to  a 
flask,  add  i  cc.  of  dilute  sulphuric  U.  S.  P.,  and  heat  on  a  water-bath 
for  one  hour,  having  connected  the  flask  with  a  reflux  condenser, 
The  resulting  turbid  liquid  is  then  shaken  out  in  a  separator  with 
three  successive  portions  of  chloroform  (10  cc.  each).  The  combined 
chloroform  solution  is  evaporated  at  150°  F.  (65.5°  C.),  the  residue 
weighed  and  multiplied  by  2.74  to  find  amount  of  strophanthin  in 
10  gms.  of  the  drug. 

LITERATURE 

G.   Fromme.      Report  of  Caesar  and  Loretz,  and  Pharm.  Ztg.    (1005), 
LXXIII,  772. 

A.  R.  L.  Dohme.    Drug.  Cir.,  1900,  132. 

ASSAY   OF    TOBACCO 

According  to  the  method  of  C.  C.  Keller  (Schweiz.  Wochenschr.  f. 
Pharm.,  1899,  309),  6  gms.  of  the  dry  tobacco  are  shaken  vigorously  in 
a  200-cc.  flask  with  60  cc.  each  of  ether  and  petroleum  ether,  10  cc 
solution  of  potassa  (20  per  cent),  and  the  shaking  repeated  during 
half  an  hour.  After  the  mixture  is  allowed  to  stand  during  three  or  four 
hours  loo  cc.  of  the  solution  are  filtered  through  a  small  plaited  filter  into 
a  loo-cc.  flask  and  a  strong  current  of  air  is  passed  through  the  filtrate 
in  order  to  drive  off  the  ammonia  (a  small  quantity  of  which  is  liberated 
during  the  process).  After  ammonia  has  been  completely  driven  off 
(as  shown  by  litmus  paper,  held  in  the  air,  which  has  passed  through 


ASSAY   OF  VERATRUM  581 

the  solution  no  longer  turning  blue)  10  cc.  of  alcohol  and  one  drop  of 
iodeosin  in  10  cc.  of  water  are  added,  the  flask  is  stoppered,  and  the 
contents  are  vigorously  shaken.  The  nicotine  and  iodeosin  are  taken 

N 
up  by  the  water  which  separates  of  a  red  color;   —  hydrochloric  acid 

(say  7  cc.)  is  then  added,'  followed,  if  necessary,  by  further  additions 
of  i  cc.  at  a  time  until  the  red  color  of  the  aqueous  layer  is  discharged; 

N 

the  excess  of  acid  is  then  determined  by  —  ammonia,  the  volume 

10 

N 
required  being  deducted  from  the  volume  of  :  -  hydrochloric  acid, 

to  ascertain  the  quantity  of  the  latter  required  to  neutralize  the  nicotine. 

N 

Each  cc.  of  —  hydrochloric  acid  corresponds  to  0.0162  gm.  of  nicotine, 
10 

and  the  total  quantity  of  the  alkaloid  is  then  ascertained  by  simple 
multiplication  with  the  number  of  cc.  of  acid  consumed.  The  product, 
multiplied  by  20,  gives  the  percentage. 

LITERATURE 

McCrae.  Chem.  Ztg.,  xxxi  (1907),  45,  and  Pharm.  Jour.,  March,  1907, 
265. 

Ame  Pictet.     Archiv.  d.  Pharm.,  Sept.,  1906,  375. 

Pictet  and  Rotschy.     Chem.  Zeitschr.,  XLVI,  p.  118. 

R.  Kissling.  Zeitschr.  f.  anal.  Chem.,  xxi,  No.  i;  and  Proc.  A.  Ph.  A., 
1882,  167. 

ASSAY  OF  VERATRUM 

There  are  several  species  of  veratrum  and  they  contain  a  number 
of  alkaloids,  the  nature  of  which  is  as  yet  imperfectly  known.  G. 
Bredemann  shows  that  veratrum  album  contains  four  alkaloids,  namely, 
protoveratrine  (C32H5iNOn),  jervine  ^eHsyNOs),  pseudojervine 
(C29H43NO7),  and  rubijervine  (C26H43NO2).  The  same  alkaloids  are 
found  in  veratrum  viride,  but  the  proportion  in  which  they  exist  differs 
in  the  different  species.  The  most  characteristic  alkaloid  is  probably 
jervine,  but  in  the  present  state  of  knowledge,  this  alkaloid  cannot 
readily  be  separated.  The  therapeutic  value  of  a  sample  of  the  drug 
cannot  therefore  be  accurately  determined  by  assay.  A  determination 
of  the  total  alkaloids  is  the  best  that  can  be  done  at  present. 

Bredemann  *  proposes  the  following  method:  12  gms.  of  powdered 
drug  are  rotated  with  120  cc.  of  a  mixture  of  equal  parts  of  chloro- 
form and  ether,  then  10  cc.  sodium  hydroxid  solution  is  added  and 
the  mixture  shaken  frequently  during  three  hours;  then  sufficient 

*  Apoth.  Ztg.,  1906,  XXI,  41-53- 


582  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

water  is  added  to  cause  coherence  of  the  drug.  The  ether-chloroform 
solution,  always  more  or  less  turbid,  is  decanted  as  completely  as 
possible,  shaken  with  magnesium  oxid  and  three  or  four  drops  of 
water,  and  then  the  liquid  poured  through  a  dry  filter  and  100  cc. 
of  the  transparent  nitrate  corresponding  to  10  gms.  drug  collected. 
The  chloroform-ether  solution  is  extracted  three  times  with  20  cc. 
of  water  containing  acetic  acid.  The  united  acetic  acid  extractions 
are  made  alkaline  with  sodium  hydroxid  and  extracted  three  times 
with  a  mixture  of  equal  parts  of  chloroform  and  ether.  The  ether- 
chloroform  solution  is  driven  off,  the  alkaloidal  residue  dried  at  100°  C. 
and  weighed.  For  the  estimation  of  alkaloid  in  the  tincture  it  is 
proposed  that  100  gms.  tincture  be  concentrated  to  one-half  its  volume 
on  a  water-bath.  Then  i  gm.  paraffin  and  about  25  cc  water  are 
added,  and  the  liquid  warmed  until  all  alcohol  has  been  expelled. 
To  the  warm  liquid  2  gms.  acetic  acid  are  added  and  the  liquid  allowed 
to  cool  with  frequent  stirring.  It  is  then  filtered  into  a  separator 
through  a  small  wet  filter.  The  paraffin  and  oil  which  remain  on  the 
filter  are  heated  on  a  water-bath  with  20  cc.  water  and  i  gm.  acetic 
acid  until  the  paraffin  melts.  The  liquid  is  then  allowed  to  cool, 
passed  through  the  filter  first  used,  and  the  dish  and  filter  washed 
thoroughly  with  water.  The  united  filtrates  are  made  alkaline  with 
sodium  hydroxid  and  extracted  with  20,  10,  and  10  cc.  chloroform. 

Volumetric  Method  of  Estimation.  According  to  G.  Bredemann 
the  alkaloids  of  Veratrum  album  may  be  conveniently  and  accurately 
estimated  by  the  volumetric  method,  employing  for  this  purpose 
Keller's  method,  with  some  insignificant  modifications.  He  has 

N 
determined  that  i  cc.  of  -  hydrochloric  acid  requires  for  saturation: 

0.00625  gm  ..............  Protoveratrine  (C32H51NOn) 

0.00411  gm  ..............  Jervine 

0.00517  gm  ..............  Pseudo  jervine 

0.00401  gm  ..............  Rubi  jervine 


Furthermore,  he  has  obtained  experimental  data  which  justify  him  in 
assuming  the  average  molecular  weight  for  the  total  alkaloids  to  be 
424,  and  that  therefore  0.00424  gm.  of  the  latter  correspond  to  i  cc. 

N 
of  the  -  acid.     In  the  practical  determinations  applied  to  thirty-six 

specimens  of  veratrum  rhizomes  (both  V.  album  and  V.  viride)  he 
found  variations  of  from  0.19928  to  0.93280  total  alkaloid.  In  two 
cases  he  found  the  rootlets  to  have  a  higher  alkaloid  content  than  the 
rhizome,  but  to  these  the  average  molecukr  weight  cannot  be  applied, 


ASSAY   OF   WILD   CHERRY   BARK  583 

since  they  contain  a  larger  proportion  of  protoveratrine,  and  therefore 
must  be  assumed  to  have  a  higher  molecular  weight. 

LITERATURE 

Wright  and  Luff.     Ph.  Jour,  and  Trans.,  May  31,  1879. 

Salzberger.    Archiv.  d.  Pharm.  (1891),  228,  462. 

Kremel.     Ph.  Post.  (1889),  227. 

Bullock.     A.  J.  Ph.  (1879),  338. 

F.  T.  Gordon.     Proc.  Pa.  Pharm.  Assoc.,  1901,  125. 

ASSAY   OF  WILD    CHERRY    BARK 

(A.  B.  Stevens,  Proceedings  A.  Ph.  A.,  1896,  215).  10  gms.  of  the 
ground  bark  are  macerated  in  100  cc.  of  water  for  twenty-four  hours, 
then  distilled  and  the  distillate  containing  the  hydrocyanic  acid  is 
passed  into  a  decinormal  solution  of  potassium  hydroxid. 

The  alkaline  solution  of  potassium  cyanid  is  then  titrated  with 
decinormal  silver  nitrate  in  the  usual  manner. 

Dr.  A.  R.  L.  Dohme  (Pharm.  Runds.,  XHI,  260}  distils  the  hydro- 
cyanic acid  by  passing  live  steam  into  the  flask  containing  the  bark 
and  water,  instead  of  using  direct  flame;  otherwise  the  process  is  the 
same  as  the  above,  except  that  he  adds  o.i  gm.  of  NaCl  to  the  distillate 
before  titrating.  This  is  deducted  from  the  final  result. 

Another  method  consists  in  distilling  by  means  of  live  steam  and 

N 
receiving  the  distillate  in  —  silver  nitrate  solution.     The  distillation 

is  known  to  be  complete  when,  upon  agitating  the  receiver,  the  dis- 
tillate no  longer  produces  a  precipitate  in  the  silver  solution.  The 
excess  of  silver  nitrate  is  then  estimated  by  Volhard's  method. 


CHAPTER  LIT 

ASSAY  OF  GALENICAL  PREPARATIONS 

(i)  J.  U.  Lloyd's  Methods,  i  gm.  of  a  solid  extract  which 
has  been  dissolved  in  5  to  8  cc.  of  an  alcoholic  menstruum  or  a  corre- 
sponding volume  of  the  tincture  evaporated  to  this  bulk,  or  5  cc.  of 
the  fluid  extract,  are  mixed  in  a  flat-bottomed  porcelain  mortar  with 
2  cc.  of  a  solution  of  perchlorid  of  iron.  To  this  is  added  sodium 
bicarbonate  with  constant  trituration  until  a  stiff  magma  results.* 
This  magma  is  extracted  by  repeated  trituration  with  chloroform,  using 
first  20  cc.  and  then  three  portions  of  10  cc.  each,  decanting  them 
severally  by  means  of  a  guiding-rod,  being  careful  that  no  suspended 
portions  of  the  magma  are  drawn  off.  In  order  to  make  sure  that  all 
of  the  alkaloid  has  been  extracted,  add  5  cc.  more  of  chloroform,  draw 
it  off,  evaporate  on  a  watch-glass,  dissolve  residue  in  dilute  sulphuric 
acid,  and  test  for  alkaloids  by  Wagner's  or  Mayer's  reagent. 

The  mixed  chloroformic  extracts  are  collected  and  may  be  esti- 
mated volumetrically  as  follows: 

Method  A.    To  be  used  if  the  chloroformic  extract  is  not  colored. 

The  chloroformic  solution  is  evaporated  to  dryness  in  a  flask  placed 
on  a  water-bath.  To  this  residue  is  added  an  accurately  measured 

N 
excess  of  —  sulphuric  acid   and   the   solution    diluted  with  a  little 

water,  the  indicator  added  and  the  excess  of  standard  acid  solution 

N 
estimated  by  titrating  with  —  potassium  hydroxid  V.  S.     The  number 

N  5° 

of  cc.  of  the  —  alkali  V.  S.  used,  divided  bv  2  and  subtracted  from 
So 

N 
the  volume  of  -  -  acid  V   S.  originally  added,  will  give  the  number 

*  The  ferric  hydroxid  which  is  produced  in  the  above  process  serves  to  attract 
most  of  the  tannates,  gums,  vegetable  acids,  and  coloring  matters,  while  the 
excess  of  sodium  bicarbonate  liberates  the  alkaloids,  which  are  then  dissolved 
by  the  chloroform.  If  the  fluid  extract  is  strongly  alcoholic  the  chloroform  will 
not  separate  easily,  in  which  case  the  addition  of  a  few  cc.  of  water  containing 
a  very  little  glucose  will  cause  a  sharp  separation. 

584 


ASSAY   OF  GALENICAL  PREPARATIONS  585 

of  cc.  of  the  latter  required  for  the  alkaloid.  This  number,  multiplied 
by  the  proper  factor,  will  give  the  total  alkaloid  present  in  the  fluid 
extract. 

Example.     The   chloroformic   residue   obtained    from    5  cc.    of   a 

N 
fluid  extract  of  hyoscyamus  was  dissolved  in  12  cc.  of  —  acfd  V.  S., 

N  25 

the  solution  titrated  with  —  alkali  V.  S.,  20.6  cc.  of  the  latter  were 

used. 

N  N 

20.6  cc.  of  —  V.  S.  is  the  equivalent  of  10.3  cc.  of  —  V.  S.     10.3  cc. 

5°  N  .  2* 

subtracted  from  12  cc.,  the  amount  of  —  acid  originally  added,  leaves 

N  2? 

1.7  cc.,  the  quantity  of  —  acid  V.  S.  which  was  required  for  the  alka- 

N 

loid.  This  multiplied  by  the  —  factor  for  total  alkaloids  of  hyoscy- 
amus, 0.011475  gm.,  gives  the  quantity  of  alkaloids  present  in  the 
5  cc.,  which  quantity  multiplied  by  20  gives  the  per  cent: 

1.7X0.011475  =  0.0194975  gm.X  20=  0.3899  per  cent. 

Method  B.  This  method  may  be  employed  if  the  chloroformic 
extract  is  highly  colored,  the  indicator  fluorescin  being  used. 

The  residue  from  the  evaporation  of  chloroform  is  dissolved  in  10  cc. 
of  acid-free  alcohol;  then  water  is  added  to  slight  turbidity,  followed 

N 
by  a  measured  excess  of  —  acid  V.  S.;  then  the  titration  is  completed 

N  2^ 

with  —  alkali  V.  S.    The  first  appearance  of  fluorescence  marks  the 

50 

completion  of  the  reaction.  This  is  best  observed  by  holding  the 
flask  over  a  dark  surface  and  viewing  by  reflected  light. 

Method  C.  This  is  to  be  used  in  the  case  of  highly  colored 
extracts.  It  consists  in  removing  the  alkaloid  in  a  pure  state  by 
shaking  out  in  a  separating  funnel  with  immiscible  solvents.  The 
chloroformic  extract  is  shaken  out  with  several  portions  of  acidulated 
water  (1:50);  this  removes  the  alkaloid,  leaving  resins,  fats,  coloring 
matters,  etc.,  in  the  chloroform.  The  acid  alkaloidal  solution  is  then 
treated  with  ether  in  a  second  separator,  ammonia  added  to  alkaline 
reaction,  and  the  alkaloid  thus  liberated  by  ammonia  dissolves  in  the 
ether,  from  which  it  is  obtained  by  evaporation  and  estimated  acidi- 
metrically. 

(2)  Lyon's  Method.*    Fluid  Extracts.    Add  ammonia  to  the  fluid 


*  "Assay  of  Drugs  and  Galenicals.' 


586  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

extract  and  shake  out  directly  with  an  immiscible  solvent;  wash  the 
alkaloid  from  this  by  means  of  acidulated  water.  Separate  the  solu- 
tions, add  ammonia  to  the  acid  solution  and  shake  out  with  the  appro- 
priate solvent.  Finally,  evaporate  this  alkaloidal  solution  on  a  water- 
bath  and  estimate  the  alkaloid  in  the  residue  alkalimetrically. 

(3)  Steiglitz's   Method.     Add  ten  to  twenty  times  its  volume  of 
weak  Prollius'  mixture,  shake  together  and  let  stand  for  some  time. 
Pour  off  an  aliquot  part  of  the  ethereal  liquid,  put  in  a  separator  and 
wash  out  with  acidulated  water.     Separate  the  latter,  add  ammonia 
in  slight  excess  and  shake  out  with  the  appropriate  solvent. 

(4)  F.  A.  Thompson's  Method.*     Into  a  capsule  place  about  5  gms. 
of  oak  sawdust,  pour  into  this  gradually  10  cc.  of  the  fluid  extract, 
mix  thoroughly,  and  dry  at  about  45°  C.  (115°  F.),  transfer  to  a  flask 
and  add   100  cc.  of   modified  Prollius'   mixture.     Shake  frequently 
during  twenty  minutes,  pour  off  exactly  50  cc.  of  the  ethereal  liquid, 
and  set  aside  in  a  warm  place  until  the  ether  has  nearly  all  evaporated. 

N 
Then  add  5  cc.  of  —  sulphuric  acid  and  10  cc.  of  ether.      Stir  to 

redissolve  in  the  ether  all  of  the  oily  or  waxy  matter  present  and  to 
insure  solution  of  the  alkaloids  in  the  acid  liquid.  Evaporate  com- 
pletely the  ether  and  any  alcohol  that  may  remain,  and  filter  the  acid 
aqueous  liquid  through  a  very  small  dry  filter  into  a  bottle.  The 
alkaloid  is  then  liberated  by  treatment  with  an  alkali  and  the  appro- 
priate solvent.^ 

(5)  J.  Katz's  Method  (Arch.  d.  Pharm.,  1898,  I,  and  Am.  Drug., 
1898,   281).     This  method  has  the  advantage  of  all  other  methods 
in  that  it  enables  one  to  rapidly  and  accurately  estimate  the  alkaloids 
in   a   preparation    without    the    necessity   of   applying   heat    for  any 
purpose  whatever  during  the  process.     The  assay  may  be  made  in 
from  one  to  three  hours.     25  cc.  of  the  tincture  of  an  alcoholic  strength 
of  about  45  per  cent  are  placed  in  a  separatory  funnel,  i  cc.  of  a 
33  per  cent  solution  of  soda  added,  and  the  mixture  shaken  for  five 
minutes  with  50  cc.  of  ether,  and  set  aside  until  the  liquids  have 
completely    separated    into    two    layers.     The    lower    dark -colored 
aqueous  layer  is  drawn  off  into  a  beaker.     The  ethereal  layer  which, 
besides  the  alkaloid,  has  taken  up  most  of  the  alcohol  and  some 
coloring    matter,   is    shaken    up    with    3   cc.    of  water,  which    after 
separating  is  drawn  off  and  added  to  the  first  aqueous  liquid.     The 
ethereal  liquid  is  then  poured  into  a  suitable  flask,  while  the  com- 
bined aqueous  liquid  is  further  treated  with  two  portions  of  ether 
(25  cc.  each),  the  ether  to  contain  about  10  per  cent  of  alcohol.     These 

*  Proc.  A.  Ph.  A.,  1892,  446. 


ASSAY   OF  GALENICAL    PREPARATIONS  587 

ethereal  extractions  are  washed  each  with  1.5  cc.  of  water;  the 
first  extraction  after  washing  may  be  added  to  the  original  ethereal 
solution,  while  the  second  is  reserved  and  later  on  employed  for 
washing  the  flask.  The  ethereal  solution,  which  contains  still  some 
traces  of  aqueous  fluid  containing  alkali,  is  dehydrated  by  treatment 
with  2  or  3  gms.  of  exsiccated  calcium  sulphate  and  finally  filtered 
into  a  glass-stoppered  flask  containing  50  cc.  of  water. 

N 
Titration  by  means  of acid  V.  S.  is  employed,  using  alcoholic 

solution  of  iodeosin  (1:250)  as  indicator. 

The  method  as  above  described  is  obviously  applicable  only  to 
such  alkaloids  as  are  readily  soluble  in  ether.  If  an  estimation  of 
alkaloids  insoluble  or  only  slightly  soluble  in  ether,  but  soluble  in 
chloroform,  is  to  be  made,  the  method  is  modified  as  follows: 

25  cc.  of  the  tincture  of  45  per  cent  alcoholic  strength  are  vigor- 
ously shaken  for  five  minutes  with  30  cc.  of  a  mixture  of  i  part  of 
chloroform  and  2  parts  of  ether.  The  solution  so  obtained  is  washed 
with  3  cc.  of  a  30  per  cent  solution  of  sodium  chlorid.  This  opera- 
tion is  repeated  twice,  using  each  time  15  cc.  of  the  ether-chloroform 
mixture  and  washing  with  1.5  cc.  of  sodium-chlorid  solution. 

//  the  separation  of  the  aqueous  and  ethereal  liquids  is  not  dis- 
tinct an  additional  2  or  3  gms.  of  sodium  chlorid  may  be  used.  This 
prevents  the  emulsification,  which  if  pure  water  were  employed 
would  occur. 

//  the  tincture  to  be  assayed  contains  more  than  45  per  cent  of 
alcohol  it  is  necessary  to  add  water  to  reduce  it  to  40  or  50  per  cent. 

Tinctures  containing  chlorophyll  or  fat  or  fatty  acids  must  first 
be  deprived  of  these  constituents,  as  these  substances,  possessing 
acid  properties  inferior  to  that  of  iodeosin,  will  act  the  part  of  an 
alkali  toward  it  and  thus  be  recorded  as  alkaloid. 

To  remove  the  chlorophyll  and  fatty  acids,  acidulate  a  mixture 
of  equal  parts  of  the  tincture  and  water  with  a  few  drops  of  sulphuric 
acid,  shake  with  talcum  during  several  hours,  and,  after  subsidence 
of  the  latter,  filter.  Of  this  filtrate  25  gms.  (not  cc.,  on  account  of 
the  admixture  of  alcohol  and  water  causing  change  of  volume)  are 
taken  and  the  alkaloid  estimated  in  the  manner  already  described, 
after  first  removing,  if  necessary,  the  last  traces  of  fat  by  a  single 
shaking  of  the  acid  solution  with  petroleum  ethe'r. 

For  the  assay  of  extracts  i  to  1.5  gms.  are  dissolved  in  from  40 
to  50  cc.  of  45  per  cent  alcohol  to  make^a  solution  containing  less  than 
3  per  cent  extractive.  For  the  assay  of  fluid  extracts  10  cc.  are  taken. 

(6)  C.  Kippenberger's  Method  (Apoth.  Ztg.,  1898,  664-674). 
The  preparation  to  be  examined,  if  strongly  alcoholic,  needs  no 


588  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

preliminary  treatment;  if,  however,  it  is  a  hydro-alcoholic  or  aqueous 
preparation — extracts  and  the  like — it  must  preliminarily  be  dis- 
solved in  strong  alcohol  to  which  a  certain  percentage  of  hydrochloric 
acid  has  been  added  (tartaric  acid  in  the  case  of  aconite)  and  filtered 
off  from  the  more  or  less  abundant  and  generally  smeary  precipitate. 
An  aliquot  part  of  the  filtrate  is  then  evaporated  on  a  water-bath, 
the  residue  dissolved  in  water,  filtered,  and  the  filtrate  transferred 
to  a  beaker,  in  which  the  precipitation  of  the  alkaloid  as  periodid  is 
then  effected  by  adding  the  prescribed  quantity  of  the  iodin  solution  * 
(usually  20  cc.  for  1-5  gms.  of  extract,  according  to  the  kind  used). 
In  most  cases  five  to  ten  minutes  are  sufficient  for  complete  deposi- 
tion of  the  precipitate;  but  if  it  is  desired  that  the  precipitate  shall 
become  crystalline  the  mixture  must  be  allowed  to  stand  from  twenty 
minutes  to  an  hour.  In  either  case  the  precipitate  is  collected  on 
a  small  plaited  filter  and  washed  two  or  three  times  with  water  to 
which  a  few  drops  of  the  iodin  solution  have  been  added;  or  a  little 
sodium  chlorid  may  be  dissolved  in  the  wash  water,  which  is  par- 
ticularly recommended  in  the  case  of  atropine.  The  washed  pre- 
cipitate is  now  dissolved  in  acetone  (pure)  (of  which  from  15  to  20 
cc.  are  required  for  1-5  gms.  of  extract  employed),  observing  that 
it  shall  completely  enter  solution  and  that  there  shall  be  no  waste. 
The  acetone  solution  is  then  treated  with  alkali  hydroxid  solution 
in  excess,  then  diluted  hydrochloric  acid  in  excess,  the  mixture  diluted 
with  water  and  shaken  out  with  petroleum  ether  to  remove  the 
acetone,  the  last  traces  of  which  are  removed  by  heating  on  a  water- 
bath.  The  liquid  is  now  supersaturated  with  alkali  hydroxid  and 
shaken  out  with  chloroform  or  with  a  mixture  of  ether  and  chloro- 
form. The  chloroformic  solution  of  pure  alkaloid  so  obtained  is 
cautiously  evaporated  and  its  amount  determined  either  by  direct 
weighing  or  by  titration. 

According  to  the  preparation  that  is  to  be  assayed,  there  must 
be  some  variation  in  the  amount  of  solvents  and  precipitant.  These 
are  indicated  by  Kippenberger  in  the  following  examples: 

Extract  of  Aconite  Root.  7.5  gms.  of  the  extract  are  dissolved  in 
75  cc.  of  water  containing  3  gms.  of  tartaric  acid  and  50  cc.  of  the 
filtrate  (=5  gms.  of  the  extract)  are  mixed  with  20  cc.  of  the  iodin 
solution. 

Tincture  of  Aconite  Root.  100  cc.  of  the  tincture  are  evaporated 
to  dryness  with  i  gm.  of  tartaric  acid,  the  residue  dissolved  in  60  cc. 


*  The  iodin  solution  is  made  by  dissolving  20  gms.  of  iodin  and  60  gms.  of 
potassium  iodid  in  water  to  make  1000  cc.  The  diluted  hydrochloric  acid  used 
is  that  of  the  German  Pharmacopoeia. 


ASSAY   OF  GALENICAL  PREPARATIONS  589 

of  water  containing  i  cc.  of  dilute  hydrochloric  acid,  and  45  cc.  of 
the  filtrate  (=75  cc.  of  the  tincture)  are  mixed  with  20  cc.  of  the 
iodin  solution. 

Extract  of  Belladonna  (either  from  leaves  or  root).  7.5  gms.  of 
the  extract  are  treated  with  75  cc.  of  alcohol  containing  2  cc.  of 
diluted  hydrochloric  acid  and  filtered.  60  cc.  of  the  filtrate  are 
evaporated  to  dryness,  the  residue  dissolved  in  60  cc.  of  water  con- 
taining i  cc.  of  diluted  hydrochloric  acid,  the  solution  filtered  and 
50  cc.  of  the  filtrate  (=5  gms.  of  the  extract)  are  mixed  with  30  cc. 
of  the  iodin  solution. 

Tincture  of  Belladonna.  100  cc.  of  the  tincture  are  mixed  with 
i  cc.  of  diluted  hydrochloric  acid  and  evaporated  to  dryness,  the 
residue  dissolved  in  50  cc.  of  water  containing  i  cc.  of  diluted  hydro- 
chloric acid,  and  40  cc.  of  the  filtrate  (=80  cc.  of  the  tincture)  are 
mixed  with  20  cc.  of  the  iodin  solution. 

Extract  of  Cinchona  (Alcoholic}.  1.5  gms.  of  the  extract  are 
dissolved  in  75  cc.  of  water  containing  3  cc.  of  diluted  hydrochloric 
acid,  and  to  50  cc.  of  the  filtrate  (=i  gm.  of  the  extract)  20  to  30 
cc.  of  iodin  solution  are  added.  (The  iodin  solution  for  this  assay 
is  best  made  with  only  30  gms.  of  potassium  iodid  per  1000  cc.). 

Extract  of  Cinchona  (Aqueous).  1.5  gms.  of  the  extract  are  treated 
with  80  cc.  of  alcohol  containing  3  cc.  of  diluted  hydrochloric  acid, 
filtered,  and  the  filtrate  evaporated  to  dryness.  The  residue  is 
dissolved  in  75  cc.  of  water  containing  2  cc.  of  diluted  hydrochloric 
acid,  and  50  cc.  of  the  filtrate  (=i  gm.  of  the  extract)  are  precipitated 
with  20  to  30  cc.  of  the  iodin  solution,  modified  as  under  alcoholic 
extract  of  cinchona. 

Tincture  of  Cinchona.  30  cc.  of  the  tincture  are  evaporated  to 
dryness,  the  residue  dissolved  in  60  cc.  of  water  containing  3  cc. 
of  diluted  hydrochloric  acid,  the  solution  filtered  and  50  cc.  of  the 
filtrate  (=25  cc.  of  the  tincture)  treated  with  20  cc.  of  the  iodin 
solution  modified  as  under  alcoholic  extract  of  cinchona. 

Extract  of  Hyoscyamus.  12  gms.  of  the  extract  are  treated  with 
1 20  cc.  of  alcohol  containing  2.5  cc.  of  diluted  hydrochloric  acid 
and  filtered,  100  cc.  of  the  filtrate  evaporated  to  dryness,  the  residue 
dissolved  in  60  cc.  of  water  containing  i  cc.  of  diluted  hydrochloric 
acid,  filtered,  and  45  cc.  of  the  filtrate  (=7.5  gms.  of  extract)  mixed 
with  20  cc.  of  iodin  solution. 

Tincture  of  Hyoscyamus.  200  cc.  of  the  tincture  and  2  cc.  of 
diluted  hydrochloric  acid  are  evaporated  to  dryness,  the  residue 
dissolved  in  50  cc.  of  water,  the  filtrate  rendered  acid  if  necessary 
by  the  addition  of  a  little  diluted  hydrochloric  acid,  and  40  cc.  of  it 
(=160  cc.  of  tincture)  are  precipitated  with  10  to  20  cc.  of  iodin 
solution. 


A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Extract  of  Nux  Vomica  (Alcoholic).  1.5  gms.  of  the  extract  are 
dissolved  in  75  cc.  of  water  containing  1.5  cc.  of  diluted  hydrochloric 
acid,  and  50  cc.  of  the  filtrate  (=i  gm.  of  extract)  are  mixed  with 
20  cc.  of  the  iodin  solution. 

Extract  of  Nux  Vomica  (Aqueous).  2  gms.  of  the  extract  are 
dissolved  in  80  cc.  of  alcohol  containing  2  cc.  of  diluted  hydrochloric 
acid;  60  cc.  of  the  filtrate  are  evaporated  on  a  water-bath,  the  residue 
is  dissolved  in  75  cc.  of  water  containing  1.5  cc.  of  diluted  hydro- 
chloric acid,  and  50  cc.  of  the  filtrate  from  this  (=i  gm.  of  extract) 
are  mixed  with  20  cc.  of  iodin  solution. 

Tincture  of  Nux  Vomica.  60  cc.  of  the  tincture  are  evaporated 
on  a  water-bath  to  dryness,  the  residue  dissolved  in  60  cc.  of  water 
containing  2  cc.  of  diluted  hydrochloric  acid,  and  50  cc.  of  the  filtrate 
(=50  cc.  of  the  tincture)  are  mixed  with  20  cc.  of  the  iodin  solution. 

(7)  Farr  and  Wright's  Method  for  Tinctures  as  Modified  by 
Seyler  (Year-book  of  Pharmacy,  1897): 

(a)  50  cc.  of  the  tincture  are  evaporated  on  a  water-bath  until 
all  the  alcohol  is  driven  off,  adding  water  from  time  to  time,  and 
finally  adding  enough  water  to  make  up  the  original  volume. 

(b)  When  the  liquid   has    cooled    off  i  cc.    of  semi-normal    sul- 
phuric acid  is  added  and  filtered  through  cotton  into  a  separator. 
The  dish  and  funnel  arc  rinsed  with  a  little  acidulated  water  and 
added  to  the  contents  of  the  separator,  which  is  then  shaken  with  15 
cc.  of  chloroform,  drawn  off  and  again  shaken  with  two  consecutive 
portions  of  5  cc.  each,  of  chloroform. 

(c)  The  colored  chloroformic  solution  is  then  washed  with  three 
successive  portions  of  acidulated  water  (20  cc.  of  water  and  2  cc. 
semi-normal  acid)  to  recover  traces  of  alkaloid,  and  the  washings  added 
to  the  original  acid  liquid  (b). 

(d)  To  the  so  purified  aqueous  liquid  2  cc.  or  more  of  ammonia- 
water  B.  P.  are  added  and  shaken  out  with  consecutive  portions  of 
chloroform  (of  10,  5,  and  5  cc.)  until  this  on  evaporation  gives  no 
precipitate  with  Mayer's  reagent. 

(e)  The  chloroformic  solution   of  crude  alkaloid  so  obtained  is 
now  shaken  out  with  three  portions  of  acidulated  water,  as  under 
(c),  the  solution  then  made  alkaline  with  ammonia-water  and  shaken 
out  once  more  with  chloroform,  using  10,  5,  and  5  cc.,  as  under  (d). 

The   chloroformic   solution    is   evaporated   on   a   water-bath   and 
the  residue  weighed  and  titrated  as  pure  alkaloid. 
For  Fluid  Extracts  take  10  to  20  cc. 

(8)  Webster's  Method.*    In  the  following  process  the  endeavor 
is  made  not   only  to  prevent  the  formation    of   emulsions    (a  most 

*  A.  J.  Ph.,  1907,  301-307. 


ASSAY   OF  GALENICAL  PREPARATIONS  591 

objectionable  feature  of  the  shaking  out  process  as  usually  carried 
out),  but  also  to  effect  the  complete  elimination  of  ammonia: 

In  the  shaking  out  process  there  are  several  drawbacks.  The 
presence  of  fat,  resin,  gums,  or  other  colloidal  bodies,  while  they 
do  not  interfere  with  the  solubility  of  the  alkaloids  and  their  salts, 
prevent,  however  (because  of  the  increased  viscosity  of  the  liquids), 
the  intimate  contact  necessary  for  their  transference  from  one  solvent 
to  another,  except  by  prolonged  and  vigorous  agitation.  Such 
treatment  often  results  in  the  formation  of  an  inseparable  emulsion 
which  interferes  with  the  completion  of  the  assay.  In  most  assay 
processes  the  purified  "ethereal"  solution  of  the  free  alkaloid  is, 
in  the  last  stage,  warmed  to  dissipate  the  solvent  together  with  adherent 
ammonia,  leaving  the  alkaloid  in  a  suitable  condition  for  weighing 
or  titrating.  The  application  of  heat  is  liable  to  occasion  loss  from 
various  causes:  Reaction  with  chloroform,  hydrolysis  of  the  alka- 
loid, as  in  the  case  of  cocaine,  evaporation  of  coniine,  spurting  of 
strychnine,  and  more  or  less  resinification  of  the  alkaloids  generally. 
The  chief  difficulty,  howrever,  is  the  elimination  of  ammonia,  which 
adheres  persistently  to  the  residue.  The  elimination  of  the  ammonia 
in  the  Webster  process  is  effected  by  the  addition  of  an  excess  of 
tartaric  acid  in  the  presence  of  absolute  alcohol.  Practically  all  of 
the  ammonia  will  be  precipitated,  as  well  as  nearly  all  of  the  albumin- 
ous and  gummy  matter.  The  filtrate  can  then  be  evaporated  to 
a  solid  extract  without  injuring  the  alkaloids,  which  may  be  dissolved 
in  acidified  water,  leaving  behind  the  resins,  chlorophyll  and  fat. 

The  process  in  detail  is  as  follows:  Add  gradually  10  cc.  of  the 
fluid  extract  to  85  cc.  of  cold  absolute  alcohol  in  which  1.5  gms.  of 
tartaric  acid  have  been  previously  dissolved;  add  absolute  alcohol 
to  make  100  cc.  (in  the  cases  of  fluid  extracts  of  gelsemium  and 
veratrum  where  the  menstruum  is  95  per  cent  alcohol,  0.75  gms.  of 
tartaric  acid  is  added  directly  to  5  cc.  of  the  fluid  extract;  the  pre- 
cipitate, if  any,  filtered  off  and  washed  with  absolute  alcohol,  etc.). 
Shake  well,  then  set  aside  for  a  few  minutes.  Filter,  transfer  50 
cc.  of  the  filtrate  to  a  shallow  porcelain  basin  of  a  diameter  of  six 
inches,  and  evaporate  carefully  on  a  water-bath,  rotating  the  contents 
of  the  basin  occasionally,  and  especially  towards  the  end  of  the 
evaporation,  so  that  the  resulting  extract  may  cover  a  large  portion 
of  the  basin.  When  dry,  cool  the  extract  by  floating  the  basin  in 
cold  water;  add  10  cc.  of  half -normal  sulphuric  acid,  and  rotate 
the  liquid  about  in  the  dish  until  the  extract  is  dissolved  or  disinte- 
grated; set  aside  for  two  minutes.  Filter  the  liquid  through  a 
small,  firmly  packed  pledget  of  cotton-wool  previously  moistened 
with  water,  into  a  large,  pear-shaped  separator.  Rinse  the  evaporat- 
ing dish  with  two  successive  5  cc.  portions  of  water,  stirring  to  dis- 


592  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

integrate  the  insoluble  substances,  and  pass  the  liquid  through  the 
filter.  To  the  separator  add  20  cc.  of  a  mixture  of  16  cc.  of  chloro- 
form and  4  cc.  of  ether;  and  4  cc.  or  sufficient  of  a  20  per  cent  solution 
of  KOH,  insert  the  stopper,  and  shake  vigorously.  When  the  fluids 
have  separated,  draw  off  the  lower  layer  into  a  second  separator 
containing  15  cc.  of  distilled  water,  and  after  agitation  and  separation 
as  before,  transfer  the  lower  layer  to  a  third  separator  also  containing 
15  cc.  of  distilled  water;  agitate,  and  allow  to  separate.  Pass  the 
lower  layer  through  a  chloroform-moistened  pledget  of  cotton-wool 
into  a  fourth  separator  of  about  150  cc.  capacity.  In  the  presence 
of  a  volatile  alkaloid  wash  the  cotton  afterwards  with  5  cc.  of  chloro- 
form; in  all  cases  the  stems  of  the  separators  should  be  washed  with 
i  cc.  of  chloroform  after  the  solution  has  been  run  off  and  the  washings 
added.  Repeat  the  extraction  till  no  more  alkaloid  is  obtained, 
with  two  or  more  portions  of  the  immiscible  solvent  which  finally 
collect,  after  washing  as  before  in  separator  No.  4.  To  the  bulked 

N          N 

ethereal  extract  add  excess  of  —  or  sulphuric  acid,  and  shake 

50         100 

thoroughly.  After  complete  separation,  reject  the  lower  layer; 
add  cochineal  T.  S.  or  iodeosin  solution  in  water-saturated  ether, 

N 
and  titrate  back  with  —  sodium  hydroxid. 

For  Solid  Extracts.    Dissolve  2  gms.  of  the  extract  in  10  cc.  of 
50  per  cent  alcohol,  and  treat  as  a  fluid  extract. 


ASSAYING   OF    PREPARATIONS    OF   ACONITE 

Assay  of  Fluid  Extract  of  Aconite  (U.  S.  P.  VIII).  Transfer 
10  cc.  of  fluid  extract  of  aconite  by  means  of  a  graduated  pipette 
to  a  porcelain  dish,  and  evaporate  it  carefully  to  dryness  on  a 
water-bath  at  a  temperature  not  exceeding  60°  C.  (140°  F.).  Add 
5  cc.  of  tenth-normal  sulphuric  acid  V.  S.  and  10  cc.  of  distilled 
water.  When  the  extract  is  dissolved,  filter  the  liquid  into  a  separator, 
washing  the  dish  and  filter,  with  about  40  cc.  of  distilled  water;  when 
this  has  passed  through,  add  25  cc.  of  ether  and  2  cc.  of  ammonia- 
water  to  the  separator,  and  agitate  for  one  minute.  Draw  off  the 
lower  layer  into  a  flask  and  filter  the  ether-solution  into  a  beaker. 
Return  the  contents  of  the  flask  to  the  separator,  add  15  cc.  of  ether, 
and  agitate  for  one  minute.  Draw  off  the  lower  layer  into  the  flask 
and  filter  the  ether-solution  into  the  beaker.  Repeat,  with  two 
other  portions  of  10  cc.  each  of  ether.  Evaporate  the  ether-solution 
to  dryness,  and  dissolve  the  residue  in  3  cc.  of  tenth-normal  sulphuric 
acid  V.  S.  diluted  with  20  cc.  of  distilled  water.  Add  to  the  solution 


ASSAY   OF  GALENICAL  PREPARATIONS  593 

5  drops  of  hematoxylin  T.  S.,  and  then  carefully  run  in  fiftieth-normal 
potassium  hydroxid  V.  S.  until  a  violet  color  is  produced,  the  transi- 
tion stages  being  as  follows:  First  yellow,  then  green,  finally  passing 
into  violet.  Divide  the  number  of  cc.  of  fiftieth-normal  potassium 
hydroxid  V.  S.  used  by  5,  subtract  this  number  from  3  (the  3  cc. 
of  tenth-normal  sulphuric  acid  V.  S.  taken),  multiply  the  remainder 
by  0.064,  and  this  product  by  10,  which  will  give  the  weight  in  grams 
of  aconitine  contained  in  one  hundred  cubic  centimeters  of  rhe  fluid 
extract  of  aconite. 

Tincture  of  Aconite  is  assayed  by  evaporating  100  cc.  to  dryness 
at  a  temperature  not  exceeding  60°  C.  (140°  F.),  and  treating  the 
resulting  extract  by  the  foregoing  method,  with  the  exception  that 
the  multiplication  of  the  product  by  10  must  be  omitted.  The 
result  will  represent  the  weight  in  grams  of  aconitine  contained  in 
100  cc.  of  the  tincture. 


ASSAYING    OF   PREPARATIONS    OF    THE    MYDRIATIC   DRUGS 

Assay   of  Extract   of   Belladonna  Leaves   (U.    S.   P.   VIII). 

Introduce  5  gms.  of  the  extract  of  belladonna  leaves  into  a  small 
beaker  and  dissolve  it  in  a  mixture  consisting  of  alcohol  5  cc.,  dis- 
tilled water  10  cc.,  ammonia-water  2  cc.,  and  chloroform  20  cc. 
When  dissolved,  transfer  it  to  a  separator,  rinsing  the  beaker  with 
a  little  alcohol  and  adding  the  rinsings  to  the  separator.  Insert  the 
stopper  securely  and  shake  the  separator  for  half  a  minute.  Draw 
off  the  chloroformic  layer  into  a  second  separator,  and  add  to  the 
first  separator  10  cc.  more  of  chloroform.  Shake  it  for  half  a  minute, 
allow  to  separate,  and  again  draw  off  the  chloroformic  layer  into 
the  second  separator.  Repeat  this  with  10  cc.  more  of  chloroform. 
To  the  united  chloroformic  liquids  in  the  second  separator,  add 
5  cc.  of  normal  sulphuric  acid  V.  S.  and  10  cc.  of  distilled  water, 
and  shake  it  for  half  a  minute.  Draw  off  the  chloroformic  layer, 
after  the  liquids  have  separated,  into  the  first  separator,  after  cleaning 
it  thoroughly,  and  the  aqueous  layer  into  a  beaker,  and  repeat  the 
process  by  adding  to  the  first  separator  10  cc.  of  distilled  water  and 
i  cc.  of  normal  sulphuric  acid  V.  S.  Draw  off  the  chloroformic  layer, 
rejecting  the  same,  and  then  run  the  acid  aqueous  layer  into  the 
beaker.  Pass  the  combined  acid  aqueous  solutions  through  a  pledget 
of  purified  cotton  into  the  first  separator,  after  cleaning  it  thoroughly, 
rinsing  the  second  separator,  the  beaker,  and  the  funnel  with  about 
10  cc.  of  distilled  water.  To  the  first  separator  add  15  cc.  of  chloro- 
form, a  small  piece  of  red  litmus  paper,  and  enough  ammonia-water 
to  produce  a  distinctly  alkaline  reaction.  Shake  the  separator  for 


594  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

half  a  minute,  and  when  the  liquids  have  separated  draw  off  the 
chloroformic  layer  into  a  beaker.  Repeat  this  process  with  two 
portions  of  10  cc.  each  of  chloroform,  and  evaporate  the  combined 
chloroformic  liquids  in  the  beaker  to  dryness  on  a  water-bath  con- 
taining warm  water;  dissolve  the  residue  in  3  cc.  of  ether  and  allow 
the  latter  to  evaporate  completely.  To  the  alkaloidal  residue  add 
5  cc.  of  tenth-normal  sulphuric  acid  V.  S.  and  5  drops  of  hasmatoxylin 
T.  S.  (or  iodeosin  T.  S.),  then  titrate  the  excess  of  acid  with  fiftieth- 
normal  potassium  hydroxid  V.  S.  Divide  the  number  of  cubic 
centimeters  of  fiftieth-normal  potassium  hydroxid  V.  S.  used,  by 
by  5,  subtract  the  quotient  from  5  (the  5  cc.  of  tenth-normal  sulphuric 
acid  V.  S.  taken),  and  multiply  the  remainder  by  0.0287,  and  this 
product  by  20,  to  obtain  the  percentage  of  mydriatic  alkaloids  con- 
tained in  the  extract  of  belladonna  leaves.  The  figure  0.0287  repre- 
sents in  weight  the  grams  of  mydriatic  alkaloids  (mainly  atropine) 
required  to  neutralize  i  cc.  of  tenth-normal  sulphuric  acid  V.  S. 

Assay  of  Fluid  Extract  of  Belladonna  Root  (U.  S.  P..  VIII). 
Transfer  10  cc.  of  fluid  extract  of  belladonna  root  by  means  of  a 
graduated  pipette  to  a  separator,  add  10  cc.  of  distilled  water,  20  cc. 
of  chloroform,  and  2  cc.  of  ammonia-water.  Shake  the  separator 
well  for  one  minute,  and  draw  off  the  lower  chloroformic  layer  into 
a  second  separator.  Repeat  the  extraction  with  two  portions  of 
10  cc.  each  of  chloroform,  and  draw  the  chloroform  solution  into 
the  second  separator.  To  the  latter  add  8  cc.  of  normal  sulphuric 
acid  V.  S.  and  20  cc.  of  distilled  water,  shaking  well  for  one  minute. 
When  perfectly  separated  draw  off  and  reject  the  lower  chloroformic 
layer,  and  filter  the  acid  aqueous  layer  into  a  clean  separator.  Wash 
the  separator  and  filter  with  10  cc.  of  distilled  water,  adding  this  to 
the  clean  separator.  To  the  latter  add  20  cc.  of  chloroform  and 
4  cc.  of  ammonia-water,  and  shake  well  for  several  minutes.  Draw 
off  the  lower  chloroformic  layer  into  a  beaker,  and  repeat  the  extrac- 
tion with  two  portions  of  10  cc.  each  of  chloroform,  adding  the 
chloroformic  solution  to  the  beaker.  Allow  the  chloroform  in  the 
beaker  to  evaporate  on  a  water-bath,  containing  warm  water,  until 
the  residue  is  perfectly  dry.  To  the  alkaloidal  residue  add  5  cc. 
of  tenth-normal  sulphuric  acid  V,  S.,  and  when  the  residual  alkaloids 
ha\e  all  disssolved,  titrate  the  solution  with  fiftieth-normal  potassium 
hydroxid  V.  S.,  using  5  drops  of  haematoxylin  or  iodeosin  T.  S.  as 
an  indicator.  Divide  the  number  of  cubic  centimeters  of  fiftieth- 
normal  potassium  hydroxid  V.  S.  used  by  5,  subtract  the  quotient 
from  5  (the  5  cc.  of  tenth-normal  sulphuric  acid  V.  S.  taken),  and 
multiply  the  remainder  by  0.0287,  and  this  product  by  10,  to  obtain 
the  weight  in  grams  of  mydriatic  alkaloids  contained  in  one  hundred 
cubic  centimeters  of  the  fluid  extract  of  belladonna  root. 


ASSAY   OF  GALENICAL   PREPARATIONS  595 

Assay  of  Tincture  of  Belladonna  Leaves.  Transfer  100  cc. 
of  tincture  of  belladonna  leaves  to  an  evaporating  dish  and  evaporate 
it  on  a  water-bath  until  it  measures  about  10  cc.  Add,  if  necessary, 
sufficient  alcohol  to  dissolve  any  separated  substance,  and  then  assay 
the  resulting  liquid  by  the  method  given  above,  using  the  same 
details  as  there  directed  for  10  cc.  of  fluid  extract  of  belladonna  root, 
with  the  exception  that  the  multiplication  of  the  product  by  10  be 
omitted.  The  result  will  represent  the  weight  in  grams  of  alkaloids  con- 
tained in  one  hundred  cubic  centimeters  of  tincture  of  belladonna  leaves. 

The  above  assay  of  fluid  extract  of  belladonna  is  a  modification 
of  Puckner's  method  (Ph.  Rev.,  1898),  which  is  as  follows: 

Measure  10  cc.  of  the  fluid  extract  into  a  separator  and  add  10  cc. 
of  water,  20  cc.  of  chloroform,  2  cc.  of  a  10  per  cent  ammonia-water, 
and  shake  well.  When  the  layers  have  separated,  draw  off  the 
heavier,  receiving  it  in  a  separator;  complete  the  extraction  of  the 
alkaloid  with  two  further  portions  of  chloroform,  10  cc.  each,  adding 
this  to  the  chloroform  solution  first  drawn  off;  shake  the  chloroform 
solution  of  the  alkaloid  with  20  cc.  of  i  per  cent  hydrochloric  acid, 
and  draw  off  the  chloroform  into  a  clean  separator,  using  a  few  cc. 
of  chloroform  to  rinse  the  stop-cock  and  outlet-tube  of  the  first,  and 
again  shake  the  chloroform  with  i  per  cent  hydrochloric  acid,  using 
only  10  cc.  this  time. 

Draw  off  and  reject  the  chloroform  and  mix  the  acid  extracts, 
add  a  few  cc.  of  chloroform,  draw  off  and  reject  the  same.  Now  add 
20  cc.  of  chloroform,  2  cc.  of  ammonia-water,  and  shake  well.  Draw 
off  the  clear  chloroformic  solution  and  complete  the  extraction  with 
two  further  portions  of  chloroform,  10  cc.  each.  Evaporate  the 
chloroform  and  titrate  the  residue  as  follows: 

Add  to  it  5  cc.  of  ether,  about  5  cc.  of  cochineal  solution,  and 

N 
gradually  a  slight  excess  of  -  -  acid.     When  complete  solution  has 

taken  place,  evaporate  the  ether  and  determine  the  excess  of  acid  with 

N 

-  alkali  V.  S.     Each  cc.  of  acid  is  taken  to  represent  0.01442  gms. 
20 

of  alkaloid.  The  advantage  of  this  method  is  that  there  is  no  neces- 
sity for  evaporating  the  fluid  extract,  the  economy  in  material,  and 
the  purity  of  the  alkaloid  extracted. 

Method  of  A.  B.  Lyons.*  While  the  pharmacopceial  process 
for  the  assay  of  fluid  extracts  of  the  mydriatic  drugs  is  fairly  satis- 
factory in  the  hands  of  skillful  workers,  it  has,  however,  its  draw- 
backs when  applied  to  preparations  which  contain  but  a  very  small 

*  Ph.  Rev.,  1908,  XXVI,  22. 


596  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

quantity  of  alkaloid,  as,  for  instance,  preparations  of  hyoscyamus. 
The  following  is  proposed  as  an  alternative: 

Put  into  a  200  cc.  measuring  flask  20  cc.  of  solution  of  lead  sub- 
acetate  (U.  S.  P.).  Add  150  cc.  of  distilled  water,  mix  and  add  with 
constant  shaking  20  cc.  of  the  fluid  extract.  Make  up  with  distilled 
water  to  exactly  200  cc.  Filter,  using  if  necessary  a  double  filter, 
and  returning  the  first  portion  of  the  filtrate  until  it  passes  through 
quite  clear.  The  filtrate  should  still  contain  some  lead  subacetate; 
it  is  not  desirable  that  the  excess  should  be  very  large,  and  it  may 
therefore  be  a  better  way  to  use  at  first  only  15  cc.  of  the  lead  solu- 
tion, then  after  adding  the  water  and  the  fluid  extract  observing 
whether  the  mixture  has  any  sweet  taste,  indicating  that  the  lead  is 
in  excess.  If  not,  more  of  the  lead  solution  should  be  added,  about 
i  cc.  at  a  time,  until  the  sweet  taste  becomes  perceptible.  The 
measure  should  then  be  made  up  to  200  cc. 

When  the  filter  has  practically  finished  dripping,  add  to  the  filtrate 
about  a  gram  of  granulated  sodium  phosphate,  and  stir  until  the  salt 
is  dissolved.  If  the  mixture  still  has  perceptible  sweetness,  add 
more  of  the  phosphate,  until  this  is  in  slight  excess.  Allow  the  mix- 
ture to  stand  at  rest  in  a  beaker  a  few  minutes  and  test  a  few  drops 
of  the  clear  liquid  that  will  shortly  separate  with  a  little  sodium 
phosphate.  If  this  produces  any  turbidity,  add  to  the  mixture  more 
of  the  phosphate  until  this  is  surely  in  slight  excess.  All  this  takes 
but  a  few  moments'  time,  although  in  the  description  it  may  seem  a 
complicated  operation. 

Filter  once  more,  when  the  filtrate  will  be  quite  clear  and  almost 
free  from  color.  Transfer  exactly  100  cc.  of  the  filtrate,  representing 
10  cc.  of  the  original  fluid  extract  to  a  separator,  add  25  cc.  of  chlo- 
form  and  about  2  cc.  of  water  of  ammonia.  Shake  carefully,  using 
a  rotary  motion.  Generally  there  is  no  serious  danger  of  emulsi- 
fication,  but  the  possibility  of  such  an  occurrence  must  be  always  borne 
in  mind.  Nearly  the  whole  of  the  alkaloid  will  pass  into  the  chloro- 
form in  this  first  washing.  Allow  the  chloroform  to  separate  com- 
pletely and  draw  it  off.  Repeat  the  extraction  with  two  additional 
portions  of  chloroform,  20  and  15  cc.  Evaporate  the  chloroform 
in  a  tared  beaker,  dry  on  a  water-bath  and  weigh.  Then  determine 
the  alkaloid  in  the  usual  way  by  titration. 

Lyons  employs  the  following  as  a  routine  method  for  such  titrations: 

The  alkaloidal  residue  is  dissolved  in  2  cc.  of  alcohol.  To  the 
solution  are  added  2  cc.  of  decinormal  sulphuric  acid,  20  cc.  of  dis- 
tilled water  and  five  drops  of  cochineal  indicator.  Into  a  small  flask 
are  introduced  at  the  same  time  2  cc.  of  the  same  alcohol,  2  cc.  of  the 
decinormal  acid,  20  cc.  of  the  same  distilled  water,  and  five  drops  of 
the  same  indicator.  Alkali  (lime  water  or  potassium  hydroxid  solution) 


ASSAY   OF  GALENICAL  PREPARATIONS  597 

is  then  added  from  a  graduated  pipette  until  the  color  changes.  The 
amount  of  alkali  consumed  is  read  off,  2  cc.  more  of  acid  is  added 
and  the  titration  repeated.  The  two  results  will  exactly  correspond 
unless  by  any  chance  the  alcohol  or  the  distilled  water  used  were  not 
strictly  neutral — a  possibility  always  to  be  kept  in  mind  in  these 
delicate  titrations.  The  second  titration  in  any  case  gives  us  the 
exact  strength  of  the  alkali  we  are  using.  If  it  differs  from  the  first, 
we  may  still  go  on  with  the  determination,  since  we  know  the  exact 
amount  of  the  error  due  to  imperfect  neutrality  of  our  solvents. 

The  next  step  is  to  titrate  the  alkaloidal  solution,  noting  the 
exact  amount  of  alkali  required  to  neutralize  the  excess  of  acid.  We 
have  now  all  the  data  for  our  calculation.  We  subtract  the  amount 
of  alkali  consumed  in  neutralizing  excess  of  acid  in  the  alkaloidal 
solution  from  the  amount  consumed  in  the  first  blank  experiment. 
The  difference  gives  the  alkalinity  of  the  alkaloidal  residue  in  units 
whose  value  is  readily  reduced  to  the  "normal"  standard.  For 
example,  suppose  we  found  our  alkaloidal  residue  to  be  the  equiv- 
alent of  4.55  cc.  of  our  "standard  "  alkali,  and  that  we  have  found 

N 
(in  our  second  blank  titration)  that   2  cc.  of  —  acid  required  for 

neutralization  7.45  cc.  of  our  "standard  "  alkali  (i.e.,  i  cc.  of  deci- 
normal  acid  corresponds  with  3.725  cc.  of  the  "standard  "  alkali). 
Then  4.55-5-3.725=1.22+  cc.  will  be  the  quantity  of  decinormal 
alkali  corresponding  with  the  alkaloid  present,  and  since  each  cc.  of 
decinormal  alkali  corresponds  with  0.0287  gm.  of  atropine,  our  residue 
contained  1.22X0.0287  =  0.035+  gm.  of  alkaloid  estimated  as  atropine. 

The  calculation  reduced  to  the  form  of  a  "rule  "  is  very  simple. 
Subtract  the  result  of  the  final  titration  expressed  in  cc.  from  the 
result  of  the  first  blank  titration,  multiply  the  remainder  by  57.4 
and  divide  the  product  by  the  result  of  the  second  blank  titration. 
The  quotient  will  be  the  quantity  of  atropine  present,  expressed  in 
mgs.  Fluid  extract  of  belladonna  root  or  of  stramonium  would  be 
assayed  in  exactly  the  same  way  as  the  fluid  extract  of  belladonna 
leaves:  For  the  assay  of  tincture  of  belladonna  or  stramonium,  take 
100  cc.  of  the  tincture,  evaporate  to  about  25  or  30  cc.,  treat  with 
10  cc.  of  solution  of  lead  subacetate  after  making  up  to  85  cc.,  add 
distilled  water  to  make  exactly  100  cc.  and  proceed  as  above  described, 
using  finally  for  the  assay  the  equivalent  of  only  50  cc.  of  the 
tincture. 

For  the  assay  of  fluid  extract  of  hyoscyamus,  use  25  cc.  each  of 
the  lead  solution  and  the  fluid  extract;  make  up  to  250  cc.  and  take 
of  the  filtrate  125  or  150  cc.  For  the  assay  of  tincture  of  hyoscyamus 
take  at  least  200  cc.,  evaporate  to  about  50  cc.,  dilute  with  water  to 
175  cc.,  add  20  cc.  of  lead  solution  and  make  up  to  200  cc. 


A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  Potassium-bismuth  lodid  Method  (Thorns  *)  is  applied 
to  preparations  of  the  mydriatic  drugs  in  the  following  manner: 
To  4  gms.  of  dry  extract  (extractum  siccum)  are  added  50  cc.  90  per 
cent  alcohol,  the  mixture  shaken  frequently  during  three  hours  and 
filtered.  Then  25  cc.  of  the  nitrate,  taken  to  represent  2  gms.  extract, 
are  heated  on  a  water-bath  until  the  alcohol  has  been  driven  off. 
The  residue  is  taken  up  with  50  cc.  water  and  to  this  added  10  cc, 
10  per  cent  sulphuric  acid  and  5  cc.  potassium-bismuth  iodid  solu- 
tion (prepared  by  pouring  a  solution  of  80  gms.  bismuth  subnitrate 
in  200  gms.  nitric  acid  (sp.gr.  1.18),  into  a  concentrated  solution  of 
272  gms.  potassium  iodid  in  water  and,  after  removal  of  the  potassium 
nitrate  crystals  formed,  diluting  to  1000  cc.).  The  precipitate  is 
collected  and  with  the  filter  placed  in  a  cylinder  and  treated  with  20  cc. 
15  per  cent  sodium  hydroxid  solution  and  10  gms.  coarsely  powdered 
crystallized  sodium  carbonate.  Next  50  cc.  ether  are  added  and  the 
whole  shaken  frequently  during  three  hours.  Now  about  100  cc. 
water,  20  cc.  ether,  and  five  drops  iodeosin  solution  are  measured 
into  a  stoppered  flask  and  any  red  color,  due  to  the  alkalinity  of  the 
glass,  destroyed  by  addition  of  a  few  drops  of  hundredth -normal 
hydrochloric  acid.  To  this,  20  cc.  of  the  ethereal  alkaloid  solution, 
representing  i  gm.  of  extract,  are  added,  and  its  alkalinity  determined 
with  one-hundredth  normal  hydrochloric  acid. 

Assay  of  Belladonna  Plaster  (Rubber  Base)  (U.  S.  P.  VIII). 
Into  a  suitable  beaker  containing  50  cc.  of  chloroform  and  3  cc.  of 
ammonia-water,  introduce  10  gms.  of  the  belladonna  plaster  cut  into 
strips.  Stir  until  the  plaster  is  entirely  removed  from  the  cloth;  then 
pour  off  the  chloroform  into  another  beaker,  wash  the  cloth  with  25  cc. 
of  chloroform  and  i  cc.  of  ammonia -water  carefully,  and  add  the 
washings  to  the  chloroformic  solution  first  obtained.  If  necessary, 
repeat  the  washing  with  25  cc.  of  chloroform,  and  add  this  also  to  the 
chloroformic  solution.  Then  dry  the  cloth  at  a  low  temperature; 
cool  and  weigh  it,  and  subtract  its  weight  from  the  original  weight 
of  the  plaster.  To  the  chloroformic  solution,  add  four  fifths  of  its 
volume  of  alcohol,  stir  gently,  and  allow  the  liquid  to  stand  until  all 
of  the  rubber  has  separated  in  a  compact  mass.  Then  pour  off  the 
supernatant  liquid  into  a  separator  of  250  cc.  capacity,  and,  having 
prepared  a  solution  of  sulphuric  acid  by  diluting  40  cc.  of  normal 
sulphuric  acid  V.  S.  with  60  cc.  of  distilled  water,  add  20  cc.  of  the 
solution  to  the  separator,  and  agitate  for  two  minutes,  rotating  gently. 
Draw  off  the  chloroformic  solution  into  another  separator,  shake  this 
with  10  cc.  of  the  sulphuric  acid  solution,  and  add  the  acid  solution 

*  H.  Thorns,  Ber.  d.  Ph.  Gesellsch.,  XV,  85;  and  Chem.  Centralbl.,  1905, 


ASSAY  OF  GALENICAL  PREPARATIONS 

to  that  in  the  first  separator.  Repeat  until  the  acid  washings  cease 
to  give  a  reaction  with  mercuric  potassium  iodid  T.  S.;  combine  the 
acid  liquids,  and,  having  rendered  this  solution  alkaline  with  ammonia- 
water,  shake  out  the  alkaloids  with  three  successive  portions  of  25, 
15,  and  10  cc.  of  chloroform.  Collect  these  in  a  flask,  distil  off  all 
of  the  chloroform  with  the  aid  of  a  water-bath.  To  the  alkaloidal 
residue  add  a  slight  excess  of  tenth-normal  sulphuric  acid  V.  S.,  noting 
the  quantity  used,  and  then  add  ten  drops  of  chloroform  and,  after 
rotating,  evaporate  the  latter  by  means  of  a  water -bath.  Then  add 
five  drops  of  haematoxylin  T.  S.,  and,  rotating,  titrate  the  excess  of 
acid  with  fiftieth-normal  potassium  hydroxid  V.  S.  Divide  the  number 
of  cc.  of  fiftieth-normal  potassium  hydroxid  V.  S.  used  by  5,  subtract 
the  quotient  from  the  number  of  cc.  of  tenth-normal  sulphuric  acid 
V  S.  first  added,  and  divide  the  difference  by  the  number  of  grams 
of  belladonna  plaster  separated  from  the  cloth.  Multiply  the  quotient 
by  0.0287,  and  this  product  by  100,  which  will  give  the  percentage 
of  mydriatic  alkaloids  in  the  belladonna  plaster. 

The  Extracts  of  Hyoscyamus,  Scopola,  and  Stramonium  are 
assayed  exactly  as  directed  for  extract  of  belladonna,  except  that  in 
the  case  of  hyoscyamus  10  gms.  are  taken,  and  in  the  assay  of  scopola 
2  gms.  are  sufficient  for  the  assay. 

The  fluid  extracts  of  these  drugs  are  assayed  in  the  manner  directed 
for  fluid  extract  of  belladonna,  except  that  50  cc.  of  the  fluid  extract 
of  hyoscyamus  are  taken,  whereas  10  cc,  is  taken  of  each  of  the  others 
for  the  assay. 

The  tinctures  of  these  drugs  are  likewise  assayed  as  is  the  tincture 
of  belladonna. 

The  German  Pharmacopoeia  Method  for  the  assay  of  extracts 
of  belladonna  and  hyoscyamus  is  as  follows:  2  gms.  of  the  extract 
are  dissolved  in  5  gms.  of  water  and  5  gms.  of  absolute  alcohol.  To 
this  solution  is  added  50  gms.  of  ether  and  20  gms.  of  chloroform, 
and,  after  briskly  shaking,  10  cc.  of  sodium  carbonate  solution  (1:3). 
The  mixture  is  then  allowed  to  stand  for  one  hour  with  frequent 
shaking,  after  which  50  gms.  of  the  clear  chloroform-ether  solution 
are  filtered  off  through  a  dry  well-covered  filter,  into  a  flask,  and  about 
half  of  it  distilled  off.  The  remaining  chloroform-ether  solution  is 
then  introduced  into  a  separatory  funnel.  The  flask  is  rinsed  with 
three  portions  of  ether  of  5  cc.  each,  and  the  mixed  solutions  shaken 

N 
out  with  20  cc.  of  hydrochloric  acid.     After  complete  separation 

of  the  solutions  (which  may  be  facilitated  by  the  addition  of  a  small 
quantity  of  ether)  the  acid  liquid  is  filtered  through  a  small  filter 
previously  moistened  with  water,  into  a  2oo-cc.  flask  of  white  glass. 
The  chloroform-ether  solution  is  then  washed  out  three  times  with 


600  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

water,  using  10  cc.  each  time,  and  the  washings  passed  through  the 
same  filter.  Sufficient  water  is  then  passed  through  the  same  filter 
to  make  the  solution  measure  100  cc.  Ether  is  then  added  to  make 
a  layer  i  cm.  in  depth,  five  drops  of  iodeosin  solution  are  now  added 

N 
and  the  mixture  titrated  with potassium  hydroxid  solution,  shaking 

after  each  addition  of  the  alkali  solution,  until  the  lower  layer  assumes 
a  pale  red  color. 


LITERATURE 

Karl  Dieterich.  Helfenberger  Annalen,  1905,  and  Ph.  Centralh., 
(1906),  XLVTI,  916. 

W.  T.  Hankey.     Am.  Drug.,  1906,  XLIX,  360. 

G.  Fromme.     Geschaft's  Bericht,  Caesar  and  Loretz,  1906,  24. 

W.  Gadd  and  S.  C.  Gadd.     Ph.  Jour.,  Sept.,  1905,  438. 

F  C.  Bird.     Ph.  Jour.,  Aug.,  1900,  195. 

Farr  and  Wright.     Ph.  Jour.,  1905,  546  (xx). 

Naylor  and  Chappel.     Ph.  Jour.,  1906,  393. 

H.  Thorns.     Berichte  d.  Ph.  Ges.,  xv,  85. 

H.  Matthes  and  O.  Ramstedt.     Ph.  Ztg.,  1906  (LI),  1031. 

Assay  of  Fluid  Extract  of  Cinchona  (U.  S.  P  VIII).  Transfer 
10  cc.  of  fluid  extract  of  cinchona  by  means  of  a  graduated  pipette 
to  an  Erlenmeyer  flask  of  200  cc.  capacity,  and  add  a  mixture  of 
100  cc.  of  ether,  25  cc.  of  chloroform,  and  10  cc.  of  ammonia -water. 
Insert  the  stopper  securely,  and  shake  the  flask  vigorously,  at  intervals, 
during  ten  minutes.  Allow  the  liquids  to  separate,  decant  into  a 
measuring  cylinder  exactly  66  cc.  of  the  supernatant  liquid  (represent- 
ing 5  cc.  of  the  fluid  extract),  and  transfer  this  to  a  separator,  rinsing 
the  cylinder  with  5  cc.  of  ether  and  adding  this  to  the  separator.  Add 
to  the  latter  about  10  cc.  of  normal  sulphuric  acid  V.  S.,  or  enough 
to  make  the  solution  distinctly  acid,  and  shake  the  separator  vigorously 
for  several  minutes,  and  when  the  liquids  have  completely  separated, 
draw  off  the  lower  layer  into  a  second  separator.  To  the  first  separator 
add  5  cc.  more  of  normal  sulphuric  acid  V.  S.,  and  5  cc.  of  distilled 
water,  shake  it  for  several  minutes,  and  when  the  liquids  have  sepa- 
rated, draw  off  the  lower  layer  into  the  second  separator.  Now  add 
5  cc.  of  distilled  water  to  the  first  separator,  shake  it,  separate  as  before, 
and  then  draw  off  the  lower  aqueous  layer  into  the  second  separator. 
To  the  second  separator,  add  25  cc.  of  ether,  a  small  piece  of  red 
litmus  paper,  and  then,  gradually,  ammonia -water,  keeping  the  tem- 
perature of  the  liquids  below  25°  C.  (77°  F.),  until  the  reaction  is 
alkaline.  Then  shake  the  separator  vigorously  for  two  minutes,  and 


ASSAY  OF  GALENICAL  PREPARATIONS  601 

allow  the  liquids  to  stand  for  ten  minutes  at  a  temperature  below 
15°  C.  (59°  F.).  Draw  off  and  reject  the  lower  aqueous  layer,  and 
then  transfer  the  ether  layer  into  a  tared  beaker.  Add  5  cc.  more  of 
ether  to  the  separator,  rinse  carefully,  and  add  the  rinsings  to  the  tared 
beaker,  and  entirely  evaporate  the  ether  at  a  moderate  heat  on  a  water- 
bath.  Then  dry  the  beaker  in  an  air-bath  at  120°  C.  (248°  F.)  for 
half  an  hour,  cool,  and  weigh.  Replace  the  beaker  in  the  air-bath, 
and  heat  again  at  the  same  temperature  for  half  an  hour,  cool,  and 
weigh,  repeating  until  the  weight  is  constant.  Multiply  the  weight 
by  20  to  obtain  the  weight  in  grams  of  anhydrous  ether-soluble  alka- 
loids contained  in  100  cc.  of  the  fluid  extract. 

To  make  the  assay  volumetrically,  dissolve  the  residue  in  15 
cc.  of  alcohol;  dilute  the  solution  with  40  cc.  of  water,  add  a 

N 
few  drops  of   haematoxylin    solution,  and -then  titrate  with  —  HC1. 

Cochineal  or  rosolic  acid  may  also  be  employed  as  indicators.  The 
decinormal  factor  for  succirubra  alkaloid  is  0.0304  gm. 

The  German  Pharmacopoeia  employs  the  same  method  for  the  ex- 
tracts of  cinchona  as  for  extract  of  belladonna  with  the  difference  that 

N 

—  solutions  are  used,  that  the  flask  is  rinsed  with  a  mixture  of  chloro- 

10 

form  (one  part)  and  ether  (three  parts)  instead  of  with  ether  alone, 
and  that  haematoxylin  is  used  as  indicator  as  follows:  Of  the  100  cc. 
of  acid  solution,  take  50  cc.,  add  a  freshly  prepared  solution  of  one 
granule  of  haematoxylin  in  i  cc.  of  alcohol,  and  then  titrate  the  mixture 
with  standard  KOH  until  the  yellowish  color  upon  shaking  turns  to 
bluish-violet. 

Fluid  Extract  of  Coca  may  be  assayed  by  the  method  described 
for  fluid  extract  of  belladonna,  U.  S.  P.  VIII,  except  that  ether  is 
used  as  the  solvent  instead  of  chloroform.  Benzin  is  also  a  very  good 
solvent  for  the  alkaloids  of  coca.  See  Assay  of  Coca  Leaves. 

Assay  of  Fluid  Extract  of  Gelsemium.  Method  of  Sayre  and 
Havenhill*  The  process  is  as  follows:  Take  15  cc.  of  the  fluid 
extract  and  evaporate  at  60°  C.  to  a  soft  extract,  or  sufficiently 
to  expel  the  alcohol.  Add  5  cc.  of  normal  sulphuric  acid,  which 
has  been  previously  diluted  with  an  equal  volume  of  water,  and 
allow  the  resulting  mass  to  disintegrate.  When  thoroughly  disin- 
tegrated transfer  to  a  i5-cc.  graduated  cylinder  and  complete  the 
dilution  to  15  cc.  Mix  thoroughly,  and  allow  the  precipitate  to 
settle  (the  addition  of  a  little  purified  talcum  will  sometimes  be 
necessary),  filter  or  decant  off  10  cc.  into  a  separatory  funnel  and 

*  L.  E.  Sayre,  Proc.  A.  Ph.  A.,  1907,  357. 


602  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

wash  the  acid  solution  with  chloroform,  using  three  portions  (10,  5, 
5  cc.),  wash  the  united  chloroformic  washings  with  about  5  cc.  of 
slightly  acidulated  water.  Unite  the  acid  solutions ;  make  the  mixture 
alkaline  with  ammonium  hydroxid,  and  shake  out  with  chloroform, 
using  three  portions  (15,  10,  and  5  cc.).  A  fourth  portion  of  10  cc. 
may  sometimes  be  necessary  to  extract  all  the  alkaloid  before  it  ceases 
to  give  alkaloidal  reaction  with  Mayer's  reagent.  Evaporate  the 
chloroformic  solution  to  constant  weight,  and  weigh  as  crude  chloro- 
form-soluble gelsemium  alkaloids. 

A  very  satisfactory  volumetric  method  is  that  described  by  M.  H. 
Webster  (A.  J.  Ph.,  1907,  301-307).  To  10  cc.  of  the  fluid  extract 
add  1.50  gms.  of  tartaric  acid;  filter  off  the  precipitate,  washing  it 
with  absolute  alcohol.  Add  sufficient  absolute  alcohol  to  make  100  cc. 
Then  proceed  as  directed  under  Webster's  method  (page  591). 

ASSAYING  OF  PREPARATIONS  OF  HYDRASTIS 

Assay  of  Fluid  Extract  of  Hydrastis.  In  the  U.  S.  P.  method, 
which  is  based  upon  that  of  W.  A.  Puckner,*  the  separation  of  the 
insoluble  berberine  hydroiodid  from  the  hydrastine  is  effected  by 
the  employment  of  potassium  iodid,  and  the  hydrastine  then 
estimated.  Puckner 's  procedure  differs,  however,  in  avoiding  the 
use  of  aliquot  portions,  and  a  more  complete  extraction  of  the  alka- 
loid is  aimed  at.  The  method  is  as  follows:  Into  a  beaker  contain- 
ing water  25  cc.  and  potassium  iodid  T.  S.  5  cc.,  fluid  extract  5  cc., 
is  measured.  The  mixture  is  stirred  well  and  filtered  through  a 
7  cm.  filter  into  a  separator.  The  precipitate  on  the  filter  is  washed 
with  two  portions,  5  cc.  each,  of  a  mixture  consisting  of  water  19  cc. 
and  potassium  iodid  T.  S.  i  cc.  When  the  precipitate  has  drained, 
it  is  transferred  with  its  filter  back  to  the  beaker  and  mixed  with  5  cc. 
of  the  wash  fluid  above  used.  This  is  poured  on  a  new  filter,  the 
filtrate  passed  into  the  separator  and  the  beaker  and  precipitate  washed 
with  the  remaining  5  cc.  of  the  wash  fluid.  The  liquid  in  the  sepa- 
rator is  made  alkaline  with  ammonia-water,  usually  o.i  cc.  or  three 
drops  being  sufficient,  and  extracted  with  three  portions  of  ether, 
20  cc.  each;  the  aqueous  solution  being  passed  successively  through 
three  separators,  each  containing  20  cc.  of  ether.  The  ether  is  passed 
successively  through  a  small  pledget  of  cotton  into  a  beaker,  and 
finally  the  neck  of  the  separator  and  stem  of  the  funnel  is  washed 
with  5  cc.  of  ether.  The  ether  is  allowed  to  evaporate  in  a  warm  place, 
and  the  residue  dried  at  95°  to  98°  as  long  as  further  loss  is  noted. 


*  Pharm.  Rev.,  XXVI,  133. 


ASSAY   OF  GALENICAL   PREPARATIONS  603 

Tincture  of  Hydrastis  is  assayed  by  evaporating  100  cc.  on  a 
water-bath  until  it  measures  10  cc.,  adding  a  sufficient  quantity  of 
alcohol  to  redissolve  any  matter  that  may  have  been  separated,  and 
then  assaying  the  resulting  liquid  as  directed  for  fluid  extract. 

HeyPs  Modification  of  Linde's  Method.*  7.5  gms.  of  fluid 
extract  are  concentrated  to  a  thick  extract  in  an  Erlenmeyer  flask. 
The  residue  is  dissolved  in  10  cc.  water,  and  then  10  gms.  petroleum 
benzin  and  50  gms.  ether  added,  the  flask  stoppered,  rotated  and 
then  2.5  gms.  10  per  cent  ammonia -water  added.  The  mixture  is 
shaken  frequently  during  one  hour  and  then  transferred  to  a  sepa- 
rator having  a  capacity  of  250  cc.  After  separation  has  occurred, 
the  aqueous  fluid  is  drawn  off.  Benzin-ether  solution  is  passed  through 
a  pledget  of  fat -free  cotton  into  a  dry  Erlenmeyer  flask  and  the  flask 
stoppered.  50  gms.  of  this  solution  is  weighed  by  difference  into  a 
separator  and  extracted  with  10  cc.  of  a  mixture  of  one  part  hydro- 
chloric acid  and  four  parts  water,  with  two  further  portions  of  water 
5  cc.  each  containing  a  few  drops  of  dilute  hydrochloric  acid,  and 
finally  with  5  cc.  water.  To  the  acid  extractions  is  added  50  gms. 
ether,  2.5  gms.  ammonia-water,  and  the  mixture  shaken  thoroughly 
and  frequently.  After  one  hour  the  fluid  is  transferred  to  a  separator, 
the  watery  solution  drawn  off,  and  the  ether  filtered  through  a  small 
plaited  filter,  the  funnel  being  kept  covered.  50  gms.  of  the  filtrate 
are  transferred  into  a  tared  vessel,  the  ether  allowed  to  evaporate 
spontaneously  and  the  residue  dried  at  105°.  As  a  precaution,  Heyl 
carried  out  all  these  operations  as  quickly  as  possible,  although  he 
has  never  observed  the  crystallization  of  hydrastine  from  ether  solution 
reported  by  other  investigators.  This  method  is  practically  the  same 
as  that  of  the  German  Pharmacopoeia. 

The  Rusting— Smeet's  Method.f  10  gms.  of  extract  are  mixed 
in  a  capacious  tared  vessel  with  20  cc.  water  and  the  contents  reduced 
by  evaporation  to  from  loto  u  gms.  Then  1.5  cc.  of  12.5  per  cent 
hydrochloric  acid  are  added  and  after  cooling  sufficient  water  to  make 
20  gms.  Now  0.5  gm.  of  infusorial  earth  is  added,  the  mixture  well 
shaken,  filtered  and  10  gms.  transferred  to  a  loo-cc.  vial.  To  this 
4  cc.  of  10  per  cent  ammonia-water  and  25  cc.  of  ether  are  added,  and 
after  thorough  shaking  for  a  few  minutes  25  cc.  of  petroleum  ether,  boil- 
ing at  50°  to  75°  C.  After  again  agitating,  1.5  gm.  of  powdered  traga- 
canth  are  added,  the  mixture  shaken  vigorously,  40  cc.  of  the  clear  liquid 
transferred  to  a  tared  flask,  and  the  contents  reduced  to  10-11  gms. 
The  flask  is  stoppered  and  kept  in  a  cool  place  for  several  hours.  Then 
the  liquid  is  carefully  poured  off,  the  crystals  washed  with  a  little 
petroleum  ether,  dried  on  a  water-bath  and  weighed. 

*  Apoth.  Ztg.,  1906  (XXI),  797. 

t  A.  W.  von  der  Haar,  Apoth.  Ztg.,  XXI,  1050  (1906). 


604  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


ASSAYING    OF    PREPARATIONS   OF   IPECAC 

Assay  of  Fluid  Extract  of  Ipecac.  The  fluid  extract  and  other 
galenical  preparations  of  ipecac  may  be  readily  assayed  by  the  Keller 
method,  or,  in  fact,  by  any  of  the  usual  methods. 

The  following  method  may  be  used:  8  gms.  of  the  fluid  extract 
are  diluted  with  8  gms.  of  water  in  an  ordinary  vial,  32  gms.  of  chlo- 
roform and  48  gms.  of  ether  are  added  and  shaken  up;  4  gms.  of 
ammonia-water  are  now  introduced  and  the  mixture  agitated  fre- 
quently during  half  an  hour.  50  gms.  of  the  chloroform -ether  solu- 
tion (representing  5  gms.  of  the  fluid  extract)  are  separated,  poured 
into  a  tared  flask,  and  the  solvent  distilled  or  evaporated  off;  the 
varnish-like  residue  is  twice  treated  with  5  to  10  cc,  of  ether  and 
evaporated  by  forcing  a  current  of  air  into  the  flask  by  means  of  a 
rubber  bulb;  the  residue  is  then  dried  on  a  water-bath  and  weighed. 
For  the  titration  the  residue  may  be  dissolved  in  a  known  quantity  of 
decinormal  hydrochloric  acid;  the  solution  may  be  assisted  by  gentle 
heat  or  the  addition  of  a  small  quantity  of  alcohol;  ten  or  twelve  drops 
of  Brazil-wood  T.  S.  are  then  added  and  the  excess  of  acid  deter- 
mined by  means  of  decinormal  alkali,  the  latter  being  added  until 
liquid  is  cardinal  to  purplish  red  in  color.  The  quantity  of  alkali 
used  is  then  subtracted  from  the  quantity  of  decinormal  acid  first 
added.  This  gives  the  quantity  of  decinormal  acid  which  was  used 
to  neutralize  the  alkaloids  present.  The  decinormal  factor,  0.0252  gm., 
is  based  upon  the  molecular  weight  of  emetine  (Kunz)  (see  Assay  of 

N 

Ipecac  Root).     This  factor,  multiplied  by  the  number  of  cc.  of  — 

10 

acid  used  gives  the  weight  of  emetine  in  5  gms.  of  the  fluid  extract, 
and  when  this  is  multiplied  by  20  the  percentage  is  obtained. 

The  decinormal  factor  for  "total  "  alkaloid  is  0.0238. 

Method  of  U.  S.  P.  VIII.  Transfer  10  cc.  of  fluid  extract  of 
ipecac  by  means  of  a  graduated  pipette  to  a  porcelain  evaporat ing- 
dish.  Evaporate  off  the  alcohol  with  the  aid  of  a  water-bath,  and, 
when  almost  cool,  add  5  cc.  of  normal  sulphuric  acid  and  stir 
the  liquid  at  intervals  for  three  minutes.  Filter  the  liquid  into  a 
separator,  rinse  the  dish,  and  wash  the  filter  successively  with  10  cc. 
and  5  cc.  of  distilled  water,  and  add  these  liquids  to  the  separator 
To  the  separator  add  20  cc.  of  ether  and  a  small  piece  of  red  litmus 
paper;  render  the  liquid  alkaline  with  ammonia -water  and  shake  the 
separator  for  one  minute.  Draw  off  the  aqueous  layer  into  a  beaker, 
and  the  ether  layer  into  another  beaker.  Return  the  aqueous  solution 
to  the  separator,  add  10  cc.  more  of  ether,  and  shake  the  liquid, 
adding  the  ether  solution  to  that  already  in  the  beaker,  and  returning 


ASSAY    OF    GALENICAL    PREPARATIONS  605 

the  aqueous  solution  to  the  separator;  repeat  the  extraction  with 
10  cc.  more  of  ether,  and  then  add  the  ether  layer  to  that  already  in 
the  beaker.  Allow  the  combined  ether  solutions  to  evaporate,  either 
spontaneously  or  with  the  aid  of  a  water-bath  containing  warm  water, 
and  then  add  10  cc.  of  tenth-normal  sulphuric  acid  Stir  the  liquid 
carefully  with  a  glass  rod  to  facilitate  the  solution  of  the  alkaloids, 
and  when  these  have  all  dissolved,  add  five  drops  of  hasmatoxylin 
T.  S.  From  a  graduated  burette  add  sufficient  fiftieth-normal 
potassium  hydroxid  to  just  cause  the  yellow  color  of  the  solution  to 
turn  purple.  Divide  the  number  of  cc.  of  fiftieth-normal  potassium 
V.  S.  used  by  5,  subtract  the  quotient  from  10  (the  10  cc.  of  tenth- 
normal  sulphuric  acid  taken),  and  multiply  the  remainder  by  0.0238, 
and  this  product  by  10,  which  will  give  the  weight  in  grams  of  alka- 
loids contained  in  each  one  hundred  cubic  centimeters  of  fluid  extract 
of  ipecac. 
See  also 

T.  C.  Bird.     Ph.  Jour.,  1900,  Feb.  24,  March  31,  and  April  21. 
Naylor  and  Chappel.     Ph.  Jour.,  1907  (iv),  393,  395. 

ASSAY   OF   NUX   VOMICA   PREPARATIONS 

Extract  of  Nux  Vomica  (Method  of  U.  S.  P.,  1890).  Extract 
of  nux  vomica  dried  at  100°  C.,  2  gms.;  alcohol;  ammonia-water. 
sp.gr.  0.960;  water,  chloroform,  decinormal  sulphuric  acid,  centi- 
normal  potassium  hydroxid,  of  each  sufficient. 

Put  2  gms.  of  the  dried  extract  of  nux  vomica  into  a  glass  separator; 
add  to  it  20  cc.  of  a  previously  prepared  mixture  of  2  volumes  of 
alcohol,  i  volume  of  ammonia -water,  and  i  volume  of  water;  shake 
the  separator  until  the  extract  is  dissolved.  Then  add  20  cc.  of 
chloroform  and  agitate  during  five  minutes.  The  chloroform  dissolves 
the  alkaloids  which  the  ammonia  liberated.  Allow  the  chloroformic 
solution  to  separate,  remove  it  as  far  as  possible,  pour  a  few  cc.  more 
of  chloroform  into  the  separator,  and  without  shaking  draw  this  off 
through  the  stop-cock  to  wash  the  outlet -tube.  Repeat  the  extraction 
with  two  further  portions  of  chloroform  of  15  cc.  each,  washing  the 
outlet-tube  each  time  as  just  directed. 

Collect  all  the  chloroformic  solutions  in  a  wide  beaker;  expose 
the  latter  to  a  gentle  heat  on  a  water-bath  until  the  chloroform  and 
ammonia  are  completely  dissipated.  Add  to  the  residue  10  cc.  of 
decinormal  sulphuric  acid  measured  accurately  from  a  burette,  stir 
gently,  and  then  add  20  cc.  of  hot  water.  When  solution  has  taken 
place  add  2  cc.  of  Brazil-wood  T.  S.  (The  sulphuric  acid  combines 
with  the  alkaloids  and  forms  sulphates  of  the  alkaloids.) 


606  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Now  carefully  run  into  this  solution,  centinormal  potassium  hydr- 
oxid  until  a  permanent  pinkish  color  is  produced,  showing  a  slight 
excess  of  the  alkali.  Divide  the  number  of  cc.  of  centinormal  potas- 
sium hydroxid  used  by  10.  Subtract  the  number  found  from  10 

N  N 

(the  10  cc.  of  —  acid  first  used),  and  the  number  of  cc.  of  the  — 

10  10 

acid  which  went  into  combination  with  the  alkaloids  is  found. 

The  two  principal  alkaloids  of  nux  vomica  are  strychnine  and 
brucine,  and  it  is  assumed  that  they  are  present  in  equa  proportions; 
and  thus  the  factor  for  total  alkaloids  is  found  by  taking  the  mean 
of  their  respective  molecular  weights: 

Strychnine,  331.73  2)723.04 

Brucine        39I-3I  361.52 

723.04 

361.52  gms.  of  the  total  alkaloids  of  nux  vomica  will  neutralize 
1000  cc.  of  normal  sulphuric  acid.  36.152  gms.  will  neutralize  1000 
cc.  of  decinormal  sulphuric  acid. 

Hence  each  cc.  of  decinormal  sulphuric  acid  used  in  the  above 
assay  represents  0.036152  gms.  of  an  equal  mixture  of  strychnine  and 
brucine.  And  by  multiplying  the  number  of  cc.  used  by  this  factor, 
the  quantity  of  these  alkaloids  in  the  2  gms.  of  extract  taken  is  obtained, 
and  this  quantity  multiplied  by  50  will  give  the  percentage. 

The  extract  should  contain  15  per  cent  of  total  alkaloids  by  the 
above  assay. 

Fluid  Extract  of  Nux  Vomica  is  evaporated  to  a  solid  extract 
and  then  assayed  by  the  above  process. 

Tincture  of  Nux  Vomica  is  assayed  by  evaporating  100  cc.  to 
dryness,  and  the  residue  then  tested  by  the  above  process.  It  should 
contain  0.3  gm.  of  alkaloids. 

The  German  pharmacopceial  method  is  identical  with  that 
described  for  extract  of  belladonna.  lodeosin  is  used  as  indicator 
and  the  total  alkaloids  determined. 

Method  of  U.  S.  P.  VIII.  Introduce  2  gms.  of  the  extract  of  nux 
vomica  into  a  beaker,  and  dissolve  it  in  25  cc.  of  a  mixture  of  16  cc.  of 
ether,  5  cc.  of  chloroform,  and  4  cc.  of  ammonia-water.  When  dissolved, 
transfer  it  to  a  separator,  rinsing  the  beaker  with  a  little  chloroform, 
and  adding  the  rinsings  to  the  separator.  Insert  the  stopper  securely 
and  shake  the  separator  carefully  for  a  few  minutes.  Draw  off  the 
aqueous  layer  into  another  separator,  washing  the  ether-solution 
and  separator  with  a  little  water,  and  adding  this  to  the  second  sep- 


ASSAY    OF    GALENICAL    PREPARATIONS  607 

arator.  Then  shake  out  the  aqueous  liquid  with  two  portions  of  15 
and  10  cc.,  respectively,  of  chloroform,  and  add  these  to  the  first 
separator.  If  a  few  drops  of  the  liquid  left  in  the  second  separator 
still  give  a  reaction  with  mercuric  potassium  iodid  T.  S.  after  acidu- 
lating, repeat  the  shaking-out  with  10  cc.  more  of  chloroform.  Now 
shake  out  the  chloroformic  solution  in  the  first  separator  with  three 
portions  of  15,  10,  and  10  cc.  of  sulphuric  acid  solution  (3  per  cent), 
and  collect  the  combined  acid  solutions  in  another  separator.  Intro- 
duce a  small  piece  of  red  litmus  paper,  add  enough  ammonia-water 
to  render  the  liquid  alkaline,  and  extract  the  mixture  with  three 
portions,  respectively,  of  15,  10,  and  10  cc.  of  chloroform.  Draw  off 
the  chloroformic  solutions  into  a  beaker,  and  evaporate  the  chloroform 
with  the  aid  of  a  water-bath.  Dissolve  the  alkaloidal  residue  in  the 
beaker  in  15  cc.  of  3  per  cent  sulphuric  acid  solution  by  the  aid  of 
a  water-bath,  and  allow  the  liquid  to  cool.  To  this  solution  add 
3  cc.  of  a  cooled  mixture  of  equal  volumes  of  nitric  acid  (sp.gr.  1.40) 
and  distilled  water,  and  after  rotating  the  liquid  a  few  times,  set  it 
aside  for  exactly  ten  minutes,  stirring  it  gently  three  times  during 
this  interval.  Transfer  the  resulting  red  liquid  to  a  separator  con- 
taining 25  cc.  of  an  aqueous  solution  of  sodium  hydroxid  (1:10), 
and  wash  the  beaker  three  times  with  very  small  amounts  of  distilled 
water,  and  add  the  washings  to  the  separator.  If  the  liquid  is  not 
quite  turbid,  add  2  cc.  more  of  the  solution  of  sodium  hydroxid. 
Now  add  20  cc.  of  chloroform  to  the  separator,  and  shake  it  well  by 
a  rotating  motion  for  a  few  minutes,  allow  the  liquids  to  separate, 
and  draw  off  the  chloroform  through  a  small  filter,  wetted  with 
chloroform,  into  a  flask.  Repeat  this  twice,  using  10  cc.  of  chloro- 
form each  time,  and  draw  off  both  portions  into  the  flask,  using  the 
same  filter.  Finally,  wash  the  filter  and  funnel  with  5  cc.  of  chloro- 
form, and  then  evaporate  all  the  chloroform  by  means  of  a  water- 
bath,  very  carefully,  to  avoid  decrepitation.  To  the  alkaloidal 
residue,  add  10  cc.  of  tenth-normal  sulphuric  acid,  5  drops  of  iodeosin 
T.  S.,  about  90  cc.  of  distilled  water,  and  20  cc.  of  ether.  When 
all  the  alkaloid  is  dissolved,  titrate  the  excess  of  acid  with  fiftieth- 
normal  potassium  hydroxid  until  the  aqueous  liquid  just  turns  pink. 
Divide  the  number  of  cc.  of  fiftieth-normal  potassium  hydroxid  used 
by  5,  subtract  this  number  from  10  (the  10  cc.  of  tenth-normal  sul- 
phuric acid  taken),  multiply  the  remainder  by  0.0332,  and  this  pro- 
duct by  50,  which  will  give  the  percentage  of  strychnine  contained 
in  the  extract  of  nux  vomica. 

The  fluid  extract  is  assayed  by  evaporating  10  cc.  on  a  water-bath 
to  dryness,  and  then  treating  as  above  described. 

The  tincture  is  assayed  by  evaporating  TOO  cc.  on  a  water-bath, 
and  treating  the  resulting  extract  as  directed  above. 


608  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

See  also 

W.  H.  Lenton.     Ph.  Jour.,  1905,  864. 

Naylor  and  Chappel.     Ph.  Jour.,  1907  (March),  393-395. 

F.  C.  Bird.     Ph.  Jour.,  1900  (Sept.),  286. 

H.  Matthes  and  O.  Ramstedt.     Archiv.  d.  Ph.,  1907  (No.  2),  112-132 


ASSAY  OF   PREPARATIONS   OF   OPIUM 

Extract  of  Opium  (Method  of  U.  S.  P.,  1890).  Extract  of 
opium  dried  at  100°  C.,  4  gms.;  ammonia-water,  2.2  cc.;  alcohol, 
ether,  water,  of  each  a  sufficient  quantity. 

Dissolve  the  extract  of  opium  in  30  cc.  of  water,  filter  the  solution 
through  a  small  filter  and  wash  the  filter  and  residue  with  water 
until  all  soluble  matters  are  extracted,  collecting  the  washings  sepa- 
rately. Evaporate  in  a  tared  porcelain  capsule  first  the  washings  to 
a  small  volume,  then  add  the  first  filtrate,  and  evaporate  the  whole 
to  a  weight  of  10  gms.  Rotate  the  concentrated  solution  about  in  the 
capsule  until  the  rings  of  extract  are  redissolved.  Pour  the  liquid 
into  a  tared  flask  and  rinse  the  capsule  with  a  few  drops  of  water  at 
a  time  until  the  entire  solution  weighs  15  gms. 

Then  add  8.5  cc.  of  alcohol,  shake  well,  add  20  cc.  of  ether,  and 
shake  again. 

Now  add  the  ammonia-water,  stopper  the  flask  with  a  sound 
cork,  shake  it  thoroughly  during  ten  minutes,  and  set  it  aside  in  a 
moderately  cool  place  for  at  least  six  hours,  or  over  night. 

At  the  expiration  of  this  time  remove  the  stopper  carefully  and 
brush  into  the  flask  any  crystals  which  may  adhere  to  the  cork. 
Place  two  rapidly  acting,  plainly  folded  filters,  one  within  the  other, 
in  a  small  funnel,  wet  them  well  with  ether  and  decant  upon  the  inner 
one,  the  ethereal  solution,  as  completely  as  possible. 

Add  10  cc.  of  ether  to  the  contents  of  the  flask,  rotate,  and  again 
decant  upon  the  filter;  repeat  this  operation  with  another  10  cc.  of 
ether.  Then  pour  the  liquid  in  the  bottle  upon  the  filter  in  small 
portions  at  a  time,  in  such  a  way  as  to  transfer  the  greater  portion 
of  the  crystals  to  the  filter.  When  the  liquid  has  passed  through 
transfer  the  remaining  crystals  to  the  filter  by  rinsing  the  flask  with 
several  small  portions  of  water,  using  not  more  than  10  cc.  in  all. 

Apply  water  to  the  crystals  drop,  by  drop,  until  they  are  practically 
free  from  mother-liquor,  and  afterwards  wash  them  with  a  saturated 
alcoholic  solution  of  morphine,  added  drop  by  drop.  When  this  has 
all  passed  through  displace  the  remaining  alcohol  by  ether,  using 
about  10  cc.  or  more  if  necessary. 

Dry  to  a  constant  weight  at  a  temperature  not  exceeding  60°  C., 
and  carefully  transfer  the  crystals  to  a  tared  watch-glass  and  weigh 


ASSAY    OF    GALENICAL    PREPARATIONS  609 

% 

them.     The  weight,  multiplied  by  25,  gives  the  percentage  of  crystallized 
morphine  present  in  the  extract. 

Instead  of  drying  and  transferring  the  crystals  to  a  watch-glass,  as 
above  directed,  the  filter  containing  them  may  be  immersed  in  some 
boiling  water  in  a  beaker,  and  an  excess  of  decinormal  sulphuric  acid 
added  to  dissolve  the  crystals  (the  quantity  being  noted) ;  a  few  drops 
of  methyl-orange  are  then  added  and  the  mixture  titrated  with  deci- 
normal potassium  hydroxid.  Deduct  the  quantity  of  the  latter  used 
from  the  quantity  of  decinormal  acid  first  added,  and  the  quantity  of 
decinormal  acid  which  combined  with  the  morphine  is  found. 

1000  cc.  of  normal  acid  represents  one  molecular  weight  of  the 
alkaloid. 

1000  cc.  of  decinormal  acid  represents  one-tenth  of  a  molecular 

N 

weight  of  the  alkaloid  (30.092  gms.);   thus  each  cc.  of  —  acid  repre- 
sents 0.03092  gm.  of  crystallized  morphine. 

The  number  of  cc.  used,  multiplied  by  this  factor,  gives  the  quan- 
tity of  morphine  present  in  the  4  gms.  of  extract  taken. 

This  multiplied  by  25  gives  the  per  cent  of  crystallized  morphine; 
it  should  contain  18  per  cent. 

Tincture  of  Opium  (Laudanum)  (Method  of  U.  S.  P.,  1890). 
Tincture  of  opium  100  cc.,  ammonia-water  3.5  cc.,  alcohol,  ether, 
water,  each  a  sufficient  quantity.  Evaporate  the  tincture  to  about 
20  cc.,  add  40  cc.  of  water,  mix  thoroughly,  and  set  the  liquid  aside 
for  an  hour,  stirring  occasionally  and  disintegrating  the  resinous  flakes 
adhering  to  the  capsule;  then  filter  and  wash  the  filter  and  residue 
with  water,  collecting  the  washings  separately.  Evaporate  first  the 
washings  to  a  small  volume,  then  add  the  first  filtrate  and  evaporate 
to  14  gms.  Pour  the  liquid  into  a  tared  flask,  rinse  the  capsule,  and 
add  the  rinsings  until  the  entire  solution  weighs  20  gms.  Then  add 
12.2  cc.  of  alcohol;  shake  well;  add  25  cc.  of  ether;  shake  again. 
Now  add  the  ammonia-water,  cork  well,  shake  for  ten  minutes,  and 
set  aside  for  at  least  six  hours  or  overnight,  so  that  the  crystals  may  form. 

At  the  expiration  of  this  time  decant  the  ethereal  layer  upon  a 
double,  plain,  rapidly  acting  filter  previously  wTet  with  ether;  add 
10  cc.  of  ether  to  the  contents  of  the  flask,  rotate,  and  again  decant. 
Repeat  this  operation  with  another  10  cc.  of  ether.  Then  pour  the 
liquid  in  the  bottle  upon  the  filter,  in  small  portions  at  a  time,  so  as  to 
transfer  the  greater  portion  of  the  crystals  to  the  filter,  and  wash  the 
remaining  crystals  on  to  the  filter  with  the  aid  of  a  small  quantity 
of  water,  using  not  more  than  10  cc.  Then  wash  the  crystals,  first 
with  a  few  drops  of  water,  then  with  an  alcoholic  solution  of  morphine, 
and  finally  with  ether  to  displace  the  alcohol.  Dry  the  crystals  to  a 
constant  weight  and  weigh  on  a  tared  watch-glass. 


610  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

If  100  gms.  of  tincture  have  been  operated  upon,  the  weight  of  the 
crystals  is  at  once  the  per  cent  of  crystallized  morphine.  The  yield 
should  be  1.3  to  1.5  gms.  of  morphine  from  100  cc.  of  tincture.  After 
weighing,  the  crystals  may  be  titrated  as  directed  under  Assay  of 
Opium. 

The  gravimetric  assay  method  of  the  U.  S.  P.  VIII  is  an  improve- 
ment upon  the  foregoing,  and  it  or  any  of  the  methods  described 
under  Assay  of  Opium,  may  be  used.  The  German  pharmacopoeia  1 
method  is  a  very  satisfactory  one. 

LITERATURE 

Mallinkrodt  and  Dunlap.     J.  A.  C.  S.,  1905,  946. 
T.  E.  Wetterstroem.     Proc.  A.  Ph.  A.,  1906,  431. 
G.  Bergstrom.     Ph.  Centralh.,  XLVII,  1906,  632. 
G.  Fromme.     Report  of  Caesar  and  Loretz,  1906,  48. 
Vanderkleed.     Apoth.,  n,  534. 

Assay  of  Extract  of  Physostigma  (U.  S.  P.  VIII).  Transfer 
i  gm.  of  the  extract  of  physostigma  to  a  small  porcelain  dish,  add 
5  cc,  of  diluted  alcohol,  and  digest  for  five  minutes  in  a  water-bath 
below  boiling  temperature;  then  add  about  5  gms.  of  very  clean, 
fine  quartz  sand,  and  evaporate  to  dryness  on  a  water-bath,  tritu- 
rating thoroughly  with  a  pestle  to  secure  uniform  admixture.  When 
dry,  carefully  transfer  the  contents  of  the  dish  to  an  Erlenmeyer  flask, 
add  100  cc.  of  ether,  and  shake  the  flask.  Then  add  10  cc.  of  an 
aqueous  solution  of  sodium  bicarbonate  (1:20),  and  shake  the 
contents  vigorously  at  intervals  for  one  hour.  Allow  the  mixture  to 
stand,  and,  when  settled,  decant  50  cc.  of  the  ether  solution  into  a 
separator,  to  which  add  a  small  piece  of  blue  litmus  paper,  sufficient 
normal  sulphuric  acid  to  render  the  liquid  acid,  and  10  cc.  of 
distilled  water.  Shake  the  separator  well  for  one  minute,  and  draw 
off  the  aqueous  layer  into  another  separator.  Repeat  the  shaking-out 
process,  using  2  cc.  of  normal  sulphuric  acid  and  8  cc.  of  dis- 
tilled water,  and  add  the  acid  aqueous  Layer  to  the  second  separator; 
again  repeat  the  extraction,  using  i  cc.  of  normal  sulphuric  acid 
and  9  cc.  of  distilled  water,  and  add  this  to  the  second  separator. 
To  the  combined  acid  liquids  in  the  second  separator,  add  25  cc.  of 
ether,  a  small  piece  of  red  litmus  paper,  and  sufficient  sodium  bicar- 
bonate solution  (1:20)  to  render  it  alkaline.  Shake  the  separator 
for  one  minute,  allow  the  liquids  to  separate,  and  draw  off  the  ether 
into  a  beaker.  Repeat  the  shaking-out  process  with  20  cc.  and  again 
with  15  cc.  of  ether  added  to  the  separator;  shake  each  time  for  one 
minute,  allow  the  liquids  to  separate,  and  draw  off  the  ether  into  the 


ASSAY    OF   GALENICAL    PREPARATIONS  611 

beaker.  Carefully  evaporate  the  ether  from  the  combined  solutions 
by  means  of  a  water-bath,  and,  when  dry,  dissolve  the  residue  in  2  cc. 
of  tenth-normal  sulphuric  acid;  rinse  the  solution  carefully  into 
a  200-cc.  flask  with  distilled  water,  add  enough  distilled  water  to  bring 
the  volume  to  about  90  cc.,  add  25  cc.  of  ether,  and  having  shaken  the 
flask,  add  five  drops  of  iodeosin  T.  S.,  then  titrate  the  excess  of  acid 
with  fiftieth-normal  potassium  hydroxid,  until,  after  shaking,  the 
aqueous  liquid  just  acquires  a  pink  color.  Divide  the  number  of 
cc.  of  fiftieth-normal  potassium  hydroxid  used  by  5,  subtract  the 
quotient  from  2  (the  2  cc.  of  tenth-normal  sulphuric  acid  taken), 
and  multiply  the  remainder  by  0.0273,  and  this  product  by  200;  the 
result  will  be  the  percentage  of  ether  soluble  alkaloids  contained 
in  the  extract  of  physostigma. 

Assay  of  Fluid  Extract  of  Pilocarpus  (U.  S.  P.  VIII).  Transfer 
10  cc.  of  fluid  extract  of  pilocarpus  by  means  of  a  graduated  pipette  to 
a  porcelain  dish  containing  a  little  clean  sand,  and  evaporate  it  to 
dryness  with  the  aid  of  a  water-bath.  Mix  the  sand  uniformly  with 
the  extract  and  transfer  the  mixture  to  an  Erlenmeyer  flask  of  about 
100  cc.  capacity,  rinsing  the  dish  with  a  mixture  of  25  cc.  of  phloro- 
form  and  2.5  cc.  of  ammonia-water.  Transfer  the  rinsings  to  the 
flask,  cork  it  securely,  and  shake  it  well  at  intervals  during  one  hour. 
Decant  the  liquid,  transfer  to  a  separator,  wash  the  sand  with  several 
portions  of  chloroform,  draw  off  and  filter  the  chloroformic  liquid  into 
another  separator.  Then  shake  out  the  chloroform  solution  with 
15  cc.  of  normal  sulphuric  acid  transferring  the  acid  aqueous  solu- 
tion to  another  separator.  Repeat  the  shaking-out  with  a  mixture 
of  5  cc.  of  normal  sulphuric  acid  and  5  cc.  of  distilled  water,  col- 
lecting the  acid  solution  in  the  second  separator.  Again  repeat  the 
shaking-out  with  10  cc.  of  distilled  water,  and  add  the  aqueous  liquid 
to  the  second  separator.  Introduce  into  the  second  separator  a  small 
piece  of  red  litmus  paper,  add  enough  ammonia-water  to  render  the 
liquid  alkaline,  and  shake  out  the  liquid  with  20  cc.  of  chloroform, 
drawing  off  the  chloroformic  solution  into  a  beaker.  Repeat  the 
shaking-out  with  two  portions  of  15  and  locc.  each  of  chloroform,  and 
add  the  chloroformic  solutions  to  the  beaker.  Evaporate  the  chloro- 
form by  means  of  a  water-bath,  and  dissolve  the  alkaloidal  residue  in 
5  cc.  of  tenth-normal  sulphuric  acid.  Add  five  drops  of  cochineal 
T.  S.  or  iodeosin  T.  S.,  and  titrate  the  excess  of  acid  with  fiftieth- 
normal  potassium  hydroxid.  Divide  the  number  of  cc.  of  fiftieth- 
normal  potassium  hydroxid  used  by  5,  subtract  the  quotient  from 
8  (the  8  cc.  of  tenth-normal  sulphuric  acid  taken),  and  multiply 
the  remainder  by  0.02,  and  this  product  by  10,  to  obtain  the 
weight  in  grams  of  alkaloids  contained  in  100  cc.  of  the  fluid 
extract  of  pilocarpus. 


612  A    MANUAL  OF   VOLUMETRIC  ANALYSIS 

Estimation  of  the  Alkaloidal  Strength  of  Scale  Salts.  4  gins, 
of  the  scales  are  dissolved  in  30  cc.  of  water  in  a  capsule  with 
the  aid  of  gentle  heat.  The  solution  is  cooled  and  transferred  to  a 
glass  separator;  an  aqueous  solution  of  0.5  gm.  of  tartaric  acid  is 
then  added,  followed  by  an  excess  of  solution  of  sodium  hydroxid. 
The  tartaric  acid  prevents  the  precipitation  of  Fe2(OH)g,  and  the 
NaOH  sets  free  the  alkaloid.  The  alkaloid  is  then  extracted  by 
shaking  up  the  mixture  with  successive  portions  of  chloroform,  15 
cc.  each  time.  The  chloroformic  layers  are  separated  each  time 
and  mixed  and  evaporated  in  a  tared  capsule  on  a  water-bath,  and 
the  residue  dried  at  100°  C.  (212°  F.),  and  weighed.  Or  the  residue 
may  be  titrated  by  adding  sufficient  decinormal  sulphuric  or  hydro- 
chloric acid  to  dissolve  the  salts  and  still  remain  in  excess,  then 
titrating  residually  with  decinormal  NaOH  or  KOH  to  determine 
the  excess  of  acid. 


CHAPTER  LIII 
ASSAY  OF  PHENOL  (CARBOLIC  ACID) 

Preparation  of  Decinormal  Bromin  (Koppeschaar's  Solution). 
7.936  gms.  in  a  liter. 

KBr=n8.22  NaBr=  102.24 

KBrO3=  165.86  NaBrO3=  149.88 

This  solution  does  not  contain  free  bromin,  but  it  contains  two  salts, 
a  bromid  and  a  bromate,  which,  when  treated  with  hydrochloric 
acid,  liberate  a  definite  quantity  of  bromin. 

It  is  made  as  follows:  Dissolve  3  gms.  of  sodium  bromate  and  50 
gms.  of  sodium  bromid  (or  3.2  gms.  of  potassium  bromate  and  50 
gms.  of  potassium  bromid)  in  sufficient  water  to  make  900  cc. 

Transfer  20  cc.  of  this  solution  by  means  of  a  pipette  into  a  bottle 
having  a  capacity  of  about  250  cc.,  provided  with  a  glass  stopper; 
add  75  cc.  of  water,  then  5  cc.  of  pure  hydrochloric  acid,  and  imme- 
diately insert  the  stopper. 

Shake  the  bottle  a  few  times,  then  remove  the  stopper  just  suffi- 
ciently to  quickly  introduce  5  cc.  of  potassium  iodid  T.  S.,  taking 
care  that  no  bromin  vapor  escape,  and  immediately  stopper  the 
bottle. 

Agitate  the  bottle  thoroughly,  remove  the  stopper,  and  rinse  it 
and  the  neck  of  the  bottle  with  a  little  water  so  that  the  washings 
flow  into  the  bottle,  then  add  from  a  burette  decinormal  sodium 
thiosulphate  until  -the  color  of  the  free  iodin  is  nearly  all  dis- 
charged, then  add  a  few  drops  of  starch  T.  S.,  and  continue  the 

N 
titration  with  —  thiosulphate  until  the  blue  color  disappears. 

10  N 

Note  the  number  of  cc.  of  the  —  sodium  thiosulphate  thus  used, 

10 

and  dilute  the  bromin   solution   so   that    equal  volumes    of   it    and 

N 

the  —  sodium  thiosulphate  will   exactly  correspond   to  each   other 
10 

under  the  above-mentioned  conditions. 

613 


614  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Example.     Assuming  that  the  20  cc.  of  bromin  solution  required 

N 
25.2  cc.  of  the  —  thiosulphate  to  completely  absorb  the  iodin,  the 

bromin  solution  must  be  diluted  in  the  proportion  of  20  to  25.2; 
that  is,  each  20  cc.  must  be  diluted  to  make  25.2  cc. 

Thus  if  850  cc.  are  left,  they  must  be  diluted  to  make  1071  cc., 
and  the  solution  is  decinormal. 

A  new  trial  should  always  be  made  after  diluting,  and  the  bromin 
solution  should  correspond,  volume  for  volume,  with  the  decinormal 
sodium  thiosulphate. 

The  first  step  in  the  preparation  of  this  solution  is  to  dissolve  the 
salts;  then  hydrochloric  acid  is  added,  which  liberates  a  definite 
quantity  of  bromin,  as  the  equation  illustrates: 


The  stopper  should  be  inserted  into  the  bottle  as  soon  as  the 
hydrochloric  acid  has  been  added,  in  order  that  no  bromin  vapor 
escape,  and  the  bottle  rotated  so  as  to  mix  the  acid  thoroughly  with 
the  liquid. 

The  next  step  is  to  determine  the  quantity  of  bromin  which  a 
definite  volume  of  solution  will  liberate.  The  bromin  solution  should 
be  of  such  strength  that  1000  cc.  of  it  will  contain  7.936  gms.  of 
available  bromin.  Bromin,  like  chlorin,  liberates  iodin  from  potas- 
sium iodid,  and  is  estimated  in  the  same  manner. 

One  atomic  weight  of  iodin  is  liberated  by  one  atomic  weight  of 
bromin  : 


Thus  by  determining  the  quantity  of  iodin  liberated  the  quantity 
of  bromin  is  found. 

N 
The  iodin  is  determined  by  the  —  sodium  thiosulphate,  one  liter 

of  which  represents  12.59  gms-  °f  iodin,  which  is  equivalent  to  7.936 
gms.  of  bromin,  as  is  shown  by  the  following  equation: 


(Br2)      =     I2  + 
20)158.72        20)251.8  20)492.92 


7.936  gms.     12.59  gms-  24.646  gms.  or  1000  cc.  —  •  V.  S. 


10 


The  Assay  of  Phenol.     1.556  gms.  of  the   carbolic   acid  to   be 
assayed   is  dissolved   in    sufficient  water  to  make  1000  cc.   25  cc.  of 


ASSAY    OF    PHENOL  615 

this  solution,  containing  0.0389  gm.  of  the  acid,  are  transferred  to 
a  glass  -stoppered  bottle  having  a  capacity  of  about  200  cc. 

To  this,  30  cc.  of  decinormal  bromin,  followed  by  5  cc.  of  hydro- 
chloric acid,  are  added,  and  the  bottle  immediately  stoppered,  and 
shaken  repeatedly  during  half  an  hour. 

Then  the  stopper  is  removed  just  sufficiently  to  introduce  5  cc.  of 
a  20  per  cent  aqueous  solution  of  potassium  iodid,  being  careful  that 
no  bromin  escape. 

The  bottle  is  then  thoroughly  shaken  and  the  neck  rinsed  with  a 
little  water,  the  washings  being  allowed  to  flow  into  the  bottle. 

The  solution  is  now  ready  for  titration,  and  the  decinormal  sodium 
thiosulphate  is  delivered  in  from  a  burette,  until  the  iodin  is  almost 
completely  absorbed;  then  add  a  few  drops  of  starch  T.  S.,  and  con- 
tinue the  titration  until  the  blue  color  is  just  discharged. 

In  the  U.  S.  P.  VIII  it  is  directed  to  use  i  cc.  of  chloroform  instead 
of  starch.  The  precipitated  tribromphenol  interferes  somewhat  with 
the  end-reaction  when  starch  is  used,  and  frequently  with  old  phenol 
solutions  *  the  precipitate  possesses  a  bluish  color  which  is  not  removed 
by  an  excess  of  sodium  thiosulphate  and  which  makes  the  end-reaction 
difficult.  This  difficulty  is  overcome  by  the  use  of  a  small  quantity 
of  chloroform  which  dissolves  the  tribromphenol  and  admits  of  a 
very  sharp  end-reaction. 

When  chloroform  is  used  alone,  the  end-reaction  is  very  clearly 
denned,  and  is  known  by  a  colorless  aqueous  solution  and  the  chloro- 
form being  free  from  any  tinge  of  pink,  due  to  traces  of  iodin. 

N 

Note   the   number  of  cc.   of  —  thiosulphate  used;    deduct   this 

10 

N 
number  from  30  cc.   (the  quantity  of  —  bromin  originally  added), 

N 

and  the   quantity  of  —  bromin  which  went    into  combination  with 
10 

the  phenol  is  obtained. 

N 
Each  cc.  of  —  bromin  represents  0.001556  gm.  of  pure  phenol. 

10  N 

Example.     Assuming  that  6  cc.  of  —  sodium  thiosulphate  were 

10 

required  to  discharge  the  color  of  the  starch  iodid,  this  deducted  from 
30  cc.  leaves  24  cc.,  the  quantity  which  combined  with  the  phenol. 

0.0015563X24=0.037344  gm. 
0.037344X100 


0.039 


of  pure  phenol. 


*  F.  X.  Moerk,  A.  J.  Ph.,  1904,  475. 


616  A   MANUAL    OF   VOLUMETRIC  ANALYSIS 

The  above  method  originated  with  Koppeschaar,  and  is  the  only 
volumetric  method  by  which  accurate  results  may  be  obtained. 

It  is  based  upon  the  fact  that  bromin  reacts  with  phenol,  pro- 
ducing an  insoluble  precipitate  of  tribromphenol. 

The  titration  is  not  made  directly;  but  the  phenol  solution  is 
treated  with  an  excess  of  standard  bromin  solution  in  the  presence 
of  some  hydrochloric  acid.  The  hydrochloric  acid  liberates  the  bromin, 
and  the  freed  bromin  then  reacts  with  the  phenol,  as  shown  by  the 
equations  : 

(a)  5NaBr+NaBrO3+6HCl=6NaCl  +  3H 

(ft)  C6H5OH  +  3Br2=C6H2Br3OH  +  3HBr. 

6)93.34        6)476.16 
io)i5.56          10)79-36 


1.556  gms.       7.936  gms.  or  IQOO  cc.  —  bromin  V.  S. 

10 

N 
Thus  each  cc.  of  the  —  bromin  represents  0.001556  gm.  of  pure 

phenol. 

The  bromin  solution  which  was  added  in  excess,  and  the  liberated 
bromin  of  which  is  not  fixed  by  phenol,  is  then  found  by  residual 

N 
titration  with  —  sodium    thiosulphate    after    the    addition    of    some 

potassium  iodid. 

The  decinormal  bromin  solution  and  the  decinormal  sodium  thio- 
sulphate solution  being  equivalent,  each  cc.  of  the  latter  consumed 
represents  one  cc.  of  the  former.  Then  by  subtracting  the  number  of 
cc.  of  the  sodium  thiosulphate  solution  used  from  the  number  of  cc. 
of  bromin  solution  originally  added,  the  quantity  of  the  latter  which 
was  actually  consumed  by  the  phenol  present  is  found.  This  number, 
when  multiplied  by  the  factor  for  phenol,  then  gives  the  quantity  of 
pure  phenol  present. 

The  hydrochloric  acid  used  in  the  above  estimation  must  contain 
no  free  chlorin.  The  potassium  iodid  must  be  free  from  iodate.  The 
starch  T.  S.  should  not  be  added  until  most  of  the  free  iodin  has  been 
taken  up,  and  the  color  of  the  solution  has  diminished  to  light  yellow. 

The  carbolic  acid  should  be  diluted  with  water  before  titration, 
and  should  never  be  stronger  than  o.i  gm.  in  25  cc. 

Mr.  H.  Beckurts  reports  that  the  precipitate  obtained  from  phenol 
and  bromin  is  not  pure  tribromphenol,  but  a  mixture  of  tribrom- 
phenol (C6H2Br3OH)  and  tribromphenol  bromid  (C6H2Br3OBr). 

Thus  the  results  obtained  by  direct  titration  are  often  too  high, 
since  in  the  formation  of  tribromphenol  only  six  atoms  of  bromin  are 


ASSAY    OF    PHENOL  617 

required,  while  for  the  production  of  tribromphenol  bromid  eight  atoms 
of  bromin  are  taken  up  by  one  molecule  of  phenol. 

The  correct  results  obtained  by  Koppeschaar's  method  are  attribu- 
table to  the  use  of  potassium  iodid,  which  decomposes  the  tribrom- 
phenol bromid,  liberating  iodin,  thus: 

C6H2Br30Br+2KI=C6H2Br30K+KBr+I2. 

The  free  iodin  is  then  estimated  by  residual  titration,  together 
with  that  liberated  by  the  excess  of  bromin  added. 

Thus  the  nature  of  the  original  precipitate  does  not  affect  the  final 
results. 

S.  J.  Lloyd  *  in  a  comprehensive  series  of  experiments  shows  that 
the  precipitate  formed  by  the  action  of  bromin  water  upon  phenol, 
is  at  first  pure  white  (tribromphenol),  but  on  standing,  the  solution 
containing  an  excess  of  bromin,  it  gradually  assumes  a  yellowish 
tint;  the  change  is  due  to  the  formation  of  tribromphenol  bromid. 
Thus  the  latter  is  not  a  primary  product  of  the  action  of  bromin- 
water  and  phenol,  but  is  formed  by  a  gradual  reaction  between  the 
precipitated  tribromphenol  and  the  excess  of  bromin. 

This  compound  once  formed  is  not  quantitatively  reconverted 
into  tribromphenol  by  potassium  iodid.  Hence  the  less  the  quantity 
of  tribromphenol  bromid  formed,  the  more  accurate  the  analysis. 
The  amount  formed  in  a  constant  interval  of  time  (say  five  minutes), 
increases  with  the  excess  of  bromin  and  with  the  volume  of  the 
reacting  mixture,  and  is  diminished  by  adding  acid  or  potassium 
bromid  or  by  lowering  the  temperature.  Hence,  in  order  to  avoid 
the  formation  of  tribromphenol  bromid  when  titrating  phenol  with 
bromin,  the  liquid  must  be  strongly  acid  or  must  contain  an  excess 
of  potassium  bromid.  The  excess  of  bromin  must  not  be  too  great, 
and  the  time  during  which  the  precipitate  of  tribromphenol  is  in 
contact  with  the  excess  of  bromin  must  not  be  too  long.  Under 
these  conditions  only  a  mere  trace  of  tribromphenol  bromid  is  formed, 
and  it  is  precisely  when  the  quantity  of  that  substance  is  small  that 
it  is  more  easily  acted  upon  by  HI,  as  shown  in  the  above  equation. 
In  view  of  these  facts,  based  upon  his  experiments,  Jloyd  f  suggests 
the  following  method: 

Lloyd's  Hypobromite  Method.  Solutions  required.  Fiftieth - 
normal  thiosulphate  and  iodin;  starch;  hydrochloric  acid  (sp.gr. 
1.2);  potassium  iodid,  17  gms.  in  100  cc.;  hyprobromite,  prepared 
by  dissolving  9  cc.  of  bromin  in  2  liters  of  a  solution  containing  28 

*  J.  A.  C.  S.,  XXVII,  17  (1905).  t  J.  A.  C.  S.,  XXVII,  24. 


618  A    MANUAL  OF    VOLUMETRIC  ANALYSIS 

gms.  of  caustic  potash.  The  hypobromite  must  be  compared  with 
the  thiosulphate  by  adding  acid,  and  potassium  iodid  and  titrating 
the  iodin  set  free. 

The  Analysis.  Introduce  the  phenol  solution  into  a  glass -stoppered 
flask  and  add  a  volume  of  acid  equal  to  about  one  third  or  one  fourth 
of  the  combined  volumes  of  the  phenol  solution  and  the  hypobromite 
that  will  probably  be  added  during  the  analysis. 

Run  in  the  hypobromite  from  a  burette,  shaking  the  flask,  until 
the  solution  becomes  permanently  yellow.  Then  add  an  excess  of 
the  hypobromite  (10  to  20  per  cent  of  that  used  already),  and  shake 
well.  Finally  add  an  excess  of  potassium  iodid,  dilute  with  water, 
add  10  cc.  of  chloroform,  and  determine  the  iodin  with  the  standard 
thiosulphate.  The  object  of  diluting  is  to  prevent  the  acid  from 
acting  on  the  potassium  iodid  or  on  the  thiosulphate;  if  10  cc.  of 
water  be  added  at  this  stage  for  ^very  cc.  of  acid  previously  added, 
the  solution  will  be  sufficiently  dilute.  The  use  of  chloroform  may 
be  dispensed  with,  if  the  mixture  be  allowed  to  stand  five  minutes 
before  adding  the  potassium  iodid;  it  is,  however,  better  to  use  the 
chloroform — carbon  disulphid  is  not  satisfactory. 

If  these  directions  be  adhered  to,  the  phenol  can  be  determined 
within  i  or  2  parts  per  thousand. 

Dr.  Waller's  Method.  Solutions  Required,  i.  A  standard  solu- 
tion of  phenol  containing  10  gms.  of  pure  phenol  in  i  liter. 

2.  Diluted  sulphuric  acid  of  15  per  cent  or  20  per  cent  strength, 
saturated  with  alum.     This  is  needed  to  facilitate  the  settling  of  the 
precipitate. 

3.  A  solution  of  bromin  in  water. 

The  Estimation.  10  gms.  of  the  sample  are  introduced  into 
a  liter  flask,  and  made  up  with  water  to  i  liter.  This  solution  is 
filtered  through  a  dry  filter,  and  10  cc.  of  the  clear  filtrate  taken  for 
analysis.  It  is  placed  into  an  8-oz.  glass-stoppered  bottle,  and  about 
30  cc.  of  the  acid-alum  solution  added.  Into  another  bottle  of  the 
same  kind  10  cc.  of  the  standard  phenol  solution  is  put,  and  to  this 
also  30  cc.  of  the  acid-alum  solution  are  added. 

The  bromin  solution  is  now  added  from  a  burette  to  the  bottle 
containing  the  standard  phenol  solution  till  no  more  precipitate  forms, 
the  bottle  being  stoppered  and  well  shaken  after  each  addition.  The 
end-reaction  is  further  indicated  by  the  appearance  of  a  yellow  color 
when  a  slight  excess  of  bromin  is  reached.  Near  the  end  the  pre- 
cipitate forms  slowly. 

The  other  solution  containing  the  sample  under  analysis  is 
titrated  in  the  same  way.  Then  the  calculation  is  made  as 
follows : 

The  number  of  cc.  of  bromin  solution  consumed  by  the  sample 


ASSAY    OF    PHENOL  619 

is  multiplied  by  100,  and  then  divided  by  the  number  of  cc.  of  bromin 
solution  used  by  the  standard  phenol  solution.  The  answer  is  the 
per  cent  of  pure  phenol  contained  in  the  sample  analyzed. 

The  amount  of  water  contained  in  a  solution  of  carbolic  acid  may 
be  determined  by  agitating  the  solution  with  an  equal  volume  of 
chloroform  in  a  graduated  cylinder.  After  standing,  the  upper  layer 
consists  of  the  water  contained  in  the  mixture. 

Crude  or  Impure  Carbolic  Acid.*  Phenol  in  crude  carbolic 
acid  is  estimated  after  separating  the  tarry  matters.  20  cc.  of  the 
crude  carbolic  acid  are  placed  in  a  beaker  with  20  cc.  of  a  strong 
solution  of  potassium  hydroxid  (sp.gr.  about  1.30).  The  mixture  is 
well  shaken  and  allowed  to  stand  for  half  an  hour;  it  is  then  diluted 
to  I  liter  with  water.  The  tarry  matters  and  other  foreign  impurities 
are  thus  set  free,  and  may  be  removed  by  nitration,  the  filter  and 
contents  being  washed  with  lukewarm  water  till  the  washings  are 
no  longer  alkaline.  The  nitrate  and  washings  are  then  slightly 
acidulated  with  hydrochloric  acid,  and  made  up  to  3  liters  with 
water. 

The  small  quantity  of  tarry  matters  which  is  left  in  the  filtrate 
does  not  interfere  in  the  titration  which  follows.  50  cc.  of  this  solution 
are  now  taken,  and  120  cc.  of  the  decinormal  bromin  are  added, 
followed  by  5  cc.  of  hydrochloric  acid,  and  the  mixture  shaken  fre- 
quently during  half  an  hour.  10  cc.  of  potassium  iodid  T.  S.  are 
then  added,  shaken,  allowed  to  rest  (not  longer  than  five  minutes), 
and  finally  titrated  with  decinormal  sodium  thiosulphate,  using  starch 
T.  S.  as  an  indicator. 

The  number  of  cc.  of  the  thiosulphate  solution  used  are  deducted 

N 
from  120  cc.,  the  quantity  of  —  bromin    originally    added,    and    the 

quantity  of  the  latter  which  was  actually  taken  up  by  the  phenol 
is  obtained.  This  figure,  when  multiplied  by  the  factor  for  phenol, 
0.001556  gm.,  gives  the  quantity  of  phenol  present  in  the  sample 
operated  upon.  It  must  be  remembered  that  the  50  cc.  of  the  diluted 
carbolic  acid  used  in  this  assay  represent  J  of  i  cc.  of  the  original 
sample. 

Example.  Let  us  assume  that  80  cc.  of  decinormal  sodium  thio- 
sulphate were  required  in  the  residual  titration.  Deducting  this 
from  1 20  leaves  40  cc.  of  bromin  which  actually  went  into  combi- 
nation with  the  phenol;  then  40X0.001556=0.06224  gm.  of  phenol 
present  in  0.33  cc.  of  the  solution  analyzed. 


*  Toth,  Zeitschr.  anal.  Chem.,  XXV,  160  (1886).     Stockmeier  and  Thurnauer, 
Chem.  Zeit.,  1893,  119-151,  recommend  a  similar  method. 


620  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Volumetric   Determination   with   Potassium   Permanganate. 

James.  F.  Tocher,*  in  search  for  a  practical  method  for  the  volumetric 
determination  of  phenol  in  the  presence  of  its  homologues,  gives  the 
details  of  his  studies  and  investigation  which  have  led  him  to  recom- 
mend potassium  permanganate  as  suitable  for  this  purpose.  He 
finds  that  phenol,  treated  with  potassium  permanganate  in  the  pres- 
ence of  normal  or  acid  sodium  carbonate,  is  oxidized  to  oxalic  acid, 
while  the  hydrated  oxids  of  manganese  are  deposited,  and  that  in 
the  absence  of  other  oxidizable  substances,  as  great,  if  not  greater, 
accuracy  can  be  attained,  volumetrically,  than  by  any  of  the  halogen 
processes  now  commonly  employed.  The  titration  may  be  carried 
out  as  follows:  i  gm.  phenol  is  dissolved  in  1000  cc.  of  water,  and 
of  this  solution  10  cc.  (=0.01  gm.  phenol)  taken  for  titration.  About 
3  to  4  gms.  NaHCO3  is  added,  together  with  a  little  water;  then  50 
cc.  decinormal  permanganate  are  added,  the  liquid  boiled  for  five 
minutes,  and  set  aside  to  cool  a  little.  Dilute  H2SO4  is  now  added 
gradually,  until  the  mixture  is  neutralized,  and  then  to  decided  excess. 
The  mixture  is  warmed  to  60°  C.,  and  decinormal  solution  of  oxalic 
acid  added,  with  stirring,  until  the  color  is  discharged.  .  If  the  phenol 
is  pure,  29.78  cc.  of  the  permanganate  solution  will  have  been  con- 
sumed by  o.oi  gm.  of  the  substance  taken. 

The  Estimation  of  Phenol  in  Pharmaceutical  Products. 
Tablets,  powders,  and  other  pharmaceutical  products  may  be  assayed 
for  phenol  in  the  presence  of  substances  which  interfere  with  a  direct 
estimation,  by  the  method  described  below. 

In  a  series  of  experiments  Puckner  and  Clark  f  have  found : 

First.  That  phenol  can  be  completely  removed  from  a  solution 
containing  much  potassium  hydroxid  by  first  saturating  with  carbon 
dioxid  and  then  distilling  with  steam  in  a  current  of  carbon  dioxid. 
Under  these  conditions  as  much  as  0.0150  gm.  of  phenol  is  obtained 
in  the  first  100  cc.  of  filtrate  distillate. 

Second.  That  sulphites,  bromates,  and  nitrates  do  not  affect  the 
estimation  by  this  method. 

Third.  That  the  U.  S.  P.  method  for  the  valuation  of  phenol  is 
entirely  satisfactory  and  also  may  be  applied  when  the  volume  of  the 
phenol  solution  is  as  great  as  50  cc.,  and  the  amount  of  phenol  present 
sufficient  to  absorb  from  10  to  90  per  cent  of  the  bromin  present. 
They  have  evolved  the  following  method: 

The  substance  containing  the  phenol  was  placed  in  a  round- 
bottomed  distilling  flask  and  water  sufficient  to  cover  it  was  added. 
The  flask  was  connected  by  means  of  a  double  perforated  rubber 

*  Ph.  Jour.,  March  25,  1901,  360,  361. 

f  Proc.  A.  Ph.  A.,  1908;  and  A.  J.  Ph.,  LXXX,  484  (1908). 


ASSAY    OF    PHENOL  621 

stopper,  with  a  Liebig  condenser  on  the  one  hand,  and  on  the  other 
with  a  tin  reservoir  containing  water.  A  current  of  carbon  dioxid 
was  then  passed  from  a  Kipp  generator  through  the  reservoir  and 
distilling  flask  for .  fifteen  minutes  or  more.  (In  the  case  of  known 
mixtures  of  phenol  and  potassium  hydroxid  V.  S.,  phenolphthalein 
was  added  and  carbon  dioxid  passed  until  colorless,  about  five  minutes 
being  sufficient.)  The  water  was  then  heated  to  boiling  and  the  dis- 
tillation continued,  a  brisk  current  of  carbon  dioxid  *  being  passed 
through  the  apparatus  continually  until  250  cc.f  of  distillate  was 
obtained.  Of  this  distillate  50  cc.  was  taken  and  placed  in  a  25o-cc. 
glass -stoppered  flask,  25  cc.  of  standard  bromin  solution  added,  and 
the  mixture  acidulated  with  5  cc.  hydrochloric  acid  U.  S.  P.;  the 
mixture  was  shaken  frequently  during  one  half  hour,  and  then  5  cc. 
potassium  iodid  T.  S.  was  quickly  introduced  and  the  mixture  well 
shaken.  The  stopper  and  neck  of  the  flask  were  rinsed  with  water, 
a  small  amount  of  chloroform  added,  and  the  iodin  titrated  with  standard 
sodium  thiosulphate  V.  S. 

Meissinger  and  Vortmann  have  devised  a  method  which  is  based 
upon  the  reaction  between  iodin  and  phenol  in  alkaline  solution. 
This  method  is  described  on  page  650. 

LITERATURE 

Bader.     Zeitschr.  anal.  Chem.,  xxxi,  58  (1892). 

Meissinger  and  Vortmann.     Ph.  Ztg.,  xxix,  759. 

Schaedler.     Ph.  Centrabl.,  xni,  225. 

Tocher.     Ph.  Jour.,  1001,  360. 

Landolt.     Berichte  d.  Chem.  Ges.,  iv,  770  (1871). 

Degener.     Zeitschr.  prakt.  Chem.  N.  F.,  xvii,  390  (1878). 

Koppeschaar.     Zeitschr.  anal.  Chem.,  xv,  233  (1876). 

Chandelon.     Bull.  Soc.  Chim.,  xxxvni,  69  (1882). 

Telle.     Chem.  News,  LXXXIII,  51  (1901). 

Waller.     Chem.  News,  XLIH,  152  (1881). 

Seubert.     Berichte  d.  Chem.  Ges.,  xrv,  1581  (1881). 

Beckurts.    J.  S.  C.  I.,  v,  546  (1886). 

*  Simple  saturation  with  carbon  dioxid  will  not  liberate  all  the  phenol,  but  a 
stream  of  the  gas  must  be  passed  during  the  distillation;  when  in  an  experi- 
ment the  supply  of  carbon  dioxid  was  cut  off  as  soon  as  the  saturation  was  com- 
plete, and  then  the  distillation  continued,  only  88.64  per  cent  of  the  phenol  was 
recovered  in  one  case,  90.48  per  cent  in  another,  and  86.68  per  cent  in  a  third. 

t  If  250  cc.  of  distillate  is  collected,  as  shown  in  an  experiment  with  pure 
phenol,  the  first  100  cc.  of  distillate  in  one  case  contained  96.48  per  cent  of  the 
phenol  taken  and  in  another  97.22  per  cent;  with  a  mixture  of  phenol,  opium, 
bismuth  subnitrate,  and  aromatic  powder,  and  containing  7.21  per  cent  phenol, 
the  first  100  cc.  distillate  contained  98.61  per  cent  of  the  phenol  present. 


622  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

].  S.  Lloyd.    J.  A.  C.  S.,  xxvii,  7  and  16  (1905). 

Toth.     Zeitschr.  anal.  Chem.,  xxv,  160  (1886). 

Kleinert.     Zeitschr.  anal.  Chem.,  xxin,  i  (1884). 

Fedeli.     Berichte  d.  Chem.  Ges.,  xxvin,  1060  (1895). 

Giascosa.     Zeitschr.  physiol.  Chem.,  vi,  43  (1878). 

Neuberg.     Zeitschr.  physiol.  Chem.,  xxvii,  123  (1899). 

Kossler  and  Penny.     Zeitschr.  physiol.  Chem.,  xvii,  117  (1889). 

Partheil.     Apoth.  Ztg.  (1896). 

Ditz  and  Cedivoda.     Zeitschr.  angew.  Chem.,  1899,  873  and  897. 

Stockmeier  and  Thurnauer.     Chem.  Zeit,  1893,  119  and  151. 

Frehrichs.     Centrabl.,  n  (1896),  2141. 

Weinreb  and  Bondi.     Monatsheft  f.  Chem.,  VI,  506  (1885). 

Werner.     Bull  Soc.  Chim.,  XLIII,  373  (1885). 

Kastle      Am  Chem.  Jour.,  xxvii,  31  (1902). 

Schryner.     J.  S.  C.  I.  (1899),  553. 


CHAPTER  LIV 

ESTIMATION  OF  GLYCERIN 

Glycerin  (Glycerol)  C3H5(OH)3  =91.37.  The  estimation  of 
glycerin,  of  fats,  etc.,  may  be  made  by  the  method  of  Benedikt  and 
Zsigmondy.  This  method  consists  in  saponifying  the  fat  and  oxidiz- 
ing the  resultant  glycerin  by  permanganate  in  alkaline  solution;  thus 
oxalic  acid,  carbon  dioxid,  and  water  are  formed.  The  excess  of 
permanganate  is  then  destroyed  by  sulphurous  acid  or  a  sulphite, 
the  liquid  filtered  to  separate  the, manganese  dioxid,  and  the  oxalic 
acid  then  precipitated  by  a  soluble  calcium  salt  in  the  presence  of 
acetic  acid,  and  the  precipitated  calcium  oxalate  then  titrated  with 
permanganate,  or  after  ignition  and  conversion  into  carbonate  titrated 
with  standard  acid  solution  in  the  usual  way. 

Aqueous  solutions  of  glycerin  may  of  course  be  assayed  by  this 
method  very  easily. 

The  reactions  are  as  follows: 


91.37  (Potassium 

oxalate) 
165.06 

then 

K2C2O4+CaCl2==2KCl+CaC2O4; 

165.06  (Calcium  oxalate) 

127.14 

then 


5CaC2O4+8H2SO4+2KMnO4=5CaSO4 
100)635.7  100)313.96  N 

6.357  gms-  3-I396  gms.  or  1000  cc.  —  V,  S. 

10 

+  2MnSO4+  K2SO4+8H2O  +  ioCO2. 

N 

Thus  1000  cc.  —   permanganate    solution  represents  6.357  g1118- 
10 

of  calcium  oxalate,  which  is  equivalent   to  8.25  gms.   of  potassium 
oxalate,  which  is  equivalent  to  4.568  gms.  of  glycerin. 

623 


624  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

Thus  each  cc.  of  the  permanganate  solution  of  decinormal  strength 
used  up  by  the  calcium  oxalate  represents  0.004568  gm.  of  glycerin. 

If  the  precipitated  calcium  oxalate  is  ignited  and  converted  into 
carbonate,  and  the  carbonate  then  titrated  with  decinormal  sulphuric 
or  hydrochloric  acid,  the  reactions  are  as  follows: 

2CaC2O4+  O2=  2CaCO3-f  2CO2; 

4)254-28  4)198-7 

i°)_6_3_.,57_  io)49-675 

6.357  gms.  4.9675  gms- 


4)198.7        4)194.7 
10)49.675     10)48.675 

4.9675  gms.  4.8675  gms.  or  looocc.  —  V.  S. 

10 

Thus  each  cc.  of  decinormal  acid  represents  0.0049675  gm.  of 
,  or  0.006357  gm.  of  calcium  oxalate,  or  .004568  gm.  of 
glycerin. 

If  Experimenting  with  Pure  Glycerin,  operate  upon  10  cc.  of 
a  2  per  cent  solution.  This  is  diluted  with  cold  water  to  about  400 
cc.,  about  10  gms.  of  caustic  potash  are  added  to  this,  and  then 
a  saturated  solution  of  potassium  permanganate  until  the  liquid  is 
no  longer  green,  but  blue  or  blackish.  An  excess  does  not  harm. 

The  liquid  is  then  boiled  for  about  one  hour,  and  a  strong  solu- 
tion of  sodium  sulphite  is  added  to  the  boiling  liquid  until  the  violet 
or  green  color  is  destroyed;  the  liquid  is  then  filtered  while  yet  hot, 
to  separate  the  precipitated  manganese  dioxid.  When  cool,  it  is 
acidified  with  acetic  acid,  and  calcium  chlorid  added  to  precipitate 
the  oxalic  acid  as  calcium  oxalate.  When  the  deposition  of  calcium 
oxalate  is  complete  it  is  separated  by  filtration,  and  titrated  either 
with  permanganate  or  after  ignition  with  standard  sulphuric  acid. 

The  former  method  is  preferable.  For  this  purpose  the  filter 
is  pierced,  and  the  precipitate  rinsed  into  a  porcelain  basin;  about 
10  cc.  of  dilute  sulphuric  acid  are  then  added  through  the  funnel 
slowly,  so  that  it  comes  into  contact  with  and  washes  through  any 
of  the  precipitate  that  may  still  cling  to  it. 

The  liquid  is  now  diluted  to  about  200  cc.,  brought  to  60°  C.,  and 
the  decinormal  permanganate  run  in  from  a  burette,  slowly,  until 
a  faint  but  distinct  pink  color  appears  and  remains  permanent  after 
stirring;  each  cc.  of  the  permanganate  thus  used  represents  0.004568 
gm.  of  glycerin. 

The  Process  for  Estimating  the  Glycerin  of  Fats  is  as  follows: 

10  gms.  of  the  fat  or  oil  are  placed  in  a  strong  small  bottle  together 


ESTIMATION   OF  GLYCERIN  625 

with  4  gms.  of  pure  potassium  hydroxid,  dissolved  in  25  cc.  of  water; 
the  bottle  is  then  closed  with  a  solid  rubber  stopper  and  tied  down 
firmly  with  wire;  it  is  then  placed  in  boiling  water  and  heated,  with 
occasional  shaking,  from  six  to  ten  hours,  or  until  the  fat  or  oil  is 
completely  saponified.  The  contents  of  the  bottle  are  then  poured 
into  a  beaker  and  diluted  with  hot  water;  this  should  give  a  clear 
solution. 

A  dilute  acid  is  then  added  to  separate  the  fatty  acids,  which  are 
filtered  out  and  the  filtrate  made  up  to  a  given  volume. 

This  solution,  which  will  usually  contain  0.2  to  0.5  gm.  of  glycerin, 
according  to  its  'origin,  is  transferred  to  a  porcelain  basin,  diluted 
with  cold  water  to  about  400  cc.,  and  the  glycerin  estimated  as 
described  under  the  experiment  with  pure  glycerin. 

The  Modified  Form  of  Herbig  and  Mangold  (Ulzer  and 
Fraenkel)  is  as  follows:  2  to  3  gms.  of  fat  are  saponified  in  pure 
methyl  alcohol  with  potassium  hydroxid.  The  alcohol  is  volatilized, 
the  residual  soap  is  dissolved  in  hot  water,  and  is  decomposed  with 
dilute  hydrochloric  acid.  It  is  then  heated  until  the  separated  fatty 
acids  form  a  clear  layer.  To  the  liquid  fat  some  paraffin  had  better  be 
added.  It  is  cooled  thoroughly,  filtered  off  into  a  liter  flask,  and 
washed  well.  The  solution  is  exactly  neutralized  with  caustic  potash 
and  phenolphthalein.  10  gms.  more  caustic  potash  are  added,  and 
as  much  5  .per  cent  potassium  permanganate  solution  is  added  in 
the  cold  as  would  represent  approximately  one  and  a  half  times  the 
theoretical  amount.  (For  every  part  of  glycerin  6.87  parts  of  potas- 
sium permanganate.) 

The  liquid  will  then  no  longer  appear  green,  but  blue  or  black. 
It  is  allowed  to  stand  one  half  hour  at  ordinary7  temperature.  Hydro- 
gen dioxid,  in  not  too  great  an  excess,  is  added,  until  the  supernatant 
liquid  becomes  colorless.  It  is  diluted  to  the  mark,  is  shaken  briskly, 
and  500  cc.  are  filtered  off  through  a  dry  filter.  To  decompose  the 
hydrogen  dioxid  the  liquid  is  boiled  for  half  an  hour,  cooled  to  about 
60°  C.,  and  after  addition  of  sulphuric  acid  the  oxalic  acid  formed 
is  titrated  with  permanganate. 

In  place  of  filtration,  the  filtrate,  after  acidifying  with  acetic  acid, 
may  be  precipitated  with  calcium  chlorid.  When  filtered  it  may  be 
determined  either  gravimetrically  by  ignition  to  calcium  oxid,  or  it 
may,  after  decomposition  writh  sulphuric  acid,  be  titrated  with  per- 
manganate, as  above. 

The  Acetin  Method.*  This  method  is  a  preferred  one  because 
of  its  simplicity  and  rapidity  as  compared  with  other  methods.  It 
depends  on  the  phenomenon  that  glycerin  on  boiling  with  acetic 

*  Benedikt  and  Cantor,  Monatsheft,  IX,  521. 


626  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

anhydrid  is  quantitatively  transformed  into  triacetin.  If  the  latter 
be  then  dissolved  in  water,  and  the  free  acetic  acid  be  neutralized 
with  sodium  hydroxid,  the  dissolved  triacetin  can  be  saponified  with 
sodium  hydroxid  and  the  excess  of  the  latter  titrated  back. 

The  reactions  occurring  are  expressed  by  the  following  equations: 


(i) 

Acetic  anhydrid.  Triacetin. 

(2) 

The  necessary  reagents  are:    (a)  Normal  hydrochloric  acid. 

(b)  Dilute  sodium  hydroxid  solution,  not  standardized,  but  con- 
taining about  20  gms.  of  sodium  hydroxid  to  the  liter. 

(c)  A  stronger  sodium   hydroxid  solution    (about    10   per  cent.) 
This  should  be  preserved  in  a  flask  provided  with  a  25  cc.  pipette. 

(d)  Phenolphthalein  T.  S. 

The  procedure,  i  to  1.5  gms.  of  the  sample  are  weighed  out  in  a 
wide-necked,  small,  round-bottom  flask  of  about  100  cc.  capacity;  7  to 
8  gms.  acetic  anhydrid  are  added,  with  about  3  gms.  dehydrated 
sodium  acetate.  The  mass  is  boiled  under  a  reflux  for  one  to  one 
and  a  half  hours. 

Triacetin  is  formed  as  per  reaction  (i).  It  is  then  allowed  to 
cool  somewhat  and  diluted  with  50  cc.  of  water  and'  the  mixture 
likewise  heated  under  a  reflux  (but  not  boiled),  until  upon  agitation 
the  triacetin  is  completely  dissolved.  The  solution  is  then  filtered 
into  a  large  flask  (500  cc.).  Usually  a  flocculent  white  precipitate 
remains  on  the  filter.  This  is  washed,  the  liquid  cooled,  phenol- 
phthalein  added,  and  the  free  acetic  acid  present  exactly  neutralized 
with  dilute  sodium  hydroxid  solution.  The  neutralization  is  known 
to  be  complete  when  the  slightly  yellowish  color  is  changed  to  reddish- 
yellow.  It  must  not  be  pink,  as  this  indicates  an  excess  of  the  alkali, 
which  excess  will  saponify  some  of  the  triacetin.  The  triacetin  is  now 
saponified  by  adding  25  cc.  of  the  stronger  sodium  hydroxid  solution 
and  boiling  for  fifteen  minutes  under  a  reflux  condenser.  (See  Reac- 
tion (2)).  This  is  then  titrated  back  with  normal  hydrochloric  acid. 
A  check  titration  is  now  made  upon  25  cc.  of  the  sodium  hydroxid 
with  normal  hydrochloric  acid,  and  from  the  difference  the 
alkali  used  by  the  triacetin  is  found.  This  is  then  calculated  into 
glycerin.  Each  cc.  of  the  normal  acid  represents  0.03045  gm.  of 
pure  glycerin.  The  difference  in  the  quantity  of  the  standard  acid 
used  in  the  check  titration,  and  in  the  actual  analysis,  multiplied 
by  the  above  factor,  gives  the  quantity  of  pure  glycerin  in  the  sample 
taken. 


ESTIMATION  OF  GLYCERIN  627 

In  order  to  obtain  accurate  results,  the  steps  should  be  conducted 
rapidly  and  continuously,  and  above  all  the  free  acetic  acid  should 
be  neutralized  with  great  care  and  an  excess  of  alkali  avoided. 

The  method  gives  results  which,  in  the  hands  of  careful  workers, 
agree  well  with  other  methods. 

O.  Heller  recommends  the  above  method  as  being  the  best. 

The  Bichromate  Method  (Hehner).*  This  method  depends 
upon  oxidizing  the  glycerin  with  potassium  dichromate  in  the  presence 
of  sulphuric  acid.  The  quantity  of  standard  dichromate  used  is 
then  determined  by  means  of  a  standard  solution  of  ammonium  ferrous 
sulphate. 

One  part  of  glycerin  is  completely  oxidized  (converted  into  CO2) 
by  7.486  parts  of  dichromate  in  the  presence  of  sulphuric  acid.  The 
method  is  very  easy  and  rapid,  and  is  considered  the  best  general 
method. 

Solutions  Required.  Standard  Potassium  Dichromate  Solution 
containing  74.86  gms.  of  the  dichromate  and  150  cc.  of  concentrated 
sulphuric  acid  in  a  liter.  One  cc.  of  this  solution  should  be  equiva- 
lent to  o.oi  gm.  of  glycerin.  The  exact  oxidizing  power  of  this  solu- 
tion must  be  ascertained  by  titration  against  a  solution  containing  a 
known  quantity  of  iron  wire,  or  pure  ferrous  ammonium  sulphate. 

Weaker  Dichromate  Solution,  made  by  diluting  the  above  so  as  to 
have  a  solution  of  one  tenth  the  strength. 

Standard  Iron  Solution.  This  should  contain  240  gms.  of  ferrous 
ammonium  sulphate  and  50  cc.  of  concentrated  sulphuric  acid  per 
liter.  It  is  standardized  against  the  dichromate  solution,  using  potas- 
sium ferricyanid  as  the  indicator. 

The  Process.  With  pure  glycerin  the  oxidation  by  this  method  is 
absolutely  quantitative.  0.2  gm.  accurately  weighed,  are  moderately 
diluted  in  a  beaker.  10  cc.  of  concentrated  sulphuric  acid  are  added, 
followed  by  30  or  40  cc.  of  the  stronger  dichromate  solution.  The  beaker 
is  then  covered  with  a  watch-glass,  placed  in  a  water-bath  and  digested 
for  two  hours.  The  excess  of  dichromate  is  then  determined  by  titra- 
tion with  the  standard  iron  solution. 

When  great  accuracy  is  required  the  weaker  dichromate  is  useful 
for  completing  the  titration.  The  operation  should  be  conducted  as 
near  as  possible  at  a  temperature  of  16°  C. 

Crude  glycerin  must  be  first  purified  from  chlorin  or  aldehyde  as 
follows:  1.5  gms.  of  the  diluted  sample  is  placed  in  a  loo-cc.  flask, 
heated  with  some  moist  silver  oxid  and  let  stand  ten  minutes;  sub- 
acetate  of  lead  solution  is  then  added  in  slight  excess  and  the  whole 

*  J.  S.  C.  I.,  1889,  4. 


628  A   MANUAL  OF   VOLUMETRIC   ANALYSIS 

diluted  to  100  cc.  and  filtered  through  a  dry  filter.     25  cc.  of  this  are 

then  digested  with  excess  of  dichromate  and  titrated  as  above  described. 

The  estimation  of  glycerin  in  fats  and  oils  is  carried  out  as  follows  : 

Saponify  3  gms.  of  fat  with  alcoholic  KOH  solution,  do  not  drive 

off  the  alcohol,  but  dilute  the  soap  solution  to  about  200  cc.  and  decom- 

pose with  dilute  sulphuric  acid;    filter  off  the  insoluble  acids,  which 

may  be  estimated  as  usual.     Vigorously  boil  the  filtrate  and  washings, 

amounting  together  to  500  cc.  in  a  covered  beaker,  down  to  one  half. 

Then  add  sulphuric  acid  and  standard  dichromate  as  described. 

The  lodic  Acid  Method  (Chaumeil).*  A.  Chaumeil  suggests 
that  'the  difficulties  of  the  dichromate  method  may  be  obviated  by 
substituting  iodic  acid,  which,  besides  being  a  more  energetic  oxidizing 
agent,  has  the  advantage  of  not  being  destroyed  by  sulphuric  acid, 
however  concentrated  the  latter  may  be.  The  oxidation  of  glycerin, 
by  iodic  acid  in  the  presence  of  sulphuric  acid  is  complete,  the  reaction 
proceeding  as  follows: 


5C3H5(OH)3+  7l2O5=  isCO2+  2oH2O+  7X2, 

from  which  it  is  seen  that  five  molecules  of  glycerin  liberate  seven 
molecules  of  iodin.  In  making  the  determination,  a  fragment  of 
marble  is  placed  in  the  distilling  flask;  the  CO2  evolved  maintains  a 
slight  pressure  in  the  apparatus,  and  prevents  absorption.  The 
receiver  contains  potassium  iodid  solution,  in  which  the  iodin  dis- 
solves when  it  comes  over.  The  iodin  is  readily  determined  by  standard 
solution  of  sodium  thiosulphate.  Where  the  glycerin  contains  chlorids, 
the  author  does  not  eliminate  the  latter  by  a  preliminary  treatment 
with  carbonate  of  silver,  as  has  been  recommended,  but  titrates  the 
glycerin  directly,  and  subtracts  from  the  volume  of  the  thiosulphate 
used  that  accounted  for  by  the  chlorids. 

ESTIMATION  OF  GLYCERIN  IN  FLUID  EXTRACTS 

Take  10  gms.  of  the  fluid  extract,  evaporate  it  at  a  low  tempera- 
ture to  5  gms.  It  is  important  that  a  low  ,  temperature  be  employed 
in  order  that  the  alcohol,  but  not  the  glycerin,  be  volatilized. 

Dissolve  the  residue  in  50  cc.  of  water,  and  add  solution  of  lead 
subacetate,  drop  by  drop,  until  precipitation  is  complete.  Allow  the 
precipitate  to  subside,  filter  the  liquid  through  a  wet  filter,  wash  the* 
precipitate  thoroughly  with  water,  add  to  the  filtrate  and  washings  a 
few  drops  of  dilute  sulphuric  acid,  then  io  gms.  of  solid  potassium 
.  j  . 

*  Bull.  Soc.  Chim.,  XXVII,  12. 


ESTIMATION  OF  GLYCERIN  629 

hydroxid,  and  an  excess  of  potassium  permanganate  solution.  Bring 
the  liquid  to  the  boiling  point  and  keep  there  for  about  one  hour,  then 
add  sufficient  of  a  strong  solution  of  sodium  sulphite  to  destroy  the 
violet  color  due  to  the  excess  of  permanganate. 

Filter  while  still  hot  to  separate  the  precipitated  MnC>2,  and  when 
cool  acidify  with  acetic  acid  and  add  calcium  chlorid  solution  to  pre- 
cipitate the  oxalic  acid  as  calcium  oxalate. 

When  precipitation  is  complete  filter  and  titrate  (after  the  addition 

N 
of  sulphuric  acid)  with  —  potassium  permanganate. 

Each  cc.  =  0.004568  gm.  glycerin. 

LITERATURE 

C.  Mangold.     Zeitschr.  angew.  Chem.,  1891,  400. 
O.  Hehner.     J.  S.  C.  I.,  vm,  4  (1889). 
Richardson  and  Jaffe.     J.  S.  C.  I.,  1898,  330. 
Benedikt  and  Cantor.     Monatscheft,  ix,  521. 
E.  Suhr.     Archiv.  f.  Hygiene,  xiv,  305. 

Chaumeil.  Bull.  Soc.  Chim.,  xxvii,  12,  and  Ph.  Jour.,  April,  1903, 
490. 

Lewkowitsch.     Analyst,  xxvin,  104,  and  Ph.  Jour.,  April,  1903,  558. 


CHAPTER  LV 
ESTIMATION  OF  TANNIN 

THE  principal  volumetric  method  for  tannic  acid  depends  upon 
the  use  of  permanganate.  The  estimation  of  tannic  acid  by  means 
of  potassium  permanganate  is  by  no  means  as  easily  carried  out  as 
is  the  titration  of  ferrous  salts  with  permanganate.  Concordant 
results  are  obtained  only  when  the  titrations  are  carried  out  in  exactly 
the  same  manner,  and  in  accordance  with  definite  directions.  The 
tannic  acid  or  tannins  found  in  various  parts  of  different  plants  (such 
as  oak  bark,  nut-galls,  cinchona,  sumach,  pine  bark,  tea,  etc.)  do 
not  possess  identical  properties,  and  in  fact,  equal  quantities  of  tannic 
acid  from  different  sources  will  reduce  different  quantities  of  per- 
manganate. Thus  it  is  evident  that  the  accurate  determination  of 
tannic  acid  is  by  no  means  a  simple  matter.  If  it  were  possible  in 
every  case  to  check  the  permanganate  against  the  particular  tannin 
to  be  estimated,  this  difficulty  would  be  overcome.  This  is,  however, 
rarely  possible.  Another  difficulty  to  overcome  is  the  fact  that  the 
various  tanning  materials  contain'  besides  tannin,  other  substances 
which  have  a  reducing  action  upon  permanganate.  The  method 
employed  to  overcome  this  difficuty  is  to  titrate  with  permanganate 
and  find  the  total  reducing  power  of  the  mixed  constituents,  and  then 
by  means  of  gelatin,  glue,  or  hide-powder  remove  the  tannic  acid  and 
then  again  titrate  with  permanganate  to  determine  the  oxidizable 
matters  other  than  tannin.  The  difference  between  the  two  titrations 
represents  the  tannin.  Lowe  (Zeitschr.  anal.  Chem.,  iv,  368)  has 
shown  that  pectinous  substances  are  frequently  present  in  tanning 
material  (especially  oak  bark)  and  that  these  must  first  be  separated 
if  accurate  results  are  to  be  obtained,  because  these  substances  are 
precipitated  by  the  substances  which  are  used  to  precipitate  tannin 
(particularly  hide-powder).  To  separate  these  substances  Lowe  evap- 
orates the  liquid  extract  of,  say,  oak  bark,  with  the  addition  of  a  drop 
of  acetic  acid,  to  dryness  on  a  water-bath.  The  residue  he  then  extracts 
with  strong  alcohol,  which  dissolves  the  tannin  but  not  the  pectinous 
substances.  He  then  evaporates  the  alcoholic  solution  on  a  water- 
bath  and  takes  up  the  dry  residue  with  water. 

630 


ESTIMATION   OF   TANNIN  631 

LowenthaPs  method,  which  is  described  below,  is  accepted  as  the 
best  volumetric  method. 


ESTIMATION   OF   TANNIN  IN  BARKS,   ETC.    (LOWENTHAI/S  METHOD) 

The  principle  of  this  method  depends  upon  the  oxidation  of  the 
tannic  acid,  together  with  other  easily  oxidizable  substances,  by 
titrating  with  potassium  permanganate. 

The  total  amount  of  such  substances  is  thus  found,  and  expressed 
by  a  known  volume  of  permanganate.  The  actual  available  tannin 
is  then  removed  by  gelatin  or  glue,*  and  another  titration  made,  to 
determine  the  amount  of  oxidizable  matters  other  than  tannin. 

The  difference  between  the  amounts  of  permanganate  solution 
used  in  the  two  titrations  gives  the  amount  of  tannin  present,  which 
is  available  for  tanning  purposes,  expressed  in  terms  of  permanganate. 

N 

Solutions  Required,    i.  —  Potassium   Permanganate    (1.05  gm. 
ou 

per  liter). 

2.  Indigo  Solution.    6  gms.  of  pure  precipitated  indigo  and  50  cc.  of 
concentrated  sulphuric  acid  are  dissolved  in  sufficient  water  to  make 
i  liter. 

3.  Glue  and  Salt  Solution.     25  gms.  of  good  transparent  glue  are 
macerated  in  cold  water,  and  then  heated  to  dissolve;   the  solution 
is  then  made  up  to  i  liter  and  saturated  with  common  salt.    The 
solution  should  be  filtered  clear  when  used. 

4.  Acidified    Solution    of   Common    Salt.    This    is    a     saturated 
solution  of  common  salt,  containing  in  i  liter  25  cc.  of  sulphuric  acid. 

The  Analysis.     20  gms.  of  the  bark  or  10  gms.  of  sumach  are 


*Neubauer  (Zeitschr.  anal.  Chem.  X,  i)  uses  animal  charcoal.  Hammer 
I  Zeitschr.  prakt.  Chem.  LXXXI,  159)  uses  hide-powder  (hide  prepared  for  tanning 
and  reduced  to  powder).  Siemand  (Zeitschr.  anal.  Chem.  XXII,  595)  recom- 
mends the  employment  of  the  glue-yielding  tissue  of  bones  or  horn-cartilage,  which 
are  prepared  as  follows:  Cut  off  the  ends  of  hollow  bones,  remove  the  marrow 
and  break  the  bones  into  large  pieces.  Digest  these  pieces  for  two  days  with  a  5 
per  cent  solution  of  sodium  carbonate,  then  brush  and  wash  them  with  water, 
leaving  them  in  contact  each  time  for  several  hours.  Then  break  the  bones  into 
smaller  pieces  and  treat  with  dilute  hydrochloric  acid,  8  liters  of  which  contain  i 
liter  of  commercial  hydrochloric  acid,  until  they  become  soft.  Then  wash  with 
water  to  remove  the  acid  and  grind  in  a  small  mill  whilst  still  moist.  Remove 
the  last  traces  of  calcium  salts  and  ferric  oxid  by  digesting  repeatedly  with 
diluted  hydrochloric  acid  (1:20)  and  thoroughly  wash  with  water,  press  and  dry. 
Horn-cartilage  (the  bony,  vascular  nucleus  of  cattle  horn).  This  is  freed  from 
calcium  salts  in  the  manner  described  above. 


632  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

boiled  with  several  portions  of  water  until  exhausted,  and  the  solution 
when  cold  is  made  up  to  i  liter. 

10  cc.  of  this  solution  are  diluted  to  1000  cc.;  25  cc.  of  the  indigo 
solution  are  added,  and  the  permanganate  solution  then  run  in,  drop 
by  drop,  from  a  burette,  stirring  constantly,  until  the  blue  color 
changes  to  yellow,  and  the  number  of  cc.  of  permanganate  solution 
consumed  noted. 

25  cc.  of  the  indigo  solution  are  now  taken  and  diluted  to  1000 
cc.,  titrated  with  permanganate,  and  the  number  of  cc.  again  noted. 
By  deducting  this  number  from  the  number  of  cc.  used  in  the  first 
titration,  the  quantity  of  permanganate  required  by  the  tannin  and 
the  other  oxidizable  substances  in  the  10  cc.  of  solution  taken  is 
found. 

The  next  step  is  to  deprive  a  portion  of  the  tannin  solution  of  its 
tannin,  and  again  titrate. 

100  cc.  of  the  tannin  solution  are  treated  with  50  cc.  of  the  glue 
and  salt  solution,  and,  after  stirring,  100  cc.  of  the  acidulated  salt 
solution  are  added,  the  mixture  stirred  again,  and  set  aside  for  several 
hours.  The  glue  absorbs  the  tannin  out  of  solution.  The  solution 
is  then  filtered.  The  filtrate  should  be  perfectly  clear. 

Of  this  filtrate  take  50  cc.  (containing  20  cc.  of  the  tannin  solution), 
mix  with  25  cc.  of  the  indigo  solution,  and  titrate  with  the  perman- 
gaifate  solution  as  before,  noting  the  number  of  cc.  consumed. 

Another  25  cc.  of  the  indigo  solution  are  now  taken,  diluted  as 
in  the  other  trial,  and  again  titrated  with  permanganate.  By  deduct- 
ing the  number  of  cc.  so  obtained  from  the  number  required  by  the 
50  cc.  of  filtrate,  the  quantity  required  by  the  oxidizable  matter  other 
than  tannic  acid  in  the  20  cc.  of  tannin  solution  is  obtained.  There- 
fore one  half  of  this  quantity,  when  deducted  from  the  quantity  of 
permanganate  solution  representing  the  total  oxidizable  matter  in 
10  cc.  of  the  tannin  solution,  gives  fhe  quantity  of  permanganate 
which  was  effected  by  the  tannin  above. 

Duplicate  titrations  should  always  be  made,  and  should  agree 
within  o.i  or  0.2  cc.  of  the  permanganate  solution. 

Thus  far  we  have  only  the  tannin  value  (expressed  in  terms  of 
permanganate),  of  10  cc.  of  the  original  solution,  representing  T^7  of 
the  material  under  examination. 

The  permanganate  solution  may  be  compared  with  a  standard 
solution  of  the  purest  gallo-tannic  acid  obtainable,  or  with  any  tannin 
of  known  value,  and  thus  a  coefficient  obtained. 

According  to  the  experiments  of  Neubauer,  63  gms.  of  pure 
crystallized  oxalic  acid  (equivalent  to  31.4  gms.  potassium  perman- 
ganate) correspond  to  41.57  gms.  of  purified  gallo-tannic  acid  (nut- 
gall  tannin).  And  Oser  found  that  63  gms.  of  oxalic  acid  correspond 


ESTIMATION   OF    TANNIN  633 

to  62.355  gms'  of 'querci-tannic  acid  (oak-bark  tannin).  These  coeffi- 
cients are  now  largely  used. 

N 
Based  upon  these  figures  each-cc.  of  -  -  permanganate  solution 

represents  .0013856  gm.  of  gallo-tannin,  or  .0020785  gm.  of  querci 
tannin.  In  most  analyses,  however,  especially  when  the  composition 
of  the  tannin  is  not  exactly  known,  it  is  expressed  as  oxalic  acid. 

Notes.  The  Quantity  of  Material  to  be  taken:  This  must  be 
such  as  will  make  a  solution  which  contains  from  0.5  to  i  gm.  of  the 
tanning  principle  per  liter.  With  this  in  view,  the  following  quan- 
tities of  the  substances  should  be  weighed  off: 

Pine  bark X 10  to  15  gms. 

Oak  bark 9  to  10  gms. 

Spanish  chestnut  wood 6  to    8  gms. 

Valonia. 3  to    4  gms. 

Sumach 6  to    8  gms. 

At  a  meeting  of  the  A.  O.  A.  C.,  November  16,  1900,  it  was 
resolved  that  such  a  quantity  of  the  material  should  be  taken  as  will 
give  about  6.8  gm.  of  total  solids  per  100  cc.  of  the  solution.  Then 
extract  in  a  Soxhlet  or  other  similar  apparatus  at  steam  heat,  for 
non -starchy  materials.  For  canaigre  and  substances  containing 
like  amounts  of  starch,  use  a  temperature  of  50°  to  55°  C.  until  near 
completion,  finishing  the  extraction  at  steam  heat.  In  the  case  of 
fluid  extracts  weigh  such  a  quantity  as  will  leave  a  residue  of  0.8  gm. 
on  evaporation  of  100  cc.  Dissolve  in  800  cc.  of  water  at  a  tempera- 
ture of  80°  C.,  allow  to  stand  twelve  hours,  and  make  up  to  i  liter. 

The  Rate  of  Titration.  The  rate  of  speed  at  which  the  permanga- 
nate is  added  influences  the  results  considerably.  At  least  four 
minutes  should  be  consumed  in  the  titration.  Towards  the  end  of 
the  titration,  the  permanganate  should  be  added,  drop  by  drop,  in 
order  to  observe  a  sharp  end-reaction. 

The  Indigo  Solution.  The  concentration  of  the  indigo  solution 
is  correct  if  20  cc.  of  it  require  about  an  equal  quantity  of  perman- 
ganate solution.  If  much  more  or  less  is  required,  the  solution  must 
be  correspondingly  diluted  or  strengthened.  The  color  of  the  solution 
changes  gradually  from  deep-blue  to  dark-green,  then  to  light -green 
and  then  to  yellowish-green.  The  last  greenish  tint  disappears  with 
the  addition  of  the  next  drop  of  permanaganate  solution.  If  this  last 
change  is  not  sharp,  it  indicates  that  the  indigo  solution  is  not  pure 
enough;  it  probably  contains  some  indigo-red,  and  cannot  be  used 
for  accurate  work. 


634  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

Estimation  by  Means  of  Egg  Albumen.  G.  Fleury  (Jour. 
Ph.  Chim.,  1892,  499)  proposes  to  use  egg  albumen  for  estimating 
tannin  in  wine  and  in  the  petals  of  red  roses. 

The  hard-boiled  egg-albumen  is  dried  at  a  moderate  temperature, 
and  powdered.  This  is  washed  with  dilute  alcohol  (10  per  cent), 
very  slightly  acidulated  with  tartaric  acid,  to  saturate  the  alkali.  The 
albumen  is  again  dried,  and  kept  in  a  well-stoppered  bottle. 

The  method  of  operation  is  as  follows: 

Albumen  powder,  equal  to  seven  or  eight  times  the  quantity  of 
tannin,  which  is  supposed  to  be  present,  is  added  to  the  liquid  in  a 
flat  dish.  The  dish  is  then  set  aside  for  forty-eight  hours,  stirring 
occasionally;  the  liquid  must  during  this  time  be  acid,  not 
alkaline. 

The  end  of  the  reaction  is  attained  when  the  liquid  ceases  to  give 
to  give  a  color  with  ferric  chlorid  T.  S. 

The  powder  is  then  collected  on  a  filter,  washed  with  very  dilute 
alcohol,  and  then  dried  at  100°  C.  At  the  same  time  a  sample  of  the 
original  powder  is  dried  and  weighed,  to  determine  the  amount  of 
water  it  contains. 

The  increase  in  weight  of  the  albumen  which  was  in  contact  with 
the  tannin,  minus  the  loss  of  weight  of  the  albumen  in  the  check 
experiment,  gives  the  weight  of  the  tannin  present. 

This  method  is  not  available  for  determining  the  tannin  in  nutgalls, 
because  the  absorption  by  the  albumen  is  incomplete  and  too  slow. 
In  testing,  it  must  be  borne  in  mind  that  gallic  acid  is  not  absorbed 
by  the  albumen,  and  consequently  still  gives  its  reaction  with  ferric 
chlorid. 

Estimation  by  Means  of  Hydrogen  Dioxid.  Thompson  (Chem. 
Ztg.,  1902,  1085)  gives  the  following  method,  in  which  tannin  is 
determined  by  the  quantity  of  oxygen  which  it  absorbs  when  in 
alkaline  solution. 

The  necessary  nascent  oxygen  is  obtained  by  the  decomposition  of 
hydrogen  dioxid  in  concentrated  alkaline  solution  upon  the  addition 
of  chemically  pure  lead  peroxid.  The  presence  of  the  tannin  does 
not  in  the  least  interfere  with  the  complete  decomposition  of  the 
hydrogen  dioxid. 

The  tannin  is  freed  of  inorganic  and  pectinous  substances  by 
treatment  with  90  per  cent  alcohol,  or  purified  90  per  cent  methyl- 
alcohol. 

The  quantity  of  tannin  is  calculated  out  of  the  difference  in  the 
quantity  of  available  oxygen  from  a  definite  quantity  of  hydrogen 
dioxid,  and  the  quantity  of  available  oxygen  still  present  after  treat- 
ment with  a  definite  quantity  of  tannin,  i  gm.  of  C.  P.  anhydrous 
tannin  absorbs  (at  o°  C.  760  mm.  pressure)  20  cc.  of  oxygen. 


ESTIMATION  OF  TANNIN  635 

Estimation  by  the  Aid  of  Safranin.  L.  Specht  and  F.  Lorenz  * 
recommend  a  method  for  the  estimation  of  tannin  which  depends 
upon  its  precipitation  with  safranin  as  a  tannin  antimony  lake  and 
the  reducibility  of  the  uncombined  safranin  by  means  of  hyposulphite. 
The  tannin  material  is  precipitated  with  tartar  emetic  and  a  known 
quantity  of  safranin,  both  in  excess,  and  the  excess  of  safranin  is  titrated 
with  hyposulphite  solution,  the  titer  of  which  is  adjusted  to  safranin. 
The  amount  of  safranin  consumed  is  determined  by  the  difference, 
and  the  amount  required  for  the  tannin  is  ascertained  by  a  blank 
experiment  made  with  tannin  of  known  purity.  To  avoid  the  inter- 
fering action  of  oxygen,  the  distilled  water  used  throughout  the  process 
must  be  previously  boiled,  while  the  hyposulphite  solution  must  be 
freshly  prepared,  as  required.  Ammonium  hyposulphite,  for  the 
preparation  of  which  specific  directions  are  given,  being  found  to  possess 
the  greatest  stability. 

The  ammonium  hyposulphite  used  in  this  process  is  made  as 
follows:  50  gms.  of  pure  zinc  dust  and  100  gms.  of  water  are  mixed 
together,  and  600  cc.  of  solution  of  ammonium  bisulphite  20°  B., 
neutralized  with  ammonia,  is  allowed  to  flow  into  this  mixture,  which 
must  be  refrigerated  so  that  the  temperature  does  not  rise  above 
36°  F.  The  solution  is  clarified  by  subsidence,  and  75  cc.  of  it  are 
diluted  with  previously  boiled  and  cooled  water  to  2000  cc.  The 
flocculent  precipitate  produced  on  dilution  is  allowed  to  subside, 
since  filtration  and  consequent  access  of  air  must  be  avoided.  The 
titer  of  this  solution  is  then  adjusted  to  safranin. 

Estimation  by  Means  of  Silk.f  Vignon  recommends  untwisted 
silk  as  superior  to  either  gut-strings  or  powdered  hide  for  the  esti- 
mation of  tannin  in  aqueous  solutions.  If  an  excess  of  silk  (about 
5  gms.)  is  immersed  in  a  solution  of  o.i  gm.  of  tannin  in  100  cc.  during 
four  or  five  hours  at  a  temperature  of  50°  C.,  the  tannin  will  be  com- 
pletely absorbed,  but  none  of  the  substances  that  usually  are  present, 
such  as  gallic  acid,  glucose,  etc.  The  quantity  of  absorbed  tannin 
may  then  be  determined  in  several  ways:  by  direct  weighing;  by  the 
difference  in  the  weight  of  extract  obtained  from  equal  weights  of 
the  solution  before  and  after  treatment,  and  by  titration  with  per- 
manganate before  and  after  treatment  with  silk — i  cc.  of  perman- 
ganate solution  containing  3.164  gms.  KMnC>4  per  liter,  correspond- 
ing to  0.004155  gm.  tannin. 

*  Chem.  Ztg.,  1900,  No.  17. 

t  Ph.  Ztg.,  Nov.  5,  1898,  791;  from  Jour,  de  Ph.  et  de  Chim.,  1898,  No.  8. 


636  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


ESTIMATION  OF  TANNIN  IN  WINES* 

(a)  Preparation  of  Reagents,      (i)    Oxalic  Acid.     Use  tenth- 
normal  solution;    10  cc.  =  0.041 57  gm.  of  tannin. 

(2)  Potassium  Permanganate  Solution.     Dissolve    1.333   gms-   of 
potassium  permanganate  in  i  liter  of  water  and  standardize  the  solu- 
tion with  the  tenth-normal  oxalic  acid  solution. 

(3)  Indigo  Solution.    Dissolve  6  gms.  of  sodium  sulphindigotate 
in  500  cc.  of  water  with  the  aid  of  heat;  cool,  add  50  cc.  of  concentrated 
sulphuric  acid,  make  the  solution  up  to  i  liter,  and  filter. 

(4)  Purified  Boneblack.     Extract  finely  pulverized  boneblack  with 
hydrochloric  acid  and  wash  with  distilled  water  until  the  acid  is 
entirely  removed.    The  boneblack  is  kept  covered  with  water. 

(b)  Determination.    Dealcoholize  100  cc,  of  the  urine  by  evapora- 
tion and  dilute  with  water  to  the  original  volume.     Transfer  10  cc.  to 
a  porcelain  dish  of  about  2  liters  capacity ;  add  about  a  liter  of  water  and 
exactly  20  cc.  of  indigo  solution.     Add  tenth-normal  potassium  per- 
manganate solution,  a  cc.  at  a  time,  until  the  blue  color  changes  to 
green ;  then  add  a  few  drops  at  a  time  until  the  color  becomes  golden 
yellow.     Designate  the  number  of  cc.  of  permanganate  solution  em- 
ployed as  a. 

Treat  10  cc.  of  the  dealcoholized  wine,  prepared  as  above,  with 
boneblack  for  fifteen  minutes;  filter  and  wash  the  boneblack  thor- 
oughly with  water.  Add  a  liter  of  water  and  20  cc.  of  indigo  solution 
and  titrate  with  permanganate  as  above.  Designate  the  number  of 
cc.  of  permanganate  employed  above  as  b. 

Then  a—b=c,  the  number  of  cc.  of  permanganate  solution  required 
for  the  oxidation  of  the  tannin  and  coloring  matter  in  10  cc.  of  wine. 


ESTIMATION   OF   TANNIN   IN   TEA  f 

Proctor's  Modification  of  Lb'wenthal's  Method. 

(a)  PREPARATION  OF -REAGENTS 

(1)  Potassium    permanganate.     Make    up    a    solution    containing 
1.33  gms.  per  liter. 

(2)  Tenth-normal  oxalic  acid.     Make  up  a  solution  containing  6.3 
gms.  per  liter. 


*  Bulletin  No.  107,  Bureau  of  Chemistry,  U.  S.  Dept.  of  Agriculture, 
t  Ibid. 


ESTIMATION   OF   TANNIN  637 

(3)  Indigo  carmine.     Make  up  a  solution  containing  6  gms.  of 
indigo  carmine  (free   from   indigo   blue)  and   50  cc.  of  concentrated 
sulphuric  acid  per  liter. 

(4)  Gelatin  solution.     Prepare  by  soaking  25  gms.  of  gelatin  for 
one  hour  in  a  saturated  sodium  chlorid  solution,  heat  until  the  gelatin 
is  dissolved,  and  make  up  to  i  liter  after  cooling. 

(5)  Mixture.     Combine    975    cc.    of    saturated    sodium    chlorid 
solution  and  25  cc.  of  concentrated  sulphuric  acid. 

(6)  Powdered  kaolin. 

(b)  DETERMINATION 

Obtain  the  value  of  the  potassium  permanganate  in  terms  of  the 
oxalic  acid.  Boil  5  gms.  of  the  tea  for  half  an  hour  with  400  cc.  of 
water;  cool,  transfer  to  a  graduated  5oo-cc.  flask,  and  make  up  to 
the  mark.  To  10  cc.  of  the  infusion  (filtered  if  not  clear)  add  25  cc. 
of  the  indigo  carmine  solution  and  about  750  cc.  of  water.  Add 
from  a  burette  the  potassium  permanganate  solution,  a  little  at  a 
time  while  stirring,  until  the  color  becomes  light  green,  then  cau- 
tiously, drop  by  drop,  until  the  color  changes  to  bright  yellow,  or 
further,  to  a  faint  pink  at  the  rim.  The  number  of  cc.  of  permanga- 
nate used  furnishes  the  value  a  of  the  formula  given  below. 

Mix  100  cc.  of  the  clear  infusion  of  tea  with  50  cc.  of  gelatin 
solution,  100  cc.  of  salt  acid  solution,  and  10  gms.  of  kaolin,  and 
shake  several  minutes  in  a  corked  flask.  After  settling  decant  through 
a  filter.  Mix  25  cc.  of  the  filtrate  (corresponding  to  10  cc.  of  the 
original  infusion)  with  25  cc.  of  the  indigo  solution  and  about  750 
cc.  of  water,  and  titrate  with  permanganate  as  before.  The  number 
of  cc.  of  permanganate  used  gives  the  value  b;  a—b=c;  c  equals 
the  amount  of  permanganate  required  to  oxidize  the  tannin.  Assume 
that  0.04157  gm.  of  tannin  (gallotannic  acid)  is  equivalent  to  0.063 
gm.  of  oxalic  acid. 

TANNIN  IN  CLOVES  AND  ALLSPICE* 

Extract  2  gms.  of  material  for  twenty  hours  with  absolute  ether. 
Boil  the  residue  for  two  hours  with  300  cc.  of  water,  cool,  make  up 
to  500  cc.,  and  filter.  Measure  25  cc.  of  this  infusion  into  a  flask 
of  about  1200  cc.  capacity,  add  20  cc.  of  indigo  solution  and  750  cc. 
of  distilled  water,  and  proceed  as  directed  under  Estimation  in  Wines. 

10  cc.  of  tenth-normal  oxalic-acid  solution  are  equivalent  to 
0.06232  gm.  of  quercitannic  acid,  or  0.008  gm.  of  oxygen  absorbed. 

*  Bulletin  No.  107,  Burean  of  Chemistry  U.  S.  Dept.  of  Agriculture. 


638  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 


LITERATURE 

• 

H.  R.  Procter.     "Leather  Industries  Laboratory  Book." 
Hunt.     Sutton's  "Volumetric  Analysis,"  8th  Ed.,  365. 
Kathreiner.     Zeitschr.  anal.  Chem.,  xvin,  113. 
A.  H.  Allen.     "Commercial  Analysis." 
Fletcher  and  Allen.     Chem.  News,  xxix,  169  (1874). 
Dreaper,  J   S.  C.  L,  xn,  412. 

Lowenthal.    Jour,   prakt.   Chem.,  m,   150  (1860);    and  Zeitschr.  anal. 
Chem.,  xvi,  33-201;  also  xx,  91. 

Fr.  Gaube.     Zeitschr.  anal.  Chem.,  x,  i. 

Siemand.     Ibid.,  xxii,  595. 

Julius  Lowe.     Ibid.,  iv,  368. 

K.  Hammer.     Jour,  prakt.  Chem.,  LXXXI,  159. 

Procter  and  Hewitt.     Zeitschr.  anal.  Chem.,  xviii,  115. 

Kramsky.     Ibid.,  1906,  776. 

Specht  and  Lorenz.     Ph.  Ztg.,  Apr.  7,  1900,  266. 

Vignon.     Ibid.,  Nov.  5,  1898,  791. 

Yocum.     Proc.  A.  Ph.  A.,  Am.  Chem.,  Soc.  Jan.  9,  1897. 

Procter.     J.  S.  C.  L,  xi,  329. 

Eitner.     Der  Gerber.,  xin,  245. 

Trimble.     Proc.  Franklin  Inst,  iv,  23. 

Trimble  and  Peacock.    A.  J.  Ph.,  xxin,  161. 

Snyder,    J.  A.  C.  S.,  1893. 


CHAPTER  LVI 

ESTIMATION  OF  FORMALDEHYDE  * 

The  Ammonia  Method  (Legler).  This  method  is  based  upon 
the  reaction  between  free  ammonia  and  formaldehyde  in  which 
hexamethylene-tetramin  is  formed.  It  is  for  ordinary  purposes 
sufficiently  accurate. 

It  is  this  method  which  is  recommended  by  Lederle  f  for  use 
in  the  laboratory  of  the  New  York  City  Health  Department,  and 
by  Prescott  in  the  laboratory  of  the  University  of  Michigan. 

The  assay  is  conducted  as  follows:  2  cc.  of  the  solution  are 
placed  in  a  glass -stoppered  bottle,  the  stopper  of  which  is  thickly 

N 
coated  with  petrolatum,  and  50  cc.  of  —  ammonia  solution  added; 

let  stand  twelve  hours,  shaking  occasionally.     Then  determine  the 

N 
excess  of  ammonia  by  titrating  with  —  sulphuric  acid  solution,  using 

rosolic  acid  or  litmus  as   indicator.     The  excess  of  ammonia  sub- 
tracted from  the  quantity  added  gives  the  quantity  which  combined 
with  the  formaldehyde,  and  thus  the  amount  of  the  latter  is  ascertained. 
The  reaction  is  represented  as  follows: 

6CH20  +  4NH3  =  (CH2)6N4  +  6H2O. 

4)178.74          4)67.72      Hexamethylenetetramin 
2)44-68          2)16.93 

22.34  gms.      8.46  gms.  or  1000  cc.  —  V.  S. 

2 
0.02234  gm.  0.00846  gm.  or   i  cc.  "      " 

N 
Assuming  that  22  cc.  of  —  sulphuric  acid  were  employed  in  the 

N 
titration,  22  cc.  of  the  —  ammonia  solution  must  have  been  in  excess, 


*  Berichte  d.  Chem.  Ges.,  XVI,  1335,  1883. 
f  Am.  Drug.,  1897,  246. 

639 


640  A    MANUAL   OF   VOLUMETRIC   ANALYSIS 

hence  28  cc.  of  the  latter  went  into  combination  with  the  formalde- 
hyde. Thus  the  2  cc.  of  formaldehyde  solution  contained  28X0.02234 
gm.=o.6255  gm. 

A.  G.  Craig*  says  that  the  chief  difficulty  in  using  the  Legler 
method  is  the  volatility  of  the  ammonia.  The  difficulty  is  not  so 
much  the  loss  of  strength  in  the  standard  solution,  but  the  loss  during 
the  determination.  He  proposes  the  following  scheme  by  which 
this  error  is  removed. 

Prepare  a  normal  solution  of  sulphuric  acid.  Make  up  an  approxi- 
mately normal  solution  of  ammonia,  the  exact  strength  being  imma- 
terial. Procure  several  three-ounce  prescription  bottles  with  smooth 
sides. and  close-fitting  soft  rubber  stoppers.  Prepare  a  methyl-orange 
solution.  Procure  a  boiler  in  which  the  bottles  may  be  immersed 
to  the  neck  without  upsetting  (a  large  beaker,  will  do).  Take  as 
much  of  the  sample  as  will  contain  0.5  gm.  of  formaldehyde.  Meas- 
ure with  the  pipette,  25  cc.  of  the  ammonia  solution  into  each  of  the 
bottles,  and  to  half  of  them  add  a  sample  of  formaldehyde;  stopper 
tightly.  If  the  necks  of  the  bottles  are  small,  the  stoppers  need  not 
be  tied  down.  Place  the  bottles  in  the  boiler,  add  cold  water  to  the 
necks,  and  heat  to  boiling.  Boil  for  one  hour,  and  cool  by  running 
in  cold  water  slowly,  being  careful  not  to  allow  the  cold  water  to 
touch  the  hot  bottles.  Titrate  with  sulphuric  acid  and  methyl-orange 
to  the  first  indication  of  a  color  change.  Take  the  difference  between 
the  readings  for  the  blanks  and  those  for  the  samples,  as  the  ammonia 
consumed  in  normal  cc.  Of  this  difference,  i  cc. =0.0601  gm.  of 
formaldehyde. 

The  Legler  method  is  also  likble  to  error  because  the  compound 
formed  is  a  weak  base,  and  as  such  combines  with  acid,  while  at  the 
same  time  it  is  liable  to  decompose  into  ammonia  and  formaldehyde 
and  thus  give  an  indefinite  end-point  when  the  residual  ammonia 
is  titrated  with  acid.  Error  is  also  liable  to  be  introduced  through 
the  presence  of  carbonic  acid  in  the  ammonia -water,  which,  with 
the  indicator  rosolic  acid,  gives  no  sharp  end-reaction. 

The  Ammonium  Chlorid  Method.  In  this  method  a  solution 
of  ammonium  chlorid  is  used,  from  which  ammonia  is  evolved  by 
treatment  with  sodium  hydroxid.  The  excess  of  alkali  is  then 
determined  by  titration  with  standard  solution  of  sulphuric  acid. 
This  method,  as  devised  by  H.  Schiff,f  and  modified  by  C.  A.  Male,t 
is  as  follows: 


*  T.  A.  C.  S.,  XXIII,  642  (1901). 
t  Chem.  Ztg.,  XXVII,  14  (1903)- 

J  Pharm.  Jour.,  June,  1905,  844.     See  also  Carl  E.  Smith,  A.  J.  Ph.,  LXX, 
86,  (1898). 


ESTIMATION   OF   FORMALDEHYDE  641 

Introduce  2  gm.  neutral  ammonium  chlorid,  dissolved  in  20  cc. 
of  water,  into  a  flask  or  bottle  of  about  200  cc.  capacity,  having  well- 
fitting  stopper.  Dilute  10  cc.  of  the  formaldehyde  solution  to  100 
cc.  with  water,  and  neutralize  with  sodium  hydroxid  solution,  as  the 
formaldehyde  solution  generally  contains  varying  quantities  of  formic 
acid.  Add  20  cc.  of  .  this  neutralized  solution  to  the  ammonium 

N 
chlorid  solution,  then  25  cc.  of  —  NaOH,  and  immediately  stopper 

the  flask,  and  leave  for  one  hour.     Afterwards  determine  the  excess 

N 
of  alkali   with   —  H2SO4,  using  rosolic   acid  or  litmus   solution  as 

indicator,  both  of  which  give  a  sharp  end-reaction.    The  reaction 
and  calculation   is   based  upon   the   following  equation: 


N 
i  cc.  —  KOH  is  equivalent  to  0.045  gm.   of  formic  aldehyde.    This 

modified  method  is  quite  as  simple,  and  gives  results  practically 
identical  with  that  of  Romijn. 

Oxidation  by  Hydrogen  Dioxid  (Blank  and  Finkenbeiner  )* 
This  method  depends  upon  the  use  of  hydrogen  dioxid  for  oxidizing 
formaldehyde  into  formic  acid,  in  alkaline  solution.  The  formic 
acid  so  produced  neutralizes  a  portion  of  the  alkali,  and  the  excess 
of  the  latter  is  then  determined  by  titration  with  standard  acid. 
The  method  gives  good  results  and  can  be  very  rapidly  carried  out. 
It  is  the  method  adopted  in  the  U.  S.  P.  VIII,  in  which  it  is  described 
as  follows: 

Transfer  3  cc.  of  solution  of  formaldehyde  to  a  well-stoppered 
Erlenmeyer  flask,  and  weigh  accurately.  Add  50  cc.  of  normal 
sodium  hydroxid  V.  S.,  and  follow  this  immediately,  but  slowly, 
through  a  small  funnel,  with  50  cc.  of  solution  of  hydrogen  dioxid, 
to  which  a  drop  of  litmus  T.  S.Jias  been  added,  and  which  has  been 
neutralized  with  normal  sodium  ^hydroxid  V.  S.  After  the  reaction 
has  ceased  and1  the  foaming  has  subsided,  rinse  the  funnel  and  sides 
of  the  vessel  with  distilled  water,  and,  after  allowing  it  to  stand  ten 
minutes,  titrate  back  with  normal  sulphuric  acid  V.  S.,  using  litmus 
T.  S.  as  indicator.  Subtract  the  number  of  cc.  of  normal  sulphuric 
acid  V.  5-  consumed,  from  50  (the  number  of  cc.  of  normal  sodium 
hydroxid  V.  S.  employed),  multiply  the  remainder  by  2.979,  and 
divide  the  product  by  the  weight  of  the  solution  taken;  the  quotient 

*Berichte  d.  Chem.  Ges.,  XXXI,  2979  (1898);   and  A.  J.  Ph.,  1899,  486. 


642  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

represents  the  percentage,  by  weight,  of  absolute  formaldehyde  in  the 
liquid. 

In  the  original  process,  double  normal  sodium  hydroxid  was  used. 
The  reaction  is  as  follows:* 

CH2O  +  H2O2+NaOH=NaOOCH+2H2O. 

The  lodometric  Method  (Romijn).f  This  method,  which  is 
considered  the  most  rapid,  most  accurate,  and  most  readily  applied, 
depends  upon  the  fact  that  iodin  in  the  presence  of  an  alkali  acts  as 
an  indirect  oxidizing  agent,  giving,  when  formaldehyde  is  present, 
the  iodid  of  the  base  and  formic  «acid: 

CH20+I2+2NaOH=2NaI+CHOOH+H2O. 

As  modified  by  L.  Reuter  J  it  is  as  follows : 

20  cc.  of  35  to  40  per  cent  formaldehyde  are  introduced  into  a 
graduated  flask  and  distilled  water  added  to  bring  up  the  volume  to 
500  cc.  Of  this  thoroughly  mixed  fluid,  5  cc.  are  introduced  into  a 
bottle  capable  of  being  perfectly  closed.  30  cc.  of  normal  sodium  or 

N  . 
potassium  hydroxid  are  added,  and  then  —  iodin  solution  allowed  to 

O 

flow  in  from  a  burette  with  constant  agitation,  until  the  fluid  remains  of 
a  bright  yellow  color  (36-60  cc.).  The  shaking  is  then  vigorously 
continued  for  one  minute,  when  40  cc.  of  normal  sulphuric  acid  are 
added,  and  then  the  excess  of  iodin  titrated  with  decinormal  thio- 

N 

sulphate  solution.     Each  cc.  of  —  iodin   solution    consumed  corre- 

10 

spends  to  0.0015  gm.  formaldehyde.  A  blank  titration  should  always 
be  made. 

A  solution  containing  as  much  as  5  per  cent  may  be  accurately 
estimated  by  this  method,  provided  not  more  than  2  gms.  be  taken. 
The  method  is,  however,  not  accurate  in  the  presence  of  other  alde- 
hydes. F.  O.  Taylor,§  commenting  upon  this  method,  says  that  it  is 
quite  satisfactory,  but  that  the  quantities  of  formaldehyde  and  reagents 
used  are  unnecessarily  large  and  cumbersome  and  the  method  is 
hence  modified  by  him  as  follows: 

From  a  weighing  bottle,  consisting  of  a  small  Erlenmeyer  flask, 
fitted  with  a  perforated  rubber  stopper  through  which  passes  a  dropper, 

*  See  C.  Allen  Lyford,  J.  A.  C.  S.,  XXIX,  1227  (1907,)  "The  Action  of  Barium 
Peroxid  and  Hydrogen  Peroxid  upon  Formaldehyde." 

f  Zeitschr.  anal.  Chem.,  XXXVI,  18-24  (1897);  ibid.,  XXXIX,  60-63  (1900). 

|  Ph.  Rev.,  1903,  207. 

§  Bull.  Ph.,  Aug.,  1903,  323. 


ESTIMATION   OF  FORMALDEHYDE  643 

and  containing  about  25  or  30  cc.  of  the  formaldehyde  solution,  weigh 
out  accurately  about  10  gms.  of  the  solution  into  a  stoppered  500  cc. 
flask  and  fill  this  to  the  mark  with  distilled  water.  For  titration  remove 
5  cc.  of  this  solution,  corresponding  to  o.oi  gm.  of  the  weighed  quantity 
of  formaldehyde,  and  put  into  a  2oocc.  Erlenmeyer  flask.  Into 
another  flask  put  5  cc.  of  water  for  a  blank  titration.  To  both  now 

add  20  cc.  of  normal  NaOH  and  then  20  cc.  of  an  approximately  — 

iodin  solution,  whose  exact  strength  need  not  be  known,  and  let  stand 
for  five  or  ten  minutes  for  the  entire  completion  of  the  reaction.  Now 

N 
add  25  cc.  of  normal  H2SO4  and  titrate  the  excess  of  iodin  with  — 

10 
Na2S2O3.  The  difference  between  the  cc.  of  thiosulphate  used  on  the 

N 
assay  and  the  cc.  of  the  blank  is  the  number  of  cc.  of  —  iodin  con- 

N  I0 

sumed  by  the  formaldehyde.     Each  cc.  of  —  iodin   so  used  equals 

0.0015  gm.  of  CH2O. 

The  Potassium  Cyanid  Method*  This  method  is  especially 
applicable  to  solutions  containing  small  quantities  of  formaldehyde. 
It  depends  upon  the  fact  that  potassium  cyanid  and  formaldehyde 
combine  to  form  an  addition  product,  in  which  one  molecule  of 
potassium  cyanid  combines  with  one  molecule  of  formaldehyde,  as 
shown  by  the  following  equation: 

H 
I 
CH20 + KCN= N= C— C— O— K. 

H 


In  the  estimation,  the  formaldehyde  is  mixed  with  a  known  quantity 
of  potassium  cyanid  (in  excess),  the  excess  of  the  latter  being  deter- 
mined by  the  use  of  standard  silver  nitrate  solution,  and  thus  the 
quantity  of  potassium  cyanid,  which  combined  with  formaldehyde 
is  found,  and  from  this  the  quantity  of  formaldehyde  is  calculated. 

N 
The  process  is  carried  out  as  follows:  f  10  cc.  of  —  silver  nitrate 

are  treated  with  six  drops  of  50  per  cent  nitric  acid  in  a  5o-cc.  flask. 
10  cc.  of  a  solution  of  potassium  cyanid  (containing  i  gm.  of  KCN 
in  500  cc.  of  water)  are  then  added  and  well  shaken.  An  aliquot  part 


*  Romijn,  Zeitschr.  anal.  Chem.,  XXXVI,  18-24  (1897). 
t  Bernard  H.  Smith,  J.  A.  C.  S.,  XXV,  1032    (1903). 


644  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

N 
of  the  filtrate,  say  25  cc.,  is  then  titrated  by  Volhard's  method  with  — 

ammonium  sulphocyanate  for  excess  of  silver. 

Another  10  cc.  of  silver  nitrate  solution  is  then  acidified  with  nitric 
acid  and  treated  with  10  cc.  of  the  potassium  cyanid  solution  to  which 
has  been  added  a  measured  quantity  of  dilute  formaldehyde  solu- 
tion. The  whole  is  made  up  to  50  cc.  and  then  filtered.  25  cc.  of 

N 
the  filtrate  are  titrated  with  —  ammonium  sulphocyanate  for  excess 

of  silver  as  before. 

The  difference  between  these  two  results  multiplied  by  2  gives 
the  amount  of  potassium  cyanid  which  was  used  by  the  formalde- 

N 
hyde,  in  terms  of  —  sulphocyanate. 

The  best  results  by  this  method  are  obtained,  if  the  solution  of 
formaldehyde  is  diluted  to  below  i  per  cent.  With  i  per  cent  solu- 
tions it  is  necessary  to  use  15  cc.  of  the  silver  solution.  In  estimating 
very  dilute  solutions,  it  is  advisable  to  use  a  2oo-cc.  flask  and  to  take 
100  cc.  of  the  filtrate  for  the  titration.  It  is  possible  by  this  method 
to  determine  with  accuracy  one  part  in  100,000. 

A  New  Method  for  the  Estimation  of  Free  Formaldehyde 
has  been  devised  by  Clowes  and  Cullens,*  depending  upon  the  fact 
that  when  formaldehyde  is  mixed  with  phloroglucin  and  hydrochloric 
acid  condensation  occurs,  resulting  in  the  separation  of  phloroglucid 
according  to  the  following  equation: 

CeHsOs + HCOH = C7H6O3 + H2O. 

The  substance  under  examination  is  mixed  with  5  cc.  of  water 
and  a  mixture  of  15  cc.  of  hydrochloric  acid  with  a  specific  gravity  of 
1.19,  15  cc.  of  water  and  a  slight  excess  of  phloroglucin  and  heated 
to  a  temperature  of  70°  to  80°  C.  on  the  water-bath  for  two  hours. 
If  the  filtrate  yeilds  phloroglucid  upon  heating  with  concentrated 
sulphuric  acid  the  hydrochloric  acid  has  not  been  sufficient  to  cause 
decomposition  of  the  methylene  derivative.  In  this  case  a  second 
sample  of  the  substance  is  heated  with  5  cc.  of  water  and  then  with 
a  mixture  of  10  to  20  cc.  of  concentrated  sulphuric  acid,  10  cc.  of 
water  and  a  slight  excess  of  phloroglucin. 

Bernard  H.  Smith,f  in  reviewing  the  various  methods,  concludes 
that  the  Blank  and  Finkenbeiner  method  is  very  satisfactory  for  strong 
solutions.  That  the  Legler  method  is  fairly  satisfactory,  but  gives 

*  Berichte  d.  Chem.  Ges.,  XXXII,  2841. 
f  J.  A.  C.  S,  XXV,  1028  (1903). 


ESTIMATION    OF  FORMALDEHYDE  645 

lower  results.  That  the  iodometric  and  the  potassium  cyanid  methods 
give  good  results  on  dilute  solutions.  The  latter  method  is  best  adapted 
to  the  estimation  of  very  small  quantities  of  formaldehyde. 

LITERATURE 

Legler.     Berichte  d.  Chem.  Ges.,  xvr,  1333. 
Losekann.     Ibid.,  xxii,  1565. 
Eschweiler.     Ibid.,  1929. 
Klar.     Ph.  Zeitg.,  XL,  611. 
Smith.     Analyst,  xxi,  148  (1896). 
G.  Romijn.    Zeitschr.  anal.  Chem.,  xxxvi,  18  (1897). 
Lederle.     Am.  Drug.,  LXX,  246  (1897). 
Orchard,     xxii,  4  (1897). 
Grutzner.     Archiv.  d.  Ph ,  ccxxxiv,  634. 
Carl  E.  Smith.     A.  J.  Ph.,  LXX,  86,  432  (1898). 
G.  L.  Taylor.    A.  J.  Ph.,  LXX,  April,  1898. 
Kebler.     Ibid.,  432  (1898). 
Legler.     Ph.  Centralh.,  xxxix,  253  (1898). 

Blank  and  Finkenbeiner.     Berichte  d.  Chem.  Ges.,  xxxi,  2979  (1898); 
ibid.,  xxxii,  2141  (1899). 

Kastle  and  Loevenhart.     J.  A.  C.  S.,  xxi,  262  (1899). 

Clowes  and  Cullens.     Berichte  d.  Chem.  Ges.,  xxxii,  2841  (1899). 

Brochet  and  Cambier.     Compt.  rend.,  cxx,  440. 

Romijn.     Zeitschr.  anal.  Chem.,  xxxix,  60  (1900). 

Legler.     Ph.  Centralh.,  XLII,  651  (1901). 

Craig.     J.  A.  C.  S.,  xxin,  638  (1901). 

Ganino.     Zeitschr.  anal.  Chem.,  XL,  587  (1901). 

Vanini  and  Seiter.     Ibid. 

Ripper.     Monatsheft,  xxi,  1079. 

Haywood.     Bull.  No.  73,  U.  S.  Dept.  Agric.,  1902. 

Pfaff.     Chem.  Zg.,  xxvi,  701  (1902). 

F.  O.  Taylor.     Bull.  Ph.,  Aug.,  1903,  323. 

Bernard  H.  Smith.     J.  A.  C.  S.,  xxv,  1028  (1903). 

H.  Schiff.     Chem.  Zg.,  xxvii,  14  ^903). 

C.  Wallnitz.     Deutsche  Gerber  Zg.,  Nos.  1-4;  6,  8,  12. 

L.  Reuter.     Ph.  Rev.,  xxi,  207  (1903). 

R.  Clauser.     Berichte  d.  Chem.  Ges.,  xxxvi,  101  (1903). 

Lemme.     Chem.  Zg.,  xxvin,  896  (1903). 

Gleissner  and  Sayre.     Drug.  Cir.,  192  (1904). 

Clemens  Kleber.     Ph.  Rev.,  xxii,  94  (1904). 

C.  A.  Male.     Ph.  Jour.,  June,  1005,  844. 

Fresenius  and  Greenhut.     Zeitschr.  anal.  Chem.,  XLIV,  13  (1905). 

R.  H.  Williams.    J.  A.  C.  S.,  xxvra,  596  (1005). 

Haywood  and  Smith.     Ibid.,  1183. 


CHAPTER  LVII 
ESTIMATION  OF  CHLOROFORM  AND  CHLORAL  HYDRATE 

Chloroform,  CHCls.  The  volumetric  estimation  of  chloroform 
is  based  upon  the  fact  that  when  chloroform  is  heated  with  an  alkali 
hydroxid  a  formate  and  a  chlorid  of  the  alkali  are  formed.  The 
reaction  between  the  chloroform  and  the  alkali  takes  place  in  definite 
proportions.  An  alcoholic  solution  of  potassium  hydroxid  gives  best 
results. 

The  process  is  carried  out  as  follows: .  A  weighed  quantity  of  chlo- 
roform, which  should  be  perfectly  neutral  in  reaction,  is  introduced 
into  a  strong  glass  flask  provided  with  a  well-fitting  glass  stopper. 
To  this  is  added  an  excess  of  normal  alcoholic  KOH  V.  S.,  the  stopper 
securely  tied  down,  and  the  flask  warmed  on  a  water-bath  to  50°  or 
60°  C.  When  reaction  is  complete  the  contents  of  the  flask  is  cooled 
and  then  titrated  with  normal  acid  V.  S.  to  find  the  excess  of  KOH. 

Each  cc.  of  normal  KOH  represents  0.02961  gm.  of  chloroform. 

CHC13  +  4KOH=3KC1+HCOOK+2H2O. 
4)118-45        4)222.96  N 

29.61  gms.       55-74=  1000  cc.  —  V.  S. 
10 

0.02961  gm.  =       i  cc.  " 

Instead  of  determining  the  excess  of  alkali  as  above  described,  a 
better  method  is  that  proposed  by  L.«fle  Saint  Martin.*  In  this  process, 
the  chloroform  is  heated  with  an  excess  of  alcoholic  potassium  hy- 
droxid in  a  sealed  tube  of  glass.  The  tube  is  kept  in  boiling  water 
for  three  hours,  then  cooled,  opened,  the  alkali  just  neutralized  with 

N 
sulphuric  acid,  and  then  the  chlorid  determined  with  —  silver  nitrate 

solution,  using  chromate  as  indicator.  W.  A.  Puckner  f  employs 
this  method  for  the  estimation  of  chloroform  in  chloroform -ether 
mixtures,  such  as  are  used  in  alkaloidal  assaying.  He  uses  instead 
of  the  sealed  tube  a  two-ounce  vial,  stoppered  with  a  sound  cork 

*  Compt.  rend.,  CVI,  492.  f  Proc.  A.  Ph.  A.,  1001,  294. 

646 


ESTIMATION  OF  CHLOROFORM  AND  CHLORAL  HYDRATE  647 

firmly  tied  down.  This  vial  is  placed  in  boiling  water  in  an  upright 
position  so  that  the  contents  do  not  come  in  contact  with  the  cork, 
which  would  be  acted  upon  by  the  alkali.  The  method  is  carried 
out  as  follows: 

To  10  cc.  of  an  approximately  normal  alcoholic  solution  of  potas- 
sium hydroxid,  either  free  from  chlorids  or  else  of  a  known  chlorid 
content,  and  contained  in  a  vial,  add  a  measured  volume  of  the  chloro- 
form-ether mixture  representing  0.05-0.2  gm.  chloroform,*  stopper 
with  a  sound  cork,  cover  with  cloth  and  tie  this  down  firmly,  mix 
the  two  liquids  by  rotation,  then  place  the  vial  in  boiling  water  in  such 
a  way  that  at  no  time  the  contents  come  in  contact  with  the  cork 
and  retain  the  temperature  for  three  hours.  Remove  the  vial  from  the 
bath,  let  cool,  add  phenolphthalein  and  then  sufficient  sulphuric  acid 
to  exactly  neutralize  the  liquid,  then  add  two  drops  of  potassium 
chromate  T.  S.  and  titrate  with  decinormal  silver  nitrate.  Or  if  Vol- 
hard's  method  of  estimation  is  preferred,  add  to  the  finished  digestion 
10  cc.  dilute  nitric  acid,  an  excess  of  decinormal  silver  nitrate,  5  cc. 
ferric  ammonium  sulphate  T.  S.  and  determine  the  excess  of  silver 
nitrate  with  decinormal  potassium  sulphocyanate.  In  either  case  i 
cc.  of  decinormal  silver  nitrate  represents  0.003948  gm.  CHCls. 

The  method  based  upon  the  determination  of  the  chlorid  formed 
is  to  be  preferred  for  accurate  work  to  the  alkalimetric  method,  for 
the  reason  that  there  is  some  decomposition  of  potassium  formate 
with  formation  of  potassium  carbonate,  which  increases  the  alka- 
linity. An  evident  decomposition  of  the  glass  has  the  same  effect, 
and  hence  the  results  by  the  alkalimetric  method  are  low. 

Chloral  Hydrate  (C2HC13O+H2O).  When  chloral  hydrate  is 
treated  with  an  alkali  it  is  decomposed  and  chloroform  and  an  alkali 
formate  are  formed.  The  reaction  must  take  place  in  the  cold,  or  at 
least  at  the  ordinary  temperature,  otherwise  the  alkali  will  attack 
and  decompose  the  chloroform  which  is  formed  and  hence  the  result 
would  indicate  too  high  a  quantity  of  chloral  hydrate. 

The  process  is  conducted  as  follows :  A  weighed  quantity  of  chloral 
hydrate  is  dissolved  in  water,  neutralized  if  it  is  acid,  as  is  frequently 
the  case,  and  then  a  measured  excess  of  normal  alkali  is  added.  When 
a  turbidity  due  to  liberated  chloroform  is  noted,  swing  until  perfectly 
clear  (one  to  two  minutes).  The  excess  is  then  determined  by  residual 

*  If  the  per  cent  of  chloroform  in  the  mixture  is  not  even  approximately 
known,  i  cc.  may  be  digested  with  25  cc.  normal  alcoholic  potassium  hydroxid 
solution  for  one  hour,  and  the  residual  alkali  determined  with  normal  acid  and 
phenolphthalein,  when  the  cc.  of  normal  alkali  which  disappeared  during  the 
digestion,  multiplied  by  0.02961,  will  give  the  amount  of  chloroform  contained 
therein  sufficiently  close  to  judge  the  quantity  to  be  taken  for  the  actual  deter- 
mination. 


648  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

titration  with  normal  hydrochloric  acid,   and  thus   the  quantity  of 
the  alkali  which  reacted  with  the  chloral  hydrate  is  found,  each  cc. 
of  which  represents  0.16497  gm.  of  chloral  hydrate. 
The  reaction  is  thus  expressed: 


N 
164.97  40        =1000  cc.  —  V,  S. 

0.16497  gm.  .040=       ice."     " 

The  use  of  barium  hydroxid  is  preferred  by  many  to  the  sodium 
hydroxid. 

lodometric  Method  of  Estimation.  E.  Rupp  *  recommends 
the  following  iodometric  method  for  the  estimation  of  chloral  hydrate: 

N  N 

To  a  mixture  of  25  cc.  of  —  iodin  and  2.5  cc.  —potassium  hydroxid 

in  a  glass  -stoppered  flask,  10  cc.  of  chloral  hydrate  solution  (1:100) 
are  added,  the  mixture  shaken  and  allowed  to  stand  five  to  ten  minutes. 
It  is  then  diluted  with  about  50  cc.  of  water,  5  cc.  of  hydrochloric  acid 

N 
added,  and  titrated  with  —  thiosulphate,  in  the  usual  manner.     The 

N 
limit  of  —  iodin  consumed  should  be  from  11.75-11.7  cc.,  as  indicated 

10  N  N 

by  13.25-13.3  cc.  of  —  thiosulphate  consumed,     i  cc.  —  1=0.008275 

gm.  chloral  hydrate. 

*  Arch.  d.  Ph.,  241,  No.  5  (July  31,  1903),  326-328. 


CHAPTER  LVIII 
ASSAYING  SURGICAL  DRESSINGS 

THE  assaying  of  surgical  dressings,  especially  those  most  frequently 
employed,  may  be  readily  done  by  volumetric  methods.  Since  the 
medicinal  content  of  such  dressings  is  usually  very  small,  a  sufficiently 
large  quantity  of  the  material  must  be  taken  for  the  assay. 

The  smallest  quantity  of  medicinal  substance  is  contained  in  the 
sublimate  dressings,  while  salicylic,  boric,  and  carbolic  dressings 
contain  larger  proportions  of  the  respective  antiseptics,  and  as  high 
as  30  per  cent  is  contained  in  some  iodoform  dressings.  Hence  in 
assaying  sublimate  dressings  a  larger  quantity  of  the  material  must 
be  taken,  while  a  comparatively  small  quantity  is  needed  of  iodoform 
dressing. 

In  taking  a  sample  for  analysis  it  is  important  that  it  be  so  selected 
from  different  parts  of  the  package  that  it  will  fairly  represent  the 
average  strength  of  the  whole.  S.  W.  Williams  suggested  that,  as 
gauzes  are  sold  by  the  yard,  it  is  evident  that  the  strength  of  the 
medication,  even  if  expressed  in  terms  per  cent  by  weight,  should 
have  some  definite  relation  to  the  measurement. 

In  the  case  of  an  expensive  medication  like  iodoform  it  would 
seem  far  more  equitable  to  give  the  strength  in  grains  per  square 
yard  or  grams  per  square  meter.  Thus  the  claims  of  the  manufac- 
turers might  be  compared  on  a  common  basis.  This  method  would 
obviate  the  confusion  attendant  upon  the  allowance  in  the  computation 
for  water  in  the  "moist  dressings." 

The  weight  of  the  water  present  in  the  so-called  "  moist  dressings  " 
is  so  varying  a  quantity  that  it  must  be  excluded  from  the  computa- 
tion, in  order  to  fairly  compare  the  strength  of  moist  dressings  with 
the  dry  kind. 

Carbolic  Acid  Dressing.  If  the  dressing  is  of  a  low  per  cent, 
10  gms.  are  taken,  if  of  a  high  per  cent  5  gms.  This  is  put  into  ? 
liter  flask,  some  water  added  and  the  whole  warmed  to  and  kept  at 
about  80°  C.  for  some  time,  rotating  the  flask  occasionally.  Then 
allow  to  cool,  dilute  to  the  icoo-cc.  mark,  and  filter.  The  paraffin, 
resin,  or  oil,  etc.,  rises  to  the  surface  of  the  aqueous  solution  and  is 
easily  separated  by  filtration.  The  carbolic  acid  is  then  estimated 

649 


650  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

by  means  of  bromin  solution,  as  described  on  page  615,  25  or  50  cc. 
of  the  carbolic  acid  solution  being  taken. 

Meissinger  and  Vortmanri's  Process  (Pharm.  Zeit.  f.  Russland, 
xxix,  759).  Transfer  25  cc.  of  the  carbolic  acid  solution  prepared 
as  above  to  a  i5o-cc.  flask,  and  add  20  cc.  of  a  i  per  cent  solution 
sodium  hydroxid  (free  from  nitrite).  Warm  the  mixture  to  60°  C. 

N 

and  add  —  iodin  solution  from  a  burette,  until  the  mixture  in  the  flask 
10 

assumes  a  permanent  brownish-yellow  color,  and  if  much  carbolic  acid 
is  present,  deposits  upon  shaking,  a  red-colored  precipitate.  A  light- 
colored  precipitate  indicates  deficiency  of  alkali.  Cool  the  contents  of 
the  flask,  acidulate  with  dilute  sulphuric  acid,  and  dilute  to  150  cc. 
with  distilled  water  and  mix. 

Remove  10  cc.  of  this  solution  by  means  of  a  pipette,  and  titrate  it 

N 
with  —  sodium  thiosulphate,  using  starch  as  indicator.     The  number 

10  N 

of  cc.  thus  found  multiplied  by  15  gives  the  cc.  of  —  iodin  solution 

10 

which  were  added  in  excess  to  the  carbolic  acid  solution.  This  de- 
ducted from  the  amount  of  iodin  added,  gives  the  number  of  cc.  of  the 
latter  which  reacted  with  the  carbolic  acid. 

N 
One  cc.  of  —  iodin  solution  is  equivalent  to  0.001556  gm.  of  car- 

10  N 

bolic  acid.     Therefore,  by  multiplying  this  by  the  cc.  of  —  iodin 

10 

solution  which  reacted  with  the  carbolic  acid,  the  quantity  of  the 
latter  in  the  25  cc.  of  solution  taken  for  analysis  is  ascertained. 

If  the  carbolic  acid  solution  is  made  from  10  gms.  of  the  dressing 
in  a  liter,  25  cc.  of  it  represents  0.25  of  the  dressing.  Hence  to  find 
the  percentage  by  weight  of  the  carbolic  acid  in  the  dressing,  multiply 
the  amount  found  by  100  and  divide  by  0.25. 

Example.  25  cc.  of  the  carbolic  acid  solution,  representing  0.25  gm. 
of  the  dressing,  were  heated  with  20  cc.  of  a  i  per  cent  solution  of  NaOH, 

N 
and  55  cc.  of  —  iodin  added,  and  the  mixture  made  up  to  150  cc. 

10  N 

10  cc.  of  this  titrated  with  —  thiosulphate  required  3.47  cc. 

10 

3.47X15=  52.05  cc.; 


2.95X0.001556=0.00459  gm. 

0.00459X100 
then  =1.83  +  per  cent. 


ASSAYING  SURGICAL  DRESSINGS  651 

In  the  above  process  the  iodin  reacts  with  the  carbolic  acid  in 
proportion  of  one  molecular  weight  of  the  latter  and  six  atomic  weights 
of  iodin;  the  greater  part  of  the  iodin  added  is,  however,  taken  up 
by  the  NaOH  to  form  sodium  iodid  and  iodate.  Upon  the  addition 
of  dilute  sulphuric  acid  these  two  salts  give  up  their  iodin,  but  the 
iodin  combined  with  the  phenol  is  not  liberated  by  the  acid. 

Salicylic  Acid  Dressings.  5  or  10  gms.  of  the  material,  accord- 
ing to  the  claimed  strength,  are  placed  in  a  beaker  or  porcelain  dish 
and  heated  with  500  cc.  of  distilled  water.  A  few  drops  of  phenol- 

N 
phthalein  T.  S.  are  then  added  and  the  solution  titrated  with  —  sodium 

hydroxid. 

In  case  resinous  or  fatty  matters  are  present  they  must  be  removed 
by  filtration  and  the  gauze  or  cotton  thoroughly  washed  and  pressed 
before  the  titration. 

N 
Each  cc.  of  —  alkali =0.01 3 7  gm.  salicylic  acid. 

Benzoic  acid  in  surgical  dressings  may  be  estimated  in  the  same 
manner  as  the  foregoing. 

N 

Each  cc.  —  alkali=o.oi22  gm.  benzoic  acid. 
10 

Boric  Acid  Gauze.  Beckurts  and  Danert  (Apoth.  Zeit.)  give 
the  following  process  for  determining  volumetrically  the  quantity  of 
boric  acid  present  in  gauze:  Cut  5  gms.  of  the  gauze  into  fine  shreds 
and  shake  with  400  cc.  of  a  mixture  of  one  part  of  glycerin  and  nine- 
teen parts  of  water  in  a  5oo-cc.  flask,  adding  later  enough  solvent  to 
make  up  to  500  cc.  Draw  off  100  cc.  of  the  clear  fluid,  and  with  the 

N 
addition  of  phenolphthalein  and  some  glycerin  titrate  with  —  sodium 

hydrate  solution.  The  number  of  cc.  of  the  solution  required,  when 
multiplied  by  p.oo6i54,  gives  the  quantity  of  boric  acid  found  in  i  gm. 
of  the  gauze;  when  multiplied  by  100  the  percentage  content  is  ob- 
tained. The  quantity  of  glycerin  added  during  titration  is  regulated 
by  the  appearance  of  alkalinity,  for  as  soon  as  the  solution  shows  an 
alkaline  reaction  glycerin  is  added;  this  is  followed  usually  at  first  by 
disappearance  of  the  red  color  until  actual  neutralization  has  taken 
place.  If  sufficient  glycerin  is  added,  the  color  reactions  are  ren- 
dered sharp. 

Sublimate  Dressings.  In  the  estimation  of  mercuric  chlorid  in 
surgical  dressings  the  available  bichlorid  only  should  be  com- 
puted. 

The  mercury  which  is  present  in  any  other  form  but  bichlorid  must 
be  excluded  from  consideration.  It  is  to  be  remembered  that  corrosive 


652  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

sublimate  reacts  and  forms  with  many  organic  substances,  compounds 
which  are  insoluble  or  which  have  no  germicidal  power. 

Moreover,  sublimated  dressings  contain  such  minute  proportions 
of  the  bichlorid  that  even  traces  of  impurities,  such  as  wood,  gum, 
oil,  fatty  acids,  etc.,  may  be  capable  of  destroying  all  of  the  bichlorid 
present.  Even  the  cotton  of  which  the  dressing  is  made  has  a  reducing 
influence  upon  the  bichlorid. 

Therefore,  in  assaying  a  sublimate  dressing,  the  mercuric  chlorid 
and  not  the  mercury  should  be  estimated.  Hence  the  dressing  should 
not  be  exhausted  for  the  assay  with  acidulated  water,  which  would 
tend  to  dissolve  some  of  the  bichlorid  combined  with  the  cellulose. 

The  available  sublimate  may  best  be  extracted  by  means  of  hot 
sodium  chlorid  solution. 

Denner's  Process,  Modified  by  Link  and  Vaswinkel.  Macerate  a 
weighed  portion  of  the  dressing,  say  20  gms.,  in  a  20-ounce  glass  - 
stoppered  bottle  with  500  cc.  of  distilled  water,  shaking  frequently 
for  one  hour.  Decant  an  aliquot  portion,  say  250  cc.,  into  an  evapo- 
rating-dish.  Add  100  cc.  of  chlorin-water  and  a  few  drops  of  hydro- 
chloric acid  and  evaporate  to  expel  the  chlorin  and  reduce  to  small 
bulk.  Treat  with  sulphureted  hydrogen,  filter  and  wash  the  pre- 
cipitate very  thoroughly.  Transfer  with  the  filter-paper  to  a  small 
glass-stoppered  bottle;  add  i  or  2  cc.  of  carbon  disulphid  and  50  cc. 

N 
of  —  iodin  solution.    Stopper  tightly  and  let  stand,  with  frequent 

So 

shaking,  until  the  black  precipitate  of  mercuric  sulphid  is  dissolved. 
The  carbon  disulphid  dissolves  the  separated  sulphur  and  thus  pre- 
vents it  from  interfering  with  the  reaction.  Open  carefully  to  avoid 

N 

loss  by  spurting,  and  add  50  cc.  of  —  sodium  thiosulphate  and  starch- 
paste  indicator.  Stopper  the  bottle,  and  shake  vigorously  until  there 
remains  no  color  indicative  of  free  iodin.  Open  carefully  again,  and 

N 
titrate  the  excess  of  thiosulphate  with  —  iodin.     This  last   result 

represents  the  amount  of  iodin  bound  by  the  mercuric  sulphid,  accord- 
ing to  the  equation: 

HgS  +  2KI+I2=(HgI2  .  2KI)  +  S, 

and  each  cc.  corresponds  to  0.002688  gm.  of  mercury  bichlorid  in  the 
250  cc.  of  solution  used  for  assay. 

The  Original  Denner's  Process  *  is  as  follows :   A  weighed  quantity 

*  Ph.  Centralh.,  XXIX.  207  (i 


ASSAYING  SURGICAL   DRESSINGS  653 

of  the  dressing  to  be  examined  is  exhausted  by  digestion  with  a  known 
quantity  of  0.7  per  cent  hot  salt  solution  and  then  in  a  definite  quantity 
of  the  filtered  solution,  faintly  acidulated,  the  sublimated  is  precipi- 
tated with  hydric  sulphid.  The  mixture  is  then  heated  to  boiling  and 
the  precipitated  mercuric  sulphid  separated  by  filtration  through  cotton 
and  thoroughly  washed.  The  precipitate  together  with  the  cotton  is 
then  put  into  a  glass-stoppered  Erlenmeyer  flask,  and  after  the  addition 

N 

of  several  cc.  of  carbon  disulphid,  a  measured   excess  of  —  iodin  is 

10 

N 

added,  and  finally  the  excess  of  the  latter  found  by  titration  with  — 

10 

sodium  thiosulphate. 

Beckurt's  Process.  Weigh  about  20  gms.  of  the  gauze  or  cotton 
to  be  assayed.  Macerate  in  a  liter  cylinder  with  250  cc.  of  warm 
distilled  water  containing  0.5  gm.  of  sodium  chlorid.  When  cool 
dilute  with  distilled  water  to  i  liter,  taking  care  that  no  air-bubbles 
are  enmeshed  in  the  material.  Shake  well  and  filter  off  500  cc.  (more 
accurately  493  cc.,  allowing  for  the  volume  of  20  gms.  of  cellulose) 
into  a  boiling  flask  with  0.2  gin.  of  ferrous  sulphate  in  solution.  Mix 
add  excess  of  sodium  hydroxid,  and  then  dilute  sulphuric  acid  in  slight 
excess.  The  sublimate  present  is  thus  reduced  to  calomel,  and  may 

N 
be  estimated  by  adding  from  a  burette  —  iodin  until  the  color  indi- 

5°  N 

cates  a  slight  excess.     Then  titrate  back  the  excess  with  —  sodium 

5° 

N 
thiosulphate  and  starch   indicator.     The  difference  between  the  — 

N  5o 

iodin  and  —  thiosulphate  used  represents  the  iodin  reacting  with  the 

mercury  present  (in  the  absence  of  other  reducing  agents),  according 
to  the  equation 

Hg2Cl2  +  6KI  +  2l  =  2(HgI2  .  2KI)  + 2KC1, 

N 
and  each  cc.  of  —  iodin   so   used   corresponds    to  0.0027688  gm.  of 

sublimate  in  the  500  cc.  of  solution  used  for  assay. 

lodoform  Dressings.  10  gms.  of  the  dressing  are  digested  with 
alcohol  until  thoroughly  exhausted.  The  alcoholic  solution  of  iodo- 
form  is  then  placed  upon  a  water-bath,  acidulated  with  a  few  drops 

N 

of  nitric  acid,  and  titrated  with  a  —  alcoholic  solution  of  silver  nitrate. 

10 

The  end-point  is  known  when  a  drop  of  the  solution  brought  in 
contact  with  a  drop  of  sodium  chlorid  solution  produces  a  turbidity. 


654  A   MANUAL   OF   VOLUMETRIC  ANALYSIS 

It  can  be  told  approximately  by  the  disappearance  of  the  greenish- 
yellow  color  of  the  solution.  It  becomes  colorless  when  the  reaction 
is  complete.  The  reaction  is  as  follows: 

CHI3 + 3  AgN03 + H20  =  3  Agl  +  3HNO3  +  CO. 

N 
Each  cc.  of  the  —  AgNO3  V.  S. =0.01302  gm.  of  iodoform. 

The  following  process  (Huss')  may  be  applied  to  all  dressings 
containing  iodin,  iodoform,  iodol,  sozoiodol,  aristol,  etc.  It  is  based 
upon  the  fact  that  such  compounds  when  heated  with  metallic  zinc 
yield  all  their  iodin  with  formation  of  zinc  iodid. 

Five  grams  of  the  dressing  are  placed  in  a  dry  test-tube,  20  gms. 
of  zinc  dust  added,  and  shaken  down,  leaving  a  layer  of  the  zinc 
2  cm.  thick  about  the  dressing.  The  whole  is  heated  some  time  in 
a  water-bath.  The  zinc  is  then  washed  out  and  transferred  to  a  5oo-cc. 
volume  flask,  which  also  bears  a  mark  at  503  cc.  The  flask  is  filled 
with  water  to  the  503 -cc.  mark  (compensating  for  the  volume  of  20  gms. 
of  zinc  dust,  sp.gr.  7.17).  250  cc.  of  the  mixture  is  boiled  with  sodium 
carbonate,  diluted  to  500  cc.,  and  250  cc.  filtered  off.  In  this  solution 

N 

the  iodin  as  sodium  iodin  is  titrated  with  —  silver  nitrate  V.  S.,  each 

10 

cc.  of  which  represents  0.01259  gm.  iodin. 

Commercial  zinc  dust  usually  contains  zinc  oxid;  it  is  purified 
by  treatment  with  dilute  hydrochloric  acid,  washing  with  water  till 
the  chlorin  reaction  disappears,  and  subsequently  with  alcohol,  and 
drying. 

Other  methods  depend  upon  digesting  the  iodoform  dressing  with 
an  alcoholic  solution  of  sodium  hydroxid,  thus  forming  sodium  iodid. 

N 
which  is  estimated  by  titrating  with  —  AgNO3  V.  S. 

Lehmann's  Method.  Martin  Lehmann  gave  in  the  Berlir 
Pharmaceutische  Zeitung  (1900,  No.  15)  a  method  for  the  volu 
metric  estimation  of  iodoform  in  surgical  dressings,  and  in  a  latei 
number  of  the  same  journal  (No.  54)  proposes  some  modifications 
which  improve  the  process.  The  process,  as  modified,  is  carried 
out  in  the  following  manner:  10  gms.  of  the  dressing  under  ex- 
amination, either  gauze  or  cotton,  are  put  into  a  glass -stoppered 
bottle,  and  200  cc.  of  spirit  of  ether  are  poured  on  it.  This 
is  allowed  to  stand  for  twenty-four  hours,  being  frequently  shaken, 
at  a  temperature  of  from  20°  to  25°  C.  The  yellow  solution  of  iodo- 
form is  gradually  decomposed  and  in  consequence  changes  from  yellow 
to  reddish  brown  from  the  liberation  of  free  iodin.  20  cc.  of  this 


ASSAYING  SURGICAL  DRESSINGS  655 

solution  are  placed  in  an  Erlenmeyer  flask  having  a  capacity  of  250  cc. 
A  quantity  of  decinormal  silver  nitrate  solution  proportionate  to  the 
amount  of  iodin  thought  to  be  present  is  added,  and  after  the  addition 
of  ten  or  fifteen  drops  of  fuming  nitric  acid,  the  flask  is  warmed  on  a 
water-bath  until  the  odor  of  both  the  ether  and  the  nitrous  acid  have 
disappeared.  After  cooling  and  diluting  with  water  the  liquid  is 
titrated,  with  decinormal  ammonium  sulphocyanate  solution,  using 
i  cc.  of  cold  saturated  ferric  alum  solution,  until  the  color  turns  from 
white  to  a  permanent  light  red.  In  examining  gauze  precisely  one 
meter  should  be  measured  off  and  carefully  weighed.  The  author 
gives  the  results  of  a  series  of  examinations,  which  are  of  interest  as 
demonstrating  the  fact  that  there  is  a  constant  loss  in  strength  by 
keeping. 

Styptic  Cotton  (C.  E.  Parker,  Drug.  Cir.,  1895,  231).  The 
iron  in  styptic  cotton  is  determined  by  macerating  3  to  5  gms.  in  50  cc. 
of  water.  The  ferric  salt  is  reduced  by  adding  stannous  chlorid  in 
hydrochloric  acid  solution  until  the  brown  color  disappears,  then 
removing  the  excess  of  stannous  chlorid  by  the  addition  of  mercuric 
chlorid  as  long  as  a  precipitate  is  produced,  and  finally  titrating  with 

N 
-  potassium  dichromate  solution  until  a  drop  tested  on  a  white  plate 

no  longer  gives  a  blue  color  with  a  drop  of  freshly  prepared  potassium 
ferricyanid  solution,  but  only  a  bluish-gray  color. 

N 
Each  cc.  of  —  K2Cr2O7= 0.00555  gin.  of  metallic  iron  or  0.016104 

gm.  of  ferric  chlorid. 


CHAPTER  LIX 
ESTIMATION  OF  COMPOUND  ETHERS 

COMPOUND  ETHERS,  also  called  esters,  correspond  in  structure  to 
the  salts  of  the  metals,  in  which  the  metal  is  replaced  by  a  hydro- 
carbon radical. 

All  compound  ethers  when  treated  with  a  strong  alkali  give  up 
their  acids  to  the  alkali,  and  set  free  the  hydroxid  of  the  hydrocarbon 
radical.  Thus  ethyl  sulphate,  (C2H5)2SO4,  when  treated  with  2KOH 
reacts  as  follows: 

(C2H5)2SO4+  2KOH= 2C2H5OH+ K2SO4. 

This  decomposition  is  termed  saponification  and  upon  it  is  based  a 
method  for  the  estimation  of  compound  ethers.  In  the  process  a 
measured  quantity  of  normal  alkali  in  decided  excess  is  brought  in 
contact  with  a  weighed  quantity  of  the  compound  ether,  and  when 
saponification  is  complete  the  excess  of  alkali  is  determined  by  re- 
titration  with  normal  acid  solution  and  thus  the  quantity  of  normal 
alkali  which  went  into  combination  with  the  ester  is  obtained,  each  cc. 
of  which  represents,  of 

Ethyl  sulphate,  (C2Hs)2SO4 0.076495  gm. 

Ethyl  acetate,     C2H5C2H3O2 0.0874  gm. 

Ethyl  chlorid,     C2H5C1 0.064  gm. 

Ethyl  nitrite,      C2HsNO2 0.07451  gm. 

In  this  process  certain  precautions  must  be  taken.  The  decom- 
position does  not  occur  immediately,  but  takes  place  slowly;  it  may 
be  hastened  by  the  application  of  heat,  but  since  most  esters  are 
volatile,  great  care  must  be  exercised  so  as  not  to  dissipate  it. 

A  quantity  of  the  ether  is  weighed  in  a  weighing-flask,  which  can 
be  well  closed  with  a  tightly-fitting  stopper.  A  measured  excess  of 
the  alkali  solution  is  then  run  in  from  a  burette,  the  solution  diluted 
with  some  water,  for  the  decomposition  is  effected  more  readily  in 
dilute  than  in  concentrated  solutions,  the  stopper  inserted,  and  the 
flask  placed  upon  a  water-bath  and  heated  to  50°  or  60°  C.  It  is 

656 


ESTIMATION  OF  COMPOUND   ETHERS  657 

kept  at  this  temperature  for  two  hours  and  frequently  shaken.  The 
flask  and  contents  are  then  cooled  and  the  stopper  withdrawn.  If 
the  saponification  is  complete,  the  characteristic  odor  of  the  ether  is 
no  longer  noticeable.* 

The  retitration  with  normal  acid  is  now  in  order.  If  barium 
hydroxid  was  used  for  the  saponification,  normal  oxalic  acid  solution 
is  indicated. 

In  the  case  of  esters  containing  volatile  acids  the  direct  method 
of  analysis  may  be  carried  out  as  follows: 

The  ester  is  boiled  with  an  excess  of  sulphuric  acid  until  complete 
saponification  is  effected.  The  liberated  volatile  acid  being  distilled 
off  in  a  current  of  steam  and  collected  in  a  measured  excess  of  standard 
alkali.  The  unneutralized  alkali  being  then  found  by  titration  with 
standard  acid,  or  if  preferred,  the  saponification  may  be  done  with 
alkali,  as  in  the  foregoing  method,  and  then  an  excess  of  sulphuric 
acid  added  and  the  liberated  acid  distilled  into  standard  alkali  solu- 
tion. 

ESTIMATION  OF  SPIRIT  OF  NITROUS  ETHER  AND  AMYL  NITRITE 

(The  Chlorate  Method) 

Spirit  of  Nitrous  Ether.  The  following  method  for  determin- 
ing the  amount  of  ethyl  nitrite  in  spirit  of  nitrous  ether  is  given  in 
the  Siiddeutsche  Apotheker-Zeitung,  1897,  306.  10  gms.  of  the  spirit 
are  treated  with  20  gms.  of  a  5  per  cent  solution  of  potassium  chlorate 
and  5  gms.  of  nitric  acid  (sp.gr.  1.153),  and  allowed  to  stand  for  an 
hour  in  a  closed  flask,  shaking  occasionally.  The  ethyl  nitrite  is 
thus  oxidized  to  ethyl  nitrate,  and  the  potassium  chlorate  reduced 
to  chlorid.  The  reaction  is  probably  as  follows: 

3C2H5N02+  KC103=  KCH-3C2H5N03. 

The  chlorid  so  formed  is  then  estimated  by  means  of  :  -  silver 
nitrate  solution. 

N 

To  the  above  mixture  25  cc.  of  —  silver  nitrate  solution  are  added, 

10 

together  with  a  few  drops  of  a  saturated  solution  of  ammonio-ferric 
sulphate,  and  then  the  excess  of  silver  nitrate  solution  determined  by 

N 
retitration   with   -  -  ammonium  sulphocyanate  or  potassium  sulpho- 


*  It  is  usually  better  to  boil  the  mixture  in  a  flask  provided  with  a  reflux 
condenser. 


658  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

cyanate.     By  deducting  the  quantity  of  sulphocyanate  solution  used 

N 
from  the  25  cc.  of  —  silver  nitrate  added,  the  quantity  of  the  latter 

which  combined  with  the  potassium  chlorid  is  found.     This  multi- 
plied by  0.022353  gives  the  weight  of  ethyl  nitrite  in  the  10  gms.  taken. 


3C2H5N02  =  KC1  =  AgN03. 

10)223.53       10)74.04     10)168.69 


N 


22-353      7-404    16.869=100000.  —  V.  S. 

10 

0.022353  gm.=  0.007404  gm.   =   ice."   " 

Carl  E.  Smith  (A.  J.  Ph.,  1898,  273)  proposes  the  following  modi- 
fication of  the  above  method:  Into  a  ico-cc.  flask  or  bottle  of  white 
glass,  provided  with  a  loosely-fitting  stopper  of  glass,  rubber,  or  cork, 
place  successively  10  cc.  of  distilled  water,  5  cc.  of  a  cold,  aqueous, 
saturated  solution  of  potassium  chlorate,  5  cc.  of  the  spirit  to  be  tested, 
and  5  cc.  of  10  per  cent  nitric  acid.  Quickly  insert  the  stopper  and 

N 
shake  frequently  during  thirty  minutes.     Then  add  10  cc.  of  —  silver 

nitrate,  shake  briskly  for  a  moment,  add  ten  drops  of  ferric  ammonium 

N 
sulphate  solution,  and  titrate  the  excess  of  silver  nitrate  with  —  potas- 

sium sulphocyanate,  without  delay,  so  that  the  darkening  of  the  precip- 
itated silver  chlorid  may  not  obscure  the  end-reaction.  This  is  reached, 
when,  after  momentary  shaking,  upon  the  addition  of  the  last  drop  of 
solution,  the  appearing  red  color  is  not  entirely  dispersed,  but  leaves 
the  liquid  faintly  reddish  throughout.  Assuming  the  spirit  to  contain 
4  per  cent  by  weight  of  ethyl  nitrite,  and  to  have  a  specific  gravity  of 

N 
0.84,  it  would  require  2.55  cc.  —  potassium  sulphocyanate  to  pre- 

N 

cipitate  the  excess  of  silver  in  solution;   and  as  each  cc.  of  —  silver 

10 

nitrate  consumed  in  precipitating  the  chlorid  formed  corresponds  to 
0.02235  gm.  of  ethyl  nitrite,  the  calculation  is  as  follows: 


(10  -2.55)Xo.o2235X  ioo 

5X0.84  =4.o  per  cent. 

If  it  is  desired  to  avoid  calculation  entirely,  however,  2.7  cc.  of 
spirit  of  nitrous  ether  and  half  the  quantities  of  the  reagents  may  be 

N 
taken,  in  which  case  each  cc.  of  —  silver  nitrate  consumed  indicates 

i  per  cent  of  ethyl  nitrite. 


ESTIMATION    OF    COMPOUND    ETHERS 

The  valuation  of  amyl  nitrite  is  made  by  first  diluting  the  amyl 
nitrite  with  alcohol  so  as  to  make  a  5  or  6  per  cent  dilution.  Then, 
using  just  double  the  quantity  of  this  (  =  10  cc.)  and  the  reagents  in 
the  same  doubled  proportion,  the  process  is  carried  out  exactly  as 
in  the  case  of  spirit  of  nitrous  ether,  assuming  the  alcoholic  dilution 
to  contain  6.037  g1115-  °f  the  sample  in  100  cc.;  the  10  cc.  taken  for 
assay  contain  0.6037  gm.  If  in  titrating  the  excess  of  silver,  5.45  cc. 

N 
of  —  potassium  sulphocyanate  are  required  (20  —  5.45)  14.55  cc-  °f 

JO 

N 

—  silver  nitrate — each  cc.  equivalent  to  0.03487  gm.  of  amyl  nitrite 

10 

— have  been  consumed  in  precipitating  the  chlorid  formed  in  the 
reaction,  and  the  calculation  is  as  follows: 

14.55X0.03487X100 

—  =84.6  per  cent. 

0.6037 


CHAPTER  LX 
URINE 

NORMAL  URINE  when  fresh  is  clear  and  transparent.  Its  color 
is  yellowish,  reddish,  or  colorless.  It  has  a  peculiar  odor,  a  distinctly 
acid  reaction,  and  its  average  specific  gravity  is  from  1015  to  1028. 

On  standing  it  generally  gives  a  slight  cloud  of  mucus,  which 
slowly  sinks  to  the  bottom,  and,  after  heavy  exercise  or  a  hearty  meal 
of  nitrogenous  food,  a  sediment  of  urates. 

If  the  urine  be  very  dilute  and  the  temperature  is  above  the  mean, 
decomposition  rapidly  takes  place,  and  the  urine  becomes  turbid, 
acquires  an  alkaline  reaction,  and  develops  a  nauseous  ammoniacal 
odor. 

Reaction.  The  acid  reaction  of  fresh  urine  is  probably  due  to 
the  presence  of  acid  phosphate  of  sodium.  If  it  has  an  alkaline  reac- 
tion when  first  voided  it  is  probably  due  to  the  conversion  of  urea 
into  ammonium  carbonate  within  the  bladder;  it  is  then  generally 
turbid,  and  indicates  an  abnormal  condition. 

The  reaction  is  best  tested  by  dropping  a  small  piece  of  a  red 
and  blue  litmus-paper  into  it.  If  both  are  found  red  in  a  few  minutes 
the  reaction  is  acid,  if  both  are  blue  it  is  alkaline,  if  both  remain 
unchanged  it  is  neutral. 

Composition.  The  average  composition  of  healthy  urine  is  as 
follows : 


Per  Cent. 

Grains 
per  Diem. 

Water 

06  oo 

CQ  fl  OZS 

Solids  as  tabulated  below  

4   OO 

1000  grs 

Urea 

2    C.O 

COO      '  ' 

Uric  acid     

o  04. 

9c  grs 

Hippuric  acid  

o  07  e, 

i«;  o    ' 

Creatinine 

o  07  c. 

T  C       O            ' 

Pigment,  mucus,  xanthin,  and  other  extractives  
Chlorids  of  potassium  and  sodium 

0.50 
o  c.o 

1o-u 
170.0     ' 
I7O  O      ' 

Sulphates  of  potassium  and  calcium  

O.II 

4.0.0 

Phosphates  of  potassium  and  sodium    . 

O    12 

AC    O 

Phosphates  of  magnesium  and  calcium  

O.8O 

7«r   «r     « 

660 


URINE  661 


Beside  these  there  have  been  found  traces  of  indican,  diastase, 
glucose,  oxalic  acid,  lactic  acid,  carbolic  acid,  and  unoxidized  sulphur 
and  phosphorus.  (From  "The  Urine,"  Holland.) 

The  composition  of  urine  is  not  constant:  it  is  influenced  by  the 
amount  of  water  and  other  fluids  taken,  by  the  temperature  of  the 
skin,  by  the  emotions,  the  blood-pressure,  the  amount  of  work  done, 
the  time  of  day,  age,  sex,  and  medicine. 

The  Quantity  passed  in  twenty-four  hours  varies  considerably. 
The  average  quantity  passed  daily  by  a  healthy  adult  is  1400  to 
1600  cc. — about  50  fl.ozs.  The  quantity  of  total  solids  contained 
in  this  is,  as  seen  in  the  table,  about  60  gms.,  or  1000  grains.  About 
one  half  of  these  solids  is  composed  of  urea. 

In  making  an  analysis  of  urine  the  analyst  looks  for  the  presence 
of  abnormal  constituents,  and  determines  the  excess  or  deficiency  of 
the  normal  constituents;  and  therefore,  since  the  composition  of 
urine  is  not  the  same  at  all  hours  of  the  day,  it  is  important  when 
accurate  results  are  desired  to  examine  a  portion  of  the  total  quantity 
of  the  urine  passed  in  twenty-four  hours.  If  this  cannot  easily  be 
obtained,  or  only  a  casual  examination  is  to  be  made,  the  first  urine 
passed  in  the  morning  may  be  used. 

Specific  Gravity.  This  varies  from  1015  to  1028,  according  to 
the  degree  of  dilution  or  concentration.  But  the  pathological  urine 
may  vary  from  almost  that  of  water  to  1050.  The  urine  of  Bright 's 
disease  is,  as  a  rule,  of  low  specific  gravity,  and  in  diabetes  of  high 
specific  gravity. 

The  specific  gravity  may  be  taken  by  any  of  the  usual  methods, 
but  the  urinometer  (a  special  hydrometer,  see  Fig.  92)  is  generally 
used  for  this  purpose.  This  instrument  is  usually  graduated  so  that 
only  the  last  two  figures  of  the  specific  gravity  appear  upon  the  stem, 
and  so  as  to  read  correctly  at  60°  F.  If  the  temperature  is  above 
60°  F.  it  will  be  sufficiently  accurate  for  ordinary  clinical  purposes  to 
add  one  degree  in  specific  gravity  for  every  10  degrees  of  temperature; 
that  is,  if  it  read  1018  at  80°  F.,  it  would  read  1020  at  60°  F.,  or  for 
every  i°  F.  above  60°  add  o.oooi  to  the  observed  specific  gravity. 
The  urinometer  is  used  as  follows:  Sufficient  urine  is  placed  in  the 
upright  jar  or  cylinder  to  float  the  urinometer,  which  is  carefully 
introduced.  When  it  has  come  to  rest  bring  the  eye  on  a  level 
with  the  surface  of  the  liquid -in  the  jar,  and  take  the  reading  at 
the  lower  edge  of  the  meniscus  formed  by  the  upper  surface  of  the 
urine. 

The  mark  on  the  instrument  which  is  cut  by  this  line,  and  which 
can  be  distinctly  seen,  is  taken  as  the  correct  reading. 

If  the  urine  be  turbid  this  method  cannot  be  employed. 


662 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


After  taking  the  specific  gravity,  reaction,  etc.,  set  a  portion  of 
the  urine  aside  in  a  conical  glass  so  as  to  allow  a  deposit  to  form, 
which  must  be  examined  microscopically  and  chemically. 

Total  Solids.  The  total  solids  in  urine  may  be  roughly  estimated 
aS  follows: 

The  last  two  figures  of  the  specific  gravity,  when  multiplied  by  the 
factor  2.33,*  will  give  the  number  of  grams  of  solid  matter  in  1000  cc. 
of  the  urine. 

From  this  it  is  easy  to  calculate  the  quantity  of  solids  passed  in 
twenty-four  hours. 

If,  for  example,  1500  cc.  of  urine  were  passed  in  twenty-four  hours, 


FIG.  92. 

and  the  specific  gravity  of  this  was  1020,  the  total  solids  would  be 

46.6X15 

20X2.33  =  46.6  gms.  in  1000  cc.     In  1500  cc.  there  will  be  

10 

=  69.9  gms.  If  it  be  desired  to  use  the  English  measures,  we  may 
determine  the  total  solids  by  multiplying  the  last  two  figures  of  the 
specific  gravity  by  the  number  of  fluid  ounces  of  urine  passed,  for  these 
last  two  figures  represent  approximately  the  grains  of  solid  matter 
in  a  fluid  ounce.  Thus  if  50  fluid  ounces  were  passed  and  the  specific 
gravity  is  1020,  the  total  solids  will  be  50X20=1000  grs.  in  twenty- 
four  hours. 


*  Neubauer,  Zeitschr.  anal.  Chem.,  I,  166.  J.  H.  Long  (J.  A.  C.  S.,  XXV,  25? 
and  871)  discusses  the  relationship  between  the  total  solids  and  the  specific  gravity 
of  urine,  and  as  a  result  of  his  investigations  establishes  the  factor  2.51.  The 
specific  gravity  of  the  urine  being  taken  at  25°  C.  instead  of  15°  C.,  as  was  done  by 
Neubauer. 


URINE  663 

A  more  exact  method  of  determining  the  total  solids  is  to  evaporate 
10  cc.  in  a  white  porcelain  dish  and  dry  in  a  water-oven  to  a  constant 
weight.  The  difference  between  the  weight  of  the  dish,  and  of  the 
dish  with  the  solids  will  be  the  weight  of  the  solids  in  10  cc.  of 
urine.  Even  by  this  method  there  is  some  loss  through  volatiliza- 
tion. 

Chlorids.  For  the  detection  of  chlorids  a  few  drops  of  nitric 
acid  are  added  to  the  urine  in  a  test-tube,  and  then  silver  nitrate  test 
solution.  A  white,  curdy  precipitate  of  silver  chlorid  forms,  which 
should  occupy  not  more  than  one  fourth  the  volume  of  the  urine 
taken.  If  it  occupies  more  the  chlorids  are  said  to  be  increased;  if 
it  occupies  less  space  than  one  fourth,  the  chlorids  are  diminished. 
It  is  always  advisable  to  compare  the  specimen  under  examination 
with  normal  urine,  subjected  to  the  same  test.  In  most  cases  such  an 
approximate  result  is  all  that  is  required  in  a  clinical  examina- 
tion. 

The  Volumetric  Estimation.  It  is  sometimes  necessary  to  make  a 
more  accurate  determination.  For  this  purpose  a  decinormal  solution 
of  silver  nitrate  is  used.  10  cc.  of  the  urine  are  diluted  with  about 
50  cc.  of  water;  a  few  drops  of  potassium  chromate  T.  S.  are  added, 
and  then  the  decinormal  silver  nitrate  V.  S.  run  in  from  a  burette 
until  a  permanent  reddish  color  is  produced.  Note  the  number  of 
cc.  of  the  V.  S.  used,  and  multiply  this  number  by  the  factor  for  chlorin, 
0.003518  gm.,  or  the  factor  for  sodium  chlorid,  0.005806  gm.  This 
will  give  the  quantity  of  chlorin  or  sodium  chlorid  in  10  cc.  of  urine. 
This  when  multiplied  by  10  gives  the  percentage.  In  highly  colored 
urines  this  method  is  sometimes  inapplicable,  because  the  change  of 
color  is  hidden  by  the  color  of  the  urine.  In  such  cases  Volhard's 
method  (see  page  122)  may  be  employed. 

Phosphates.  Phosphoric  acid  exists  in  the  urine  combined  with 
the  alkalies  and  with  the  alkali  earths.  These  phosphates  are, 
therefore,  generally  distinguished  by  the  terms  alkali  and  earthy 
phosphates.  By  adding  an  alkali  to  normal  urine  the  earthy  phos- 
phates (calcium  and  magnesium)  are  precipitated. 

The  earthy  phosphates  may  be  approximately  estimated  by  adding 
a  few  drops  of  ammonia-water  to  the  urine  and  observing  the  amount 
of  turbidity  produced  after  boiling.  By  comparing  this  with  the 
amount  obtained  by  the  same  treatment  of  normal  urine  the  excess 
or  deficiency  is  determined.  The  precipitate  is  Ca^PO^  and 
MgNH4PO4. 

The  alkali  phosphates  may  be  detected  in  the  filtrate  from  the 
earthy  phosphates  by  the  addition  of  a  few  drops  of  magnesium  sul- 
phate solution  and  some  ammonium  chlorid.  The  precipitate  will 
be  much  more  voluminous  than  that  produced  by  the  earthy  phos- 


664  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

phates,  and  the  excess  or  deficiency  may  be  determined  by  comparison 
with  normal  urine.  The  precipitate  has  the  composition  MgNH4PO4. 

The  Quantitative  Estimation  of  Phosphate  is  rarely  required 
but  may  be  made  by  the  volumetric  process  with  uranium  nitrate 
(see  also  page  314). 

Total  Phosphates.  50  cc.  of  the  urine  are  poured  into  a  beaker 
and  5  cc.  of  a  saturated  sodium  acetate  solution  are  added,  together 
with  a  slight  excess  of  acetic  acid.  The  mixture  is  warmed  over  a 
water-bath  to  near  boiling,  and  then  titrated  with  the  uranium  solution, 
which  is  added,  drop  by  drop,  as  long  as  a  precipitate  falls,  or  until  a 
drop  of  the  hot  solution  brought  in  contact,  on  a  white  porcelain  plate, 
with  a  drop  of  freshly  prepared  potassium  ferrocyanid  solution,  produces 
a  brown  color  due  to  the  formation  of  uranic  ferrocyanid. 

The  quantity  of  uranic  solution  used  is  next  read  off  and  the  phos- 
phoric acid  calculated. 

Each  cc.  of  uranic  solution  represents 

0.005    gm.  of  P2O5; 
0.0069  "     "  H3PO4. 

A  second  and  even  a  third  titration  should  always  be  made,  and 
all  the  above-named  conditions  should  in  every  case  be  strictly  ad- 
hered to. 

The  Earthy  Phosphates  are  found  by  adding  to  200  cc.  of  the  urine 
a  sufficient  quantity  of  ammonia-water  to  render  it  strongly  alkaline. 
This  causes  the  earthy  phosphates  to  precipitate.  After  the  mixture 
has  stood  for  twelve  hours,  the  precipitate  is  collected  on  a  filter  and 
thoroughly  washed  with  dilute  ammonia-water  (1:3).  A  hole  is  then 
made  in  the  bottom  of  the  filter  and  the  precipitate  washed  through 
it  into  a  beaker  and  dissolved  in  the  smallest  possible  quantity  of  hot 
acetic  acid.  To  this  solution  are  added  5  cc.  of  the  sodium  acetate 
solution  and  water  sufficient  to  make  50  cc.,  and  then  treated  as  for 
total  phosphates.  The  amount  of  earthy  phosphates  thus  found 
subtracted  from  the  total  phosphates  gives  the  alkali  phosphates. 

Sulphates.  There  are  two  kinds  of  sulphates  in  urine:  the 
mineral  sulphates  (K2SO4,  Na2SO4,  and  MgSO4)  and  the  ethereal 
sulphates.  The  mineral  sulphates  constitute  about  nine  tenths  of  the 
total  sulphates,  the  remaining  one  tenth  is  in  the  form  of  phenol, 
cresol,  indoxyl,  and  skatoxyl  sulphates  of  potassium.  These  latter 
are  products  of  proteid  putrefaction,  absorbed  from  the  intestinal 
canal.  Their  normal  amount  varies  between  0.12  and  0.3  gm.  Any 
increase  is  roughly  indicative  of  intestinal  indigestion. 

About  30  grains,  or  2  grams,  of  sulphates  are  daily  discharged  in 
the  urine. 


URINE  .  665 

Test.  A  few  drops  of  hydrochloric  acid  are  added  to  the  urine 
in  a  test-tube  to  prevent  the  formation  of  barium  phosphate.  Barium 
chlorid  T.  S.  is  now  added,  which  causes  a  white  precipitate  in  the 
presence  of  sulphates. 

Volumetric  Estimation.  This  is  done  by  the  use  of  a  standard 
solution  of  barium  chlorid.  (See  also  page  331.) 

The  Gravimetric  Method.  Take  100  cc.  of  urine,  add  5  cc.  HC1 
and  heat  to  near  boiling,  then  add  barium  chlorid  T.  S.  in  slight  excess ; 
place  the  beaker  containing  the  mixture  on  a  water-bath  until  the  pre- 
cipitate has  subsided,  decant  the  clear  liquid  carefully  from  the  pre- 
cipitate, add  hot  water,  and  when  the  precipitate  has  again  settled 
decant  again;  continue  this  until  the  decanted  liquid  no  longer  gives 
a  cloudiness  with  sulphuric  acid.  Then  dry  the  precipitate  and  weigh 
carefully.  This  gives  the  quantity  of  BaSO4  which  is  precipitated  out 
of  the  urine  by  barium  chlorid. 

207.7  parts  of  barium  sulphate  represent  98  parts  of  sulphuric 
acid.  Therefore  by  multiplying  the  weight  obtained  by  98  and  divid- 
ing by  207.7  the  number  of  grams  of  sulphuric  acid  in  the  100  cc.  of 
urine  taken  is  obtained.  From  this  we  can  easily  calculate  the 
quantity  eliminated  in  twenty-four  hours. 

Estimation  of  Ethereal  Sulphates  Only.  Take  200  cc.  of  urine, 
add  an  equal  volume  of  barium  chlorid  solution,  made  alkaline  with 
barium  hydroxid,  filter  off  the  precipitate,  and  the  clear  filtrate  will 
contain  the  ethereal  sulphates.  These  are  not  precipitable  by  barium 
chlorid  until  they  are  decomposed  by  boiling  with  hydrochloric  acid. 
Take  200  cc.  of  this  filtrate  (representing  100  cc.  of  urine),  add  20  cc. 
of  hydorchloric  acid  and  boil  ten  minutes.  Filter,  dry  the  precipitate, 
and  weigh. 

Total  Acidity.  Place  50  cc.  of  the  urine  in  a  beaker,  add  three 
or  four  drops  of  phenolphthalein,  and  then  run  into  the  beaker  care- 
fully from  a  burette  decinormal  sodium  hydroxid  V.  S.  until  a  faint 
permanent  red  color  appears.  The  number  of  cc.  of  the  decinormal 
alkali  used,  multiplied  by  0.006255,  gives  the  acidity  of  50  cc.  of  the 
urine  expressed  in  grams  of  oxalic  acid.  From  this  the  total  acidity 
is  determined  by  multiplying  by  the  quantity  of  urine  passed  in  twenty- 
four  hours  and  dividing  by  50. 

If  the  urine  is  highly  colored  the  end-reaction  is  sometimes  difficult 
to  see.  In  such  a  case  the  color  may  be  removed  by  shaking  up  a 
portion  of  the  urine  with  coarsely  powdered  animal  charcoal,  then 
filtering.  The  urine  is  thus  decolorized,  and  the  pink  color  produced 
by  the  indicator  at  the  completion  of  the  reaction  is  easily  seen. 

Joulie's  Method.  The  degree  of  acidity  of  the  urine  is  a  measure 
of  the  degree  of  the  acidity  of  the  blood.  This  acidity  in  both  cases 
is  due  to  acid  phosphates,  salts  of  HsPC^,  that  is,  salts  which,  like 


666  A   MANUAL   OF   VOLUMETRIC   ANALYSIS 

NaH2PO4,  CaHPO4,  and  MgHPO4,  are  still  capable  of  taking  up 
one  or  more  atoms  of  base.  It  is  therefore  obvious  that  the  real 
urinary  acidity  is  phosphoric  acid. 

The  specific  gravity  of  the  urine  to  be  examined  is  taken  at  15°  C. 
If  the  temperature  differs  from  this  figure  a  correction  is  made  accord- 
ing to  a  printed  table.  The  total  acidity,  that  is  to  say,  the  acidity 
from  all  sources,  is  now  calculated,  and  is  expressed  in  terms  of  the 
amount  present  in  a  hundred  parts  of  the  excess  of  the  urine  over  that 
of  distilled  water  at  the  same  temperature.  In  this  way  the  errors 
inseparable  from  the  varying  amount  of  water  in  different  samples 
of  the  same  person's  urine  are  altogether  avoided.  The  degree  of 
dilution  of  a  specimen,  always  hitherto  a  matter  of  difficulty,  may 
thus  be  disregarded.  Thus,  if  the  specimen  in  question  shows  a 
specific  gravity  of  1.020,  the  excess  (which  is  called  E)  is  20,  because 
the  density  of  the  water  at  15°  C.  is  known  to  be  i.ooo.  The  total 
acidity  is  calculated  by  means  of  precipitation  with  a  standard  solution 
of  sucrate  of  calcium  (10  gms.  of  powdered  chalk  and  20  of  sugar  to  a 
liter  of  distilled  water).  The  figure  thus  obtained  is  called  A.  Thus 

A 
we  have  —  Xioo=RA  (i.e.,  the  percentage  of  total  acidity  in  E).    The 

tii 

normal  standard  for  RA,  as  worked  out  by  Joulie,  is  between  4  and  5 
— as  nearly  as  possible  4.5.  But,  as  already  seen,  this  total  acidity 
is  a  matter  of  secondary  importance.  What  we  wish  to  ascertain  is 
the  amount  which  the  acid  phosphates  contribute  to  this  acidity;  in 
other  words,  how  much  of  this  is  serviceable  phosphoric  acidity,  and 
how  much  of  it  is  organic,  fortuitous,  and  undesirable. 

The  next  step  is  therefore  to  determine  the  amount  of  phosphoric 
acid,  combined  and  uncombined,  which  is  present  in  the  given  speci- 
men. This  is  calculated  by  the  classical  method  with  nitrate  of 
uranium  and  ferrocyanid  of  potassium,  and  is  expressed  in  terms  of 
the  amount  present  in  100  parts  of  the  excess  of  the  urinary  density 
over  that  of  water  at  the  same  temperature.  If  we  call  the  total  phos- 
phatic  contents  thus  obtained  P,  we  have  the  following  formula 
p 
—  Xioo=RP  (i.e.,  the  ratio  of  the  phosphates;  in  other  words,  the 

JTl* 

percentage  of  total  phosphatic  contents  in  E).  The  normal  standard 
for  RP  as  estimated  by  Joulie  is  between  n  and  11.5,  as  nearly  as 
possible  11.17.  In  order  to  find  what  we  are  in  search  of,  namely, 
the  amount  which  the  phosphates  contribute  to  the  total  acidity,  it  is 

necessary  to  divide  RP  by  RA,  thus:   TTT= =2.45.     This  repre- 

•KA        4.5 

sents  the  normal  percentage  of  active  phosphoric  acid,  that  is,  the 
amount  which  is  capable  of  neutralizing  an  equal  atomic  weight 


URINE  667 

of  base,  in  E,  which  is  the  excess  of  urinary  density  over  that  of 
water. 

When  the  readings  show  that  both  RA  and  RP  are  below  the  normal, 
the  explanation  is  simple  enough.  It  is  that  there  is  an  insufficient 
quantity  of  phosphoric  acid  in  the  blood  and  the  symptoms  are  there- 
fore due,  at  least  in  part,  to  the  deposition  of  earthy  phosphates  in 
certain  organs  and  tissues. 

Urea  (CO(NH2)2).  This  is  the  most  important  constituent  of 
the  urine,  as  it  is  the  chief  condition  in  which  the  nitrogen  leaves  the 
body.  It  may  be  detected  by  evaporating  a  few  drops  of  urine  on  a 
glass  slide,  moistening  with  nitric  acid,  allowing  it  to  crystallize,  and 
examining  the  crystals  of  urea  nitrate  under  a  microscope  of  low 
power.  As  urea  is  generally  looked  upon  as  an  index  of  the  retro- 
grade changes  going  on  in  the  body,  or  of  the  eliminating  power  of  the 
kidneys,  its  quantitative  estimation  is  a  matter  of  great  import- 
ance. 

The  quantity  of  urea  eliminated  in  twenty-four  hours  has  been 
put  as  being  30  to  33  gms.,  or  from  430  to  550  grains. 

The  Quantitative  Estimation  of  Urea  is  effected  by  treating  it  with 
alkaline  hypochlorites  or  hypobromites  which  decompose  the  urea 
into  CO2,  N,  and  H2O.  (See  Part  IV,  Chapter  LXV.) 

S.  R.  Benedict  and  F.  Gephart  Q.  A.  C.  S.,  xxx,  1760  (1908)) 
recommend  the  following  procedure  for  the  accurate  estimation  of 
urea  in  urine:  5  cc.  of  urine,  together  with  an  equal  volume  of  dilute 
hydrochloric  acid  (made  by  adding  four  volumes  of  distilled  water  to 
one  of  concentrated  acid)  are  introduced  into  a  rather  wide  test-tube. 
The  mouth  of  the  tube  is  covered  with  a  cap  of  lead  foil  and  is  then 
placed  in  a  small  autoclave  which  is  heated  to  a  temperature  of  150° 
to  155°  (corresponding  to  a  pressure  of  6  kilograms  per  square  centi- 
meter). This  temperature  is  maintained  for  an  hour  and  an  half. 
After  the  autoclave  has  cooled,  the  contents  of  the  tube  are  washed 
into  an  8oo-cc.  Kjeldahl  distillation  flask,  diluted  to  about  400  cc., 
treated  with  20  cc.  of  10  per  cent  NaOH  solution  and  distilled  for  about 
forty  minutes  into  an  excess  of  standard  acid.  The  residue  of  acid 
is  titrated,  and  the  urea  nitrogen  calculated  (after  subtracting  the 
previously  determined  Ammonia  nitrogen). 

The  process  gives  accurate  results  and  compares  favorably  with 
the  Kjeldahl  process  and  with  Folin's. 

Uric  Acid  (C5H4N4O3)  occurs  in  urine,  sometimes  in  a  free  state, 
but  oftener  in  combination  with  potassium,  sodium,  or  ammonium, 
and  occasionally  with  calcium  and  magnesium.  These  are  called 
urates.  It  is  detected  microscopically,  and  varies  in  quantity  from 
0.4  to  0.8  gm.  (6  to  12  grs.)  in  twenty-four  hours.  The  crystals  are 
sometimes  large  enough  to  be  seen  by  the  naked  eye.  It  deposits, 


668  A    MANUAL   OF    VOLUMETRIC   ANALYSIS 

upon  standing,  in  the  form  of  a  brick-colored  precipitate,  commonly 
called  brick-dust. 

Qualitative  Chemical  Tests.  The  Murexid  Test.  A  portion  of 
the  urine  is  evaporated  to  dry  ness  in  a  porcelain  dish  upon  a  water- 
bath.  The  residue  is  then  moistened  with  nitric  acid,  and  after  evap- 
orating off  the  nitric  acid  the  residue  is  moistened  with  ammonium 
hydroxid.  If  uric  acid  is  present  the  residue  assumes  a  beautiful 
purple-red  color,  due  to  the  formation  of  murexid. 

The  Silver  Carbonate  Test.  Make  the  urine  alkaline  with  Na2CO.3 
or  K2CO3,  and  moisten  a  filter-paper  with  the  liquid.  Now  touch 
the  moistened  paper  with  a  solution  of  AgNO3.  In  the  presence  of 
uric  acid  a  distinct  gray  stain  is  produced. 

Quantitative  Estimation  of  Uric  Acid.  Acidulate  a  portion  of  the 
urine  with  HC1,  and  set  aside  for  twenty-four  hours.  The  uric  acid 
is  thus  set  free,  and,  being  insoluble,  precipitates  and  adheres  to  the 
bottom  and  sides  of  the  vessel.  It  is  collected  on  a  weighed  filter, 
washed  thoroughly,  dried,  and  weighed.  The  heat  used  should  not 
be  over  100°  C.  (212°  F ).  The  weight  of  the  filter  and  its  contents, 
minus  the  weight  of  the  filter  alone,  gives  the  weight  of  uric  acid  in 
the  volume  of  urine  taken.  The  quantity  eliminated  in  twenty-four 
hours  can  then  be  calculated. 

A  volumetric  method,  depending  upon  the  separation  of  uric  acid 
by  hydrochloric  acid,  and  its  subsequent  titration  with  potassium 
permanganate  is  as  follows,  and  is  a  fairly  reliable  and  rapid  process : 
200  cc.  of  the  urine  are  acidulated  with  a  few  drops  of  concentrated 
hydrochloric  acid,  and  evaporated  on  a  water-bath  to  about  half  the 
volume.  It  is  then  transferred  to  a  stoppered  flask,  and  5  cc.  of  con- 
centrated hydrochloric  acid  added,  and  the  mixture  violently  shaken 
for  a  few  minutes.  It  is  then  set  aside  for  a  half  hour  to  settle,  and 
decanted  onto  a  filter  of  smooth,  hard  texture.  The  sediment  in  the 
flask  is  then  washed  with  about  20  cc.  of  cold  water  and  the  water 
passed  through  the  same  filter.  Then  after  washing  the  filter  with 
a  like  quantity  of  cold  water,  a  hole  is  made  in  its  apex  and  any 
adhering  precipitate  washed  into  the  original  flask.  To  this  is  then 
added  10  cc.  of  10  per  cent  potassium  hydroxid  solution  and  the 
mixture  slightly  warmed  until  a  clear  solution  is  obtained:  90  cc.  of 
cold  water  and  20  cc.  of  dilute  sulphuric  acid  are  then  added  and 

N 
the  titration  with  —  potassium  permanganate  carried  out.     Each  cc. 

N 

of  —  permanganate  is  equivalent  to  0.0075  gm.  of  uric  acid. 
10 

Hartley's  Method.*     This   is   based  upon  the  precipitation  of 
*  J.  A.  C.  S.,  1897,  649. 


URINE  669 

uric  acid  by  means  of  silver  nitrate  in  the  presence  of  magnesia  mixture 
and  a  decided  excess  of  ammonia.  When  precipitation  is  complete, 
the  slightest  trace  of  silver  in  solution  is  shown  by  a  dark  color  pro- 
duced in  a  drop  of  the  clear  solution  by  a  soluble  sulphid.  The  follow- 
ing description  of  the  process  is  from  Hartley's  "Clinical  Chemistry:" 
"The  solutions  required  are: 

"i.  A  —  normal  solution  of  AgNO$,  made  by  diluting  one  volume 

N 

of  a  —  solution  with  four  volumes  of  distilled  water. 
10 

"2.  Magnesium  mixture,  made  to  contain  about  10  gms.  of  crystal- 
lized MgSO4,  12  gms.  of  NH4C1,  and  100  cc.  of  aqua  ammonia  (U. 

S.  P.). 

"3.  A  solution  of  ammonium  sulphydrate,  or  potassium  sulphid. 
This  solution  should  be  freshly  made,  and  of  such  strength  that  its 
color  is  nearly  that  of  the  urine. 

"When  the  urine  contains  a  sediment  of  uric  acid,  or  acid  urates, 
it  is  to  be  put  in  solution  by  warming  with  a  few  drops  of  NaOH  solu- 
tion before  beginning  the  process,  and  the  excess  of  alkali  neutralized 
with  acetic  acid.  In  very  dark  fever-urines  it  is  best  to  dilute  with 
an  equal  volume  with  water.  The  titration  is  performed  in  a  hot 
solution  to  prevent  the  precipitation  of  the  xanthin  bases  by  silver 
nitrate. 

"  The  process  is  conducted  as  follows :  To  50  cc.  of  the  clear  urine 
add  5  cc.  of  the  magnesium  mixture  and  about  10  cc.  of  ammonium 
hydroxid  (sp.gr.  0.960),  or  enough  to  give  a  decided  excess.  Warm 

N 
the  solution  on  a  water-bath,  and  add  from  a  burette  a  —  solution  of 

5° 

silver  nitrate.  From  time  to  time  a  drop  is  removed  from  the  solu- 
tion, by  means  of  a  dropper-pipette,  with  a  bit  of  absorbent  cotton 
wound  tightly  over  the  end,  so  as  to  make  an  efficient  filter,  and  after 
removing  the  cotton  filter,  bring  a  drop  of  the  solution  in  contact  with 
a  drop  of  the  weak  potassium  sulphid  solution  on  a  white  porcelain 
surface.  Experiments  with  pure  water  showed  that  it  required  J  cc. 
of  the  silver  solution  in  50  cc.  to  give  a  marked  reaction.  This  amount 
must  therefore  be  deducted  from  the  reading.  The  titration  is  con- 
tinued until  a  dark  ring  or  cloud  is  seen  at  the  line  of  contact  of  the 
two  drops,  showing  the  presence  of  silver  in  the  solution.  Each  cc. 
of  silver  solution  corresponds  10.0.00336  gm.  of  uric  acid,  and  the 
number  of  cc.  used  (less  i  cc.  for  each  50  cc.  of  urine),  multiplied 
by  this  factor,  gives  the  number  of  grams  of  uric  acid  in  the  urine 
taken.  From  this  we  may  easily  calculate  the  amount  in  100  cc.  or 
that  excreted  in  twenty -four  hours. 


670  A    MANUAL  OF   VOLUMETRIC   ANALYSIS 

"As  soon  as  the  process  is  complete,  the  precipitate  rapidly  settles, 
and  it  is  best  to  draw  off  a  drop  or  two  from  this  clear  supernatant 
liquid  and  test  it  carefully  again.  We  may  also  check  our  work  by 
running  in  another  drop  of  the  silver  solution,  to  see  if  it  produces  a 
cloud,  or  to  see  if  the  precipitation  be  complete.  As  there  is  no  excess 
of  silver  in  the  hot  liquid  at  any  time,  there  can  be  no  reduction  of 
silver. 

"If,  after  the  titration  is  complete,  the  solution  be  cooled,  it  will 
usually  be  found  that  it  will  require  from  i  to  3  cc.  of  the  silver  solu- 
tion to  again  produce  the  end-reaction,  because  of  the  precipitation 
of  the  xanthin  bases  as  silver  compounds.  The  formula  of  the  xanthin 

N 
silver  compound  is  Ag2O  .  C5H4N4O2.     The  factor  for  the  —  AgNOs 

5o 

solution  is  0.0015  —  that  is,  if  we  calculate  them  all  as  xanthin,  each 
cc.  of  silver  solution  used  in  the  cold  solution,  more  than  is  required 
by  the  hot  solution,  corresponds  to  the  above  amount  of  xanthin 
bases. 

"By  making  two  titrations,  the  one  in  the  hot  and  the  other  in  the 
cold  urine,  we  may  estimate  both  the  uric  acid  and  the  xanthin  bases, 
the  latter  by  the  difference  in  the  results  of  the  two  titrations." 

The  lodic  Acid  Method  (Merck).*  This  method  depends  upon 
the  fact  that  minute  quantities  of  uric  acid  will  react  with  iodic  acid, 
in  acidulated  solutions,  with  liberation  of  iodin.  This  reaction  is 
decidedly  quantitative  and  if  the  liberated  iodin  is  removed  by  chloro- 
form and  titrated  with  standard  sodium  thiosulphate  solution,  very 
good  results  are  obtained.  Every  molecule  of  iodic  acid  (HIOs) 
oxidizes  five  molecules  of  uric  acid  to  diuric  acid,  with  liberation  of 
iodin  (one  atom),  as  per  the  equation: 


2HI03  = 

Uric  acid  (1669.1)    Iodic  acid  (349.08)  251.8 

As  seen  by  this  equation,  251.8  gms.  of  iodin  are  liberated  from 
iodic  acid  by  1669.1  gms.  of  uric  acid.  Therefore  125.9  gms.  of  iodin 

N 
are  liberated  by  834.55  gms.  of  uric  acid.     Each  cc.  of  —   sodium 

thiosulphate  (2.465  gms.  per  liter)  equals  0.001259  gm.  of  iodin,  and 
0.008345  gm.  of  uric  acid. 

The  Process.  50  to  100  cc.  of  urine  are  put  into  a  glass  -stoppered 
separatory  burette  |  (if  the  urine  is  of  high  specific  gravity  it  is  diluted 

*  Ph.  Zg.,  1905,  791. 

t  This  should  be  of  200  to  300  cc.  capacity,  and  provided  at  its  lower  end 
with  a  4  to  5  cm.  long  neck  of  0.5  cm.  caliber,  and  below  the  stop-cock  a  tip 
0.5  to  i  cm.  in  length. 


URINE  671 

with  an  equal  volume  of  water  to  prevent  emulsification)  acidulated 
with  5  to  9  cc.  of  50  per  cent  tartaric  or  citric  acid  solution,*  and 
then  for  every  10  cc.  of  diluted  urine,  0.4  to  0.7  cc.f  of  iodic  acid 
solution  (0.37  gm.  per  liter)  are  added,  the  mixture  well  shaken  and 
let  stand  for  one  or  two  minutes. 

The  liberated  iodin  is  then  shaken  out  with  three  to  five  successive 
portions  of  chloroform,  2.5  to  3  cc.  each. 

The  chloroform  iodin  solution  is  then  collected  and  any  iodic 
acid  present  washed  out  with  water,  the  aqueous  liquid  being  sepa- 
rated by  means  of  a  pipette. 

After  washing,  the  chloroform  iodin  solution  is  mixed  with  three 
to  four  times  its  volume  of  50  per  cent  alcohol,  in  which  a  few  crystals 
of  potassium  iodid  are  dissolved,  and  after  the  addition  of  starch 

N 

solution  titrated  with  soldium  thiosulphate. 

100 

If  any  albumen  is  present  it  must  be  removed  by  boiling.  If  iodid 
is  present  in  the  urine  its  quantity  must  be  determined  by  testing 
with  NaNOs  and  acetic  acid,  or  by  means  of  hydrogen  dioxid  and 
sulphuric  acid,  and  the  difference  between  the  quantity  of  iodin  so 
found  and  the  total  iodin  liberated  gives  the  quantity  which  was 
liberated  by,  and  hence  represents,  the  uric  acid  in  the  urine.  See 
also  Gasometric  Method,  Part  IV. 

The  Iodic  Acid  Method  (Bouillet).j:  This  differs  from  the  fore- 
going in  determining  the  amount  of  iodic  acid  consumed  by  the  uric 
acid.  It  is  carried  out  as  follows:  Precipitate  the  uric  acid  in  the 
form  of  insoluble  urate  of  barium  by  adding  to  100  cc.  of  urine, 
previously  neutralized  with  soda,  chlorid  of  barium  until  no  more 
precipitate  is  formed,  then  acidulate  with  5  cc.  of  acetic  acid  i :  100 
let  stand  for  fifteen  or  twenty  minutes,  filter,  and  wash  the  precipitate, 
which  consists  of  urate,  phosphate,  and  sulphate  of  barium.  Transfer 
this  precipitate  into  a  porcelain  crucible,  using  about  100  to  150  cc. 
of  water  for  this  purpose;  add  20  cc.  of  H2SO4  i :  10,  so  as  to  set  free 
the  uric  acid,  then  boil.  At  this  moment  add  10  cc.  of  the  titrated 
solution  of  ^Os,  and  keep  boiling  gently  until  the  whole  of  the  iodin 
is  liberated.  Sometimes  the  liquid  remains  yellow,  on  account  of  the 
presence  of  the  last  traces  of  iodin,  which  are  difficult  to  remove;  this, 
however,  can  be  easily  effected  by  adding  a  little  marble — the  carbonic 
acid  given  off  carries  the  iodin  with  it.  After  cooling,  titrate  the 
undecomposed  ^Os;  for  this  purpose,  10  cc.  of  HC1  1:10  and  30  cc. 

*  These  acids  do  not  absorb  iodin,  even  upon  standing  for  days, 
t  It  is  best  to  make  one  or  two  trials,  because  the  presence  of  tartaric  or  citric 
acid  may  cause  liberation  of  HC1  from  the  NaCl. 

J  Bull.  Soc.  Chim.  (3),  XXV,  No.  5;  and  Chem.  News,  April  19,  1901. 


672  A    MANUAL   OF   VOLUMETRIC  ANALYSIS 

of  KI  i :  10  are  successively  added  to  the  cold  solution,  and  the  iodin 

N 
set  at  liberty  is  titrated  with  —  hyposulphite.     The  difference  between 

the  original  volume  of  the  hyposulphite  solution,  V,  and  the  present 
volume,  v,  gives  the  amount  of  iodic  acid  decomposed;  this  difference, 
multiplied  by  the  factor  0.007,  giyes  tne  weight  of  uric  acid.  In  a 
general  way,  the  formula  is  (F— 2;)  X  0.007  =  uric  acid.  The  method 
is  simple  and  rapid,  and  has  proven  extremely  accurate. 


LITERATURE  ON  THE  ESTIMATION  OF  URIC  ACID  AND  UREA 

E.  Riegler.     Zeitschr.  anal.  Chem.,  1896,  31. 
Tunnicliffe  and  Rosenheim.     Centrabl.  f.  Physiol.,  1897,  xi,  434. 
Rudisch  and  Boroschek.     J.  A.  C.  S.,  xxrv,  562  (1902). 
Ludwig.     Zeitschr.  anal.  Chem.,  xxi,  148  (1882). 
Salowski.     Zeitschr.  physiol.  Chem.,  xiv,  50,  1890. 
Folin  and  Shaffer.     Ibid.,  xxxn,  553,  (1901). 
Haycraft.     Zeitschr.  anal.  Chem.,  xxv,  169  (1886). 
Hermann.     Zeitschr.  physiol.  Chem.,  xii,  497. 
Kruger.     Ibid.t  xxi,  311. 
Czapek.     Ibid.,  xn,  502. 

Folin.    Ibid.,  xxxn,  504;  xxxvi,  333;  xxxvn,  548. 
W.  Braeutigam.     Ph.  Ztg.,  Nov.  13,  1901. 
B.  Merck.     Ibid.,  1905,  791. 
Neubauer  and  Vogel.     "Urine  Analysis." 
Cammerer.     J.  Chem.  Soc.,  LVT,  1040. 
Hopkins  and  Groves.     Chem.  News,  LXVT,  107. 
Dimmock  and  Branson.     Ph.  Jour.,  Jan.  12,  1907. 

H.  Bouillet.  Bull.  Soc.  Chim.  (3),  xxv,  No.  5;  and  Chem.  News,  April 
19,  1901. 

J.  F.  Tocher.     Trans.  Brit.  Ph.  Conf.,  1902,  405-415. 

A.  H.  Allen.     Chem.  News,  Feb.  28,  1896. 

Benedict  and  Gephart.    J.  A.  C.  S.,  xxx,  1760  (1908). 

E.  H.  Bartley.     Ibid.,  1897,  649. 

Sellier.     Ph.  Ztg.,  Aug.  5,  1903,  627. 

Dimmock  and  Branson.    Trans.  Brit.  Ph.  Conf.,  1903,  439. 

Albumen.  In  all  cases  the  urine  should  be  clear  before  applying 
the  tests  for  albumen.  If  not  clear,  it  should  be  filtered. 

(a)  Boiling  Test.  About  10  cc.  of  the  clear  urine  are  placed  in  a 
narrow  test-tube,  one  drop  of  acetic  or  nitric  acid  is  added,  and  the 
tube  heated  over  a  small  flame  in  such  a  way  that  the  upper  portion 
of  the  liquid  only  will  be  heated.  In  the  presence  of  albumen  the 
urine  will  become  turbid,  more  or  less  so  in  proportion  to  the  amount 
of  albumen  present. 


URINE  673 

If  the  acetic  or  nitric  acid  is  not  added  before  heating,  a  turbidity 
will  be  produced  by  the  phosphates;  this,  however,  will  again  dis- 
appear upon  adding  the  acid. 

(b)  The  Nitric  Acid  Test.     About  2  cc.  of  pure  nitric  acid  are 
placed  in  a  test-tube,  and  the  tube  being  inclined  to  one  side,  the 
urine  is  carefully  run  down  the  side  of  the  tube  so  that  it  will  float 
upon  and  not  mix  with  the  acid.     An  opaque-white  zone  will  appear 
at  the  line  of  contact  of  the  two  liquids,  if  albumen  is  present. 

A  mixture  of  nitric  acid  one  volume,  and  saturated  solution  of 
magnesium  sulphate  five  volumes,  is  sometimes  used  instead  of  pure 
nitric  acid  in  the  above  test,  and  is  used  in  the  same  way. 

(c)  Ferrocyanid  Test.     A  small  portion  of  the  urine  is  acidulated 
with  acetic  acid,  and  filtered  if  much  of  a  precipitate  forms.     This 
acidulated  urine  is  then  floated  on  a  solution  of  potassium  ferroycanid . 
A  white  precipitate  appears  if  albumen  is  present.     This  is  a  very 
delicate  and  reliable  test;   peptone,  mucin,  or  alkaloids  are  not  pre- 
cipitated by  it.     This  is  known  as  Bodeker's  Test. 

(d)  Picric  Acid  Test.     A  cold  saturated  solution  of  picric  acid  may 
be  used  in  the  same  way  as  the  nitric  acid — by  contact.     A  white  zone 
appears   at    the    line   of   contact.     Alkaloids,    mucin,    peptones,   and 
urates  are,  however,  precipitated  as  well  as  albumen  in  this    test,  and 
the  solution  should  be  heated  to  redissolve  these. 

(e)  Sodium  Tungstate  Test.     The  reagent  is  made  by  mixing  equal 
parts  of  a  cold  saturated  solution  of  sodium  tungstate  and  citric  acid 
solution.     This  is  a  very  delicate  test,  and  is  applied  in  the  same  way 
as  the  nitric  acid  and  the  above.     Peptones,  alkaloids,  mucin,  and 
urates  are  also  precipitated  by  this  reagent,  but  these  are  redissolved 
upon  boiling. 

(f )  Potassio-mercuric  lodid  Test,  or  Tanrefs  Test.     The  reagent  is 
prepared  as  follows:    Mercuric  chlorid,  1.35  gms.;    potassium  iodid, 
3.32  gms.;   acetic  acid,  20  cc.;   distilled  water,  80  cc.     The  two  salts 
are  separately  dissolved  in  water,  and  then  the  solutions  mixed  and 
the  acetic  acid  added.     This  solution  is  also  used  by  the  contact 
method.     It  is  very  delicate,  detecting  one  part  of  albumen  in  20,000 
parts  of  urine.     It  is  necessary  to  heat  in  order  to  dissolve  the  alka- 
loids, mucin,  and  peptone,  which  are  precipitated  together  with  the 
albumen. 

(g)  Acidulated-brine  Test.     The  reagent  is  made  by  adding  one 
fluid  ounce  of  hydrochloric  acid  to  a  pint  of  a  saturated  solution  of 
common  salt  and  filtering. 

It  is  used  as  follows:  The  solution  is  heated  to  boiling,  and  the 
urine  added  by  the  contact  method.  A  white  zone  appears  at  the 
line  of  contact  if  albumen  is  present.  Peptone,  alkaloids,  etc.,  are 
not  precipitated  by  this  reagent. 


674  A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

The  Quantitative  Estimation  of  albumen  is  of  great  importance, 
but  comparative  tests  are,  as  a  rule,  sufficient.  An  easy  comparative 
test  is  to  heat  a  given  quantity  of  urine  in  a  test-tube,  add  a  few  drops 
of  nitric  acid,  and  set  aside  for  about  twelve  hours,  and  then  note 
the  volume  occupied  by  the  precipitated  albumen.  This  is  generally 
spoken  of  as  volume  per  cent,  and  has  we  relation  to  actual  percentage. 

More  accurate  results  are  obtained  with  Esbach's  Albuminometer. 
This  is  a  graduated  glass  tube  (Fig.  93).  Fill  the  tube  to  U  with  the 
urine,  then  to  R  with  the  reagent.  Close  the  tube  with  a  rubber 
stopper,  shake,  and  set  aside  for  twenty-four  hours.  Then  note  the 
height  of  the  precipitate,  as  indicated  by  the  graduations.  Each  of 
the  numbered  divisions  represents  a  gram  of  albumen  in  1000  cc.  of 
urine.  The  reading  should  be  taken  at  the  middle  of  the 
albuminous  surface.  The  reagent:  Picric  acid,  10  gms.; 
citric  acid,  20  gms.;  water,  1000  gms. 

Blood.  A  small  quantity  of  the  urine  is  mixed  in  a 
test-tube  with  an  equal  volume  of  a  mixture  of  freshly 
prepared  tincture  of  guaiac  and  spirit  of  turpentine, 
which  has  been  exposed  to  the  air  for  some  time.  If 
blood-coloring  matter  is  present  the  mixture  assumes  an 
indigo-blue  color,  the  rapidity  of  formation  of  which 
depends  upon  the  amount  of  blood-coloring  matter  present. 
Pus,  saliva,  and  salts  of  iodin  also  give  a  blue  color  with 
this  test ;  but  it  appears  only  after  a  considerable  lapse  of 
time,  and  it  is  seldom  likely  to  mislead.  Instead  of 
the  spirit  of  turpentine,  peroxid  of  hydrogen  may  be 
used. 
FIG.  93.  Pus.  The  presence  of  pus  is  easily  revealed  by  the 

microscope , 

Urine  containing  pus  is  always  turbid  to  the  naked  eye,  and  deposits 
a  white  or  greenish-white  sediment,  which  resembles  urates  or  earthy 
phosphates.  If  heated  the  sediment  does  not  disappear — difference 
from  urates,  neither  is  it  dissolved  by  dilute  acids — difference  from 
earthy  phosphates.  It  dissolves,  however,  in  strongly  alkaline  solu- 
tions, giving  a  gelatinous,  ropy  liquid.  Pus  effervesces  with  hydrogen 
peroxid. 

Sugar,  (a)  Bismuth  Test.  A  few  cc.  of  urine  are  placed  in  a 
test-tube,  and  an  equal  volume  of  sodium  hydroxid  solution  and  a 
little  bismuth  subnitrate;  mix  well,  and  boil  for  a  few  minutes.  A 
black  precipitate  is  produced  if  sugar  is  present. 

If  albumen  is  present  it  must  be  removed  before  applying  the  test, 
as  it  is  decomposed  by  boiling  with  the  alkali,  forming  a  black  sulphid 
of  bismuth. 

(b)  Nylander's  Test  is  a  modification  of  the  above.     A  solution  is 


URINE  675 

made  of  bismuth  subnitrate  2  gms.,  Rochelle  salt  4  gms.,  sodium 
hydroxid  8  gms.,  and  distilled  water  100  cc. 

Heat  the  urine  to  boiling,  and  add  a  few  drops  of  this  alkaline 
solution  of  bismuth,  continuing  the  boiling.  If  sugar  is  present,  the 
mixture  turns  black. 

This  is  a  very  delicate  test,  but,  as  in  the  previous  one,  any  albumen 
must  be  removed. 

(c)  Moore's  Test.     Add  one  part  of  liquor  soda  to  two  parts  of 
urine, .and  boil.     If  sugar  is  present  the  urine  will  become  blackish 
brown.     Albumen  must  be  removed  before  applying  the  test. 

(d)  Picric  Acid  Test.     About  5  cc.  of  the  urine  are  mixed  with  half 
as  much  of  picric  acid  solution  and  about  2  cc.  of  liquor  potassa,  and 
boiled.     A  dark  mahogany-red  color  is  developed  in  the  presence  of 
sugar.     Albumen  will  cause  turbidity,  but  will  not  interfere  with  the 
test. 

(e)  Trommels  Test.     5  cc.  of  urine  are  mixed  in  a  test-tube  with 
one  half  of  its  volume  of  liquor  soda,  and  one  or  two  drops  of  a  solu- 
tion of  CuSO4  (i :  10).     In  the  presence  of  sugar  a  clear,  deep-blue 
color  is  obtained.     Heat  the  solution  now  almost,  though  not  quite, 
to  boiling.     At  first  a  greenish  then  a  yellow  turbidity  forms,  which 
rapidly  changes  to  a  reddish-yellow  color,  and  precipitates  red  cuprous 
oxid.     An  excess  of  the  copper  solution  should  not  be  used. 

(/)  Haines'  Test.  The  reagent  used  is  a  solution  of  copper  sul- 
phate in  a  mixture  of  equal  parts  of  glycerin  and  water. 

To  about  5  cc.  of  urine  add  a  few  drops  of  this  reagent,  and  then 
add  sodium  hydroxid  solution  until  the  liquid  assumes  a  deep-blue 
color.  The  mixture  is  then  gradually  heated  to  boiling.  If  sugar  is 
present  the  color  changes  to  yellow,  and  finally  brick-red. 

(g)  Indigo-carmine  Test.  The  reagent  is  made  by  mixing  one  part 
of  dried  commercial  extract  of  indigo  with  thirty  parts  of  pure  dry 
sodium  carbonate. 

The  test:  Add  enough  of  this  powder  to  5  cc.  of  the  urine  to  give 
it  a  transparent  blue  color,  and  heat  to  boiling.  If  sugar  is  present, 
the  color  changes  to  violet,  cherry-red,  and  finally  yellow.  On  gently 
agitating  the  tube  the  colors  appear  in  the  reversed  order. 

(h)  Molisch's  Test.  Put  i  cc.  of  the  urine  in  a  test-tube,  add 
2  cc.  of  a  saturated  solution  of  alpha-naphthol,  mix  well,  and  then 
add  an  excess  of  sulphuric  acid.  A  deep  violet  color  is  produced  if 
sugar  is  present.  On  dilution  with  water  a  blue  precipitate  occurs. 

Thymol  or  menthol  may  be  used  instead  of  naphthol.  The  color 
then  produced  is  deep  red. 

Quantitative  Estimation.  This  is  generally  effected  by  the  use 
of  FehJing's  or  Pavy's  solution.  The  process  is  described  on 
page  492. 


676 


A    MANUAL    OF    VOLUMETRIC   ANALYSIS 


By  Fermentation.  The  Densimetric  Method.  This  is  performed 
by  adding  a  small  quantity  of  yeast  to  a  certain  volume  of  urine  and 
setting  aside  for  about  twenty-four  hours.  As  the  sugar  is  decom- 
posed the  specific  gravity  of  the  urine  becomes  less.  Therefore  by 
taking  the  specific  gravity  of  the  urine  before  and  after  fermentation 
a  fairly  accurate  estimation  of  sugar  present  may  be  made,  provided 
the  quantity  be  not  less  than  0.5  per  cent.  Each  degree  of  the  urinom- 
eter  indicates  0.219  Per  cent  °f  sugar.  If  the  specific  gravity  of  a  sample 
of  urine  is  found  to  be  1032,  and  after  subjecting  it  to  fermentation  it 
is  1022,  the  quantity  of  sugar  present  in  the  sample  is  ten  times 
0.219=2.19  per  cent. 

Estimation  of  Sugar  by  Dr.  Einhorn's  Fermentation  Saccha- 
rometer.  Take  i  gm.  of  commercial  compressed  yeast  (or  TV  of  a  cake 

of  Fleischmann 's  yeast),  shake  thoroughly 
in  the  graduated  test-tube  with  10  cc.  of 
the  urine  to  be  examined.  Then  pour  the 
mixture  into  the  bulb  of  the  saccharometer 
(Fig.  94).  By  inclining  the  apparatus  the 
mixture  will  easily  flow  into  the  cylinder, 
thereby  forcing  out  the  air.  Owing  to  the 
atmospheric  pressure  the  fluid  does  not 
flow  back,  but  remains  there. 

The  apparatus  is  to  be  left  undisturbed 
for  twenty  to  twenty-four  hours  in  a  room 
of  ordinary  temperature. 

If  the  urine   contains   sugar,  the   alco- 
holic fermentation  begins  in  about  twenty 
to   thirty  minutes.     The  evolved  carbonic 
acid    gas    gathers     at     the    top    of    the 
cylinder,  forcing  the  fluid  back  into  the  bulb. 

On  the  following  day  the  upper  part  of  the  cylinder  is  filled  with 
carbonic  acid  gas.  The  changed  level  of  the  fluid  in  the  cylinder 
shows  that  the  reaction  has  taken  place,  and  indicates  by  the  numbers 
— to  which  it  corresponds —  the  approximate  quantity  of  sugar  present. 
If  the  urine  contains  more  than  i  per  cent  of  sugar,  then  it  must 
be  diluted  with  water  before  being  tested. 

Diabetic  urines  of  straw  color,  and  a  specific  gravity  of  1018-1022 
may  be  diluted  twice;  of  1022-1028,  five  times;   1028-1038,  ten  times. 
The  original   (not   diluted)   urine  contains   in  proportion   to  the 
dilution  two,  five,  or  ten  times  more  sugar  than  the  diluted  urine. 

In  carrying  out  the  fermentation  test  it  is  always  recommendable 
to  take,  besides  the  urine  to  be  tested,  a  normal  one,  and  to  make  the 
same  fermentation  test  with  it. 

The  mixture  of  the  normal  urine  with  yeast  will  have  on  the  follow- 


FIG.  94. 


URINE  677 

ing  day  only  a  small  bubble  on  the  top  of  the  cylinder.  That  proves 
at  once  the  efficacy  and  purity  of  the  yeast. 

If  there  is  likewise  in  the  suspected  urine  a  small  bubble  on  the 
top  of  the  cylinder,  then  no  sugar  is  present;  but  if  there  is  a  much 
larger  gas  volume,  then  we  are  sure  that  the  urine  contains  sugar. 

Tests  for  Bile,  (a)  Oliver's  Test.  Dissolve  2  gms.  of  fresh 
peptone  (Savory  and  Moore's  Pulverized),  0.25  gm.  salicylic  acid, 
and  2  cc.  of  33  per  cent  acetic  acid  in  water  to  make  200  cc.  The 
solution  should  be  rendered  perfectly  clear  by  nitration. 

The  urine  should  also  be  clarified  by  filtration,  and  diluted  to  a 
specific  gravity  of  1008.  i  cc.  of  this  urine  is  added  to  3  cc.  of  the 
above  reagent.  If  biliary  salts  are  present  a  distinct  opalescence  at 
once  appears,  which  becomes  more  intense  in  about  five  minutes. 
This  opalescence  will  be  more  or  less  distinct  in  proportion  to  the 
quantity  of  bile  present. 

(b)  Gmelin's    Test.     2   or   3   cc.   of  partially   decomposed   yellow 
nitric  acid  are  placed  in  a  test-tube,  and  an  equal  volume  of  the  urine 
is  cautiously  poured  on  top.     In  the  presence  of  bile  pigments  a  play 
of  colors  will  appear,   beginning  with  green,  then  passing  through 
blue,  violet,  red,  and  yellow. 

The  nitric  acid  may  be  prepared  for  this  test  by  adding  a  fragment 
of  zinc  to  ordinary  nitric  acid. 

(c)  Pettenkofer's   Test.     Mix  equal  parts  of  urine  and  sulphuric 
acid,  add  one  drop  of  simple  syrup,  and  apply  a  gentle  heat.     The 
color  will  change  from  cherry-red  to  purple  if  biliary  acids  are  present. 

(d)  Ultzmann's  Test.     5  cc.  of  urine  are  mixed  with  2  cc.  of  a 
strong  solution  of  KOH  (1:3)  and  then  an  excess  of  pure  HC1  added. 
The  mixture  will  become  emerald-green  if  biliary  pigments  are  present. 

(e)  Tincture  of  lodin   Test.     A  few  drops  of  iodin  tincture  are 
floated  upon  the  surface  of  the  urine.     If  biliary  pigments  are  present, 
there  will  appear  at  the  line  of  contact  of  the  two  liquids,  after  a  few 
minutes,  a  beautiful  emerald-green  zone. 


PART    IV 

A   FEW   GASOMETRIC   ANALYSES 


CHAPTER  LXI 
THE  NITROMETER 


(6) 


(d) 


FOR  general  gas  analysis,  and  for  the  rapid  estimation  of  such 
substances  as  ethyl  nitrite,  hydrogen   dioxid,  urea,  bleaching-powder, 
manganese  dioxid,  etc.,  an  instrument  called  the  nitrometer  is  used. 
The  apparatus  in  its  simplest  form  is  shown  in  Fig.  95.     It  consists 
of  a  measuring  tube,  <z,  of  50  or  100  cc.  capacity, 
and  graduated  in  tenths  of   a  cc.     This  is  con- 
nected by  means  of  a  stout  rubber  tube  with  an 
open    equilibrium  tube  b,  also    called    "control- 
tube,"    "pressure-tube,"   or    "level-tube;"    both 
tubes   are  preferably  provided    with    a    globular 
expansion  near  the  lower  end,  and  are  held  by 
suitable  clamps  upon  a  stand,  in  such  a  manner 
that    either    tube    may   be   readily   and   quickly 
clamped  at  a  higher  or  lower  level.     The  measur- 
ing tube  is  fitted  at  the  top  with  a  stop-cock,  r, 
and   a   graduated   glass   tube  or  cup,  d.     Some 
nitrometers  are  provided  with  a  three-way  stop- 
cock, so  arranged  that  according  to  the  way  it 
is    turned,  it  will  discharge  the  contents  of  the 
cup  either  into  the  measuring  tube  below,  or  out 
into  the  waste  opening  which  is  usually  placed  at 
e,  or  it  will  discharge  the  contents  of  the  measur- 
ing tube  into  the  waste  opening. 
With  this  apparatus  gases  can  be  rapidly  and  accurately  measured 
at  definite  temperature  and  pressure. 

In  measuring  the  gas  the  instrument  is  filled  with  some  liquid  in 
which  the  gas  is  insoluble  —  generally  mercury.  In  many  cases  a 
saturated  solution  of  salt  may  be  used. 

678 


FIG.  95. 


THE   NITROMETER  679 

Suppose  we  fill  the  instrument  with  mercury  in  such  quantity  that 
when  the  stop-cock  is  opened  and  the  control-tube  raised,  the  mercury 
will  rise  as  far  as  the  top,  and  about  two  inches  in  the  control- 
tube. 

The  top  is  now  closed,  the  control-tube  lowered,  and  a  little  car- 
bonic acid  gas  admitted  through  E.  The  top  is  then  again  closed, 
and  the  instrument  allowed  to  stand  until  its  contents  have  acquired 
the  temperature  of  the  room.  A  centigrade  thermometer  suspended 
to  the  stand  will  then  give  the  temperature  of  the  gas. 

The  control-tube  is  now  raised  or  lowered  so  as  to  make  the  level 
of  the  liquid  in  both  tubes  the  same.  This  makes  the  pressure  in  the 
tube  the  same  as  the  atmospheric  pressure  outside,  and  by  referring  to  a 
barometer  standing  near  this  pressure  is  ascertained. 

We  now  have  a  definite  volume  of  the  gas  at  a  known  temperature 
and  pressure. 

It  now  only  remains  to  read  off  the  volume  of  the  gas,  and  correct 
it  to  the  normal  temperature  and  pressure  by  Charles*  and  Bodies' 
laws,  respectively. 

The  normal  temperature  and  pressure  is  o°  C.  and  760  mm.  pressure. 

The  weight  of  the  gas  in  grams  may  then  be  calculated  from  its 
volume  by  multiplying  the  number  of  cc.  at  the  normal  temperature 
and  pressure,  by  the  weight  of  i  cc.  of  the  gas  in  grams. 

This  weight  may  be  found  as  follows: 

1000  cc.  of  hydrogen  at  normal  temperature  and  pressure  weigh 
0.0896  gm.  i  cc.  of  H  then  weighs  0.0000896  gm. 

One  cc.  of  oxygen  weighs  sixteen  times  as  much,  and  i  cc.  of  nitrogen 
weighs  fourteen  times  as  much.  Therefore,  by  multiplying  the  weight 
of  i  cc.  of  H  by  the  atomic  weight  of  an  elementary  gas,  or  half  the 
molecular  weight  of  a  compound  gas,  the  weight  of  i  cc.  of  that  gas 
is  obtained. 

According  to  the  law  of  Charles,  the  volume  of  a  gas  under  con- 
stant pressure  varies  directly  with  the  absolute  temperature. 

All  gases  expand  or  contract  by  yfs  °f  thdr  volume  for  each  centi- 
grade degree  of  temperature,  increased  or  decreased. 

We  may  regard  a  gas  at  o°  C.  as  having  passed  through  273°  C. 
In  other  words,  273°  below  zero  must  be  regarded  as  the  absolute 
zero,  and  o°  C.  as  273°  absolute  temperature. 

Thus  the  absolute  temperature  centigrade  is  the  observed  tem- 
perature +  2  73°. 

Example.  A  given  volume  of  oxygen  gas  at  15°  C.  measures  20  cc. 
What  will  it  measure  at  o°  C.  ? 

0°+273°X20  273°X20 

150+273'       °r      ^88~=I8'95CC' 


680  A   MANUAL  OF    VOLUMETRIC   ANALYSIS 

Boyle's  Law.  The  volume  of  a  confined  gas  is  inversely  pro- 
portional to  the  pressure  brought  to  bear  upon  it.  That  is,  the  less  the 
pressure  the  greater  the  volume,  and  vice  versa. 

Rule.  Multiply  the  observed  volume  by  the  observed  pressure, 
and  divide  by  the  normal  pressure. 

Example.  A  given  volume  of  gas  at  750  mm.  pressure  measures 
20  cc.  What  will  it  measure  at  760  mm.  (the  normal  pressure)  ? 

75oX2occ. 

76o        =  19-73  cc.     Ans. 

Now  let  us  take  an  example  in  which  both  laws  are  involved. 

A  given  volume  of  oxygen  at  15°  C.  subjected  to  a  pressure  of 
750  mm.  measures  20  cc.  What  will  it  measure  at  the  normal  tem- 
perature and  pressure? — i.e.,  o°  C.  and  760  mm. 

In  the  first  example  we  find  that  20  cc.  of  oxygen  at  15°  C.  will 
measure  at  o°  C.  18.95  cc-  Then 

750X18.950:. 

=18.700:.     Ans. 

760 

Now  to  find  the  weight  of  this  volume  of  oxygen  we  proceed  as 
follows: 

i  cc.  of  H  weighs  0.0000896  gm.; 

i  cc.  of  O  weighs  16X0.0000896  =  0.0014336  gm.; 

18.70  cc.  weigh  0=18.70X0.0014336  gm.,  or  0.02680832  gm. 

The  method  of  using  the  nitrometer  for  gasometric  assay  of  various 
substances  is  illustrated  in  the  assay  of  spirit  of  nitrous  ether. 
(See  page  683.) 

The  following  tables,  from  the  U.  S.  P.  VIII,  give  the  factors  for 
corrections  for  temperature  and  barometric  pressure.  These  factors 
may  be  used  in  order  to  obtain  reasonably  accurate  results  when  the 
temperature  and  pressure  are  not  nearly  normal.  The  barometric 
correction  is  important  at  any  locality  more  than  250  meters  above 
sea-level. 

Example.  Assuming  that  the  volume  of  gas  read  off  was  44.5  cc. 
at  32°  C.  (89.6°  F.),  and  it  is  desired  to  ascertain  the  corresponding 
volume  at  25°  C.  (77°  F.),  barometric  pressure  not  being  taken  into 
consideration,  then  the  44.5  cc.  must  be  reduced  in  the  proportion  of 


THE   NITROMETER 


681 


i  to  0.977  (see  temperature  correction  factors  above),  or  44.5  must  be 
multiplied  by  0.977.  The  result  will  be  43.48  (43.4765)  cc.  as  the 
equivalent  volume  of  gas  at  25°  C.  (77°  F.). 

FACTORS  FOR  TEMPERATURE  CORRECTIONS 

(Normal  Temperature,  25°  C.) 


Temperature. 

Factor. 

Temperature. 

Factor. 

Temperature. 

Factor. 

i5°C. 

-035 

22°  C. 

I.  010 

29°  C. 

0.987 

i6°C. 

.031 

23°  C. 

1.007 

30°  c. 

0.983 

17°  C. 

.028 

24°  C. 

1.003 

31°  c. 

0.980 

i8°C. 

.024 

25°  c. 

I.OOO 

32°  C. 

0.977 

19°  C. 

.021 

26°  C. 

0.997 

33°  C. 

0.974 

20°  C. 

.017 

27°  C. 

o  993 

34°  C. 

0.971 

21°  C. 

014 

28°  C. 

0.990 

35°  C. 

0.968 

FACTORS  FOR  CORRECTION  FOR  BAROMETRIC  PRESSURE 

(Normal  Barometer.  760  mm.) 


Barometer  Reading. 

Factor. 

Barometer  Reading. 

Factor. 

Mm. 

Inches. 

Mm. 

Inches. 

790 

31  .10 

1.039 

660 

25.98 

0.868 

78o 

30.71 

1.026 

650 

25-59 

0-855 

770 

30-31 

1.013 

640 

25.20 

0.842 

76o 

29.92 

I  .000 

630 

24.80 

0.829 

75° 

29-53 

0.987 

620 

24.41 

0.816 

740 

29   13 

0.974 

610 

24.02 

0.803 

73° 

28.74 

0.961 

600 

23.62 

0.789 

720 

28.35 

0.947 

590 

23-23 

0.776 

710 

27-95 

0-934 

58o 

22.83 

0.763 

700 

27.56 

o,  921 

570 

22.44 

0.750 

690 

27  .17 

0.907 

56o 

22.05 

o  737 

680 

26.77 

0.895 

550 

21.65 

0.724 

670 

26.38 

0.882 

Example.  Assuming  that  the  volume  of  gas  read  off  was  43.48 
(43.4765)  cc.  at  590  mm.  barometric  pressure,  and  it  is  desired  to 
ascertain  the  corresponding  volume  at  normal  barometric  pressure 
(760  mm.),  temperature  not  being  taken  into  consideration,  then  the 
43.48  cc.  must  be  reduced  in  the  proportion  of  i  to  0.776  (see  baro- 
metric correction  factors  above),  or  43.48  must  be  multiplied  by  0.776. 
The  result  will  be  33.74  cc.  as  the  equivalent  volume  of  gas  at  normal 
barometric  pressure. 


682 


A   MANUAL   OF   VOLUMETRIC  ANALYSIS 


ESTIMATION  OF  NITROGEN  DIOXID 

NO  =  29.81;  i  liter  at    o°  C.  and  760  mm.  =  1.3396  gms. 
at  25°  C.  and  760  mm.  =  1.2272  gms. 

ONE  CUBIC  CENTIMETER  OF  NITROGEN  DIOXID  is  THE  EQUIVALENT  OF: 


At  o°  C,  and 
760   mm. 
Gram. 

At  25°  C.  and 
760  mm. 
Gram. 

Nitrogen  dioxid  NO=    29.81  .  . 

o  ooi  3306 

o  0012272 

Amyl  nitrite    C5HuNO-=  116.24  

O   OO  ^2234 

o  00478^1 

Ethyl  nitrite,  C2H5NO2=   74-Si  

O.OO33482 

o  0030673 

Sodium  nitrite  NaNO2=   68.57  

o  0030813 

o  0028227 

CHAPTER  LXII 

ASSAY  OF  NITRITES 

Spirit  of  Nitrous  Ether.  This  is  an  alcoholic  solution  of  ethyl 
nitrite  (C2H5NO2=  74.51),  yielding  when  freshly  prepared  and  tested 
in  the  nitrometer  not  less  than  n  times  its  own  volume  of  nitrogen  dioxid 
(NO  =  29.8i),U.  S.  P. 

When  nitrites  are  mixed  with  an  excess  of  KI  and  acidulated  with 
H2SC>4,  iodin  is  liberated,  and  all  the  nitrogen  of  the  nitrite  is  evolved 
in  the  form  of  NO.  as  shown  in  the  equation 


149.02  59-62 

The  process  is  conducted  as  follows: 

Open  the  stop  -cock  of  the  measuring  tube,  raise  the  control-tube, 
and  pour  into  the  latter  a  saturated  solution  of  NaCl  until  the  measuring 
tube,  including  the  bore  of  the  stop-cock,  is  completely  filled.  Then 
close  the  stop-cock  and  fix  the  control-tube  at  a  lower  level.  Now 
introduce  into  the  funnel  at  the  top  of  the  measuring  tube  a  weighed 
quantity  (about  4  gms.)*  of  spirit  of  nitrous  ether;  open  the  stop-cock, 
and  allow  the  spirit  to  run  into  the  nitrometer,  being  careful  that  no 
air  enters  at  the  same  time.  10  cc.  of  potassium  iodid  T.  S.  are  now 
added  in  the  same  manner,  and  followed  by  10  cc.  of  normal  sulphuric 
acid  V.  S.  Effervescence  takes  place  immediately,  and  after  thirty 
to  sixty  minutes,  when  the  volume  of  gas  has  become  constant,  the 
control-tube  is  lowered  so  as  to  make  the  level  of  the  liquid  in  both 
tubes  the  same,  and  the  volume  of  the  gas  in  the  graduated  tube  read 
off. 


*  It  is  convenient  to  take  5  cc.  accurately  measured,  and  calculate  its  weight 
by  multiplying  by  the  specific  gravity.  The  U.  S.  P.  directs  to  transfer  30  gms. 
of  the  spirit  of  nitrous  ether,  which  has  been  previously  shaken  with  0.5  gm. 
of  KHCO3  to  a  tared  flask,  and  weigh  it  accurately.  Add  sufficient  alcohol  to 
bring  the  volume  to  exactly  100  cc.,  and  mix  thoroughly.  Introduce  into  the 
nitrometer  exactly  10  cc.  of  the  alcoholic  solution. 

683 


684  A   MANUAL  OF  VOLUMETRIC  ANALYSIS 

This  volume,  multiplied  by  0.0030673  gm.  gives  the  weight  of  ethyl 
nitrite  in  the  spirit  taken  for  analysis.  The  product  multiplied  by 
100,  and  then  divided  by  the  weight  of  the  spirit  taken,  gives  the  per 
cent  of  pure  ethyl  nitrite  present. 

The  temperature  correction  is  one  third  of  one  per  cent  of  the  total 
percentage  found,  for  each  degree,  additive,  if  the  temperature  is 
below  25°  C.  The  barometric  correction  is  -£$  of  one  per  cent  for 
each  millimeter,  additive  if  abo\e,  subtract ive  if  below  760. 

The  volume  of  NO  generated  at  the  ordinary  indoor  temperature 
(assumed  to  be  at  or  near  25°  C.,  77°  F.)  should  not  be  less  than  55  cc. 
if  5  cc.  of  the  spirit  are  taken,  corresponding  to  about  4  per  cent  of 
pure  ethyl  nitrite. 

Sodium  chlorid  solution  is  used  in  the  above  assay,  because,  owing 
to  its  density,  the  spirit  will  float  on  top,  and  the  gas  evolved  will  not 
dissolve  in  it.  At  the  same  time  the  expense  of  using  mercury  is 
saved.  It  is  important  that  no  air  be  allowed  to  get  into  the  measuring 
tube,  because  this  would  convert  the  NO  into  a  higher  oxid  of  nitrogen, 
which  would  dissolve  in  the  salt  solution,  and  thus  vitiate  the  result. 

It  is  required  to  correct  the  volume  of  gas  evolved  at  higher  tem- 
peratures, to  its  corresponding  volume  at  o°  C.  The  calculations 
involved  are  as  explained  below: 

Example.  5  cc.  of  spirit  of  nitrous  ether  (sp.gr.  0.823)  are  treated 
in  a  nitrometer,  and  the  NO  evolved  measures  55  cc. 

The  temperature  at  which  the  operation  is  conducted  is  25°  C., 
and  the  atmospheric  pressure  normal. 

What  per  cent  of  ethyl  nitrite  is  present  in  the  sample  ? 

By  consulting  the  equation  given  above,  it  will  be  seen  that  one 
molecular  weight  of  NO  =  29.81  is  evolved  from  one  molecular  weight 
of  ethyl  nitrite,  74.51. 

Now  reduce  the  volume  of  the  gas  liberated  at  25°  C.  to  its  corre- 
sponding volume  at  o°  C.  Thus 

273°+25°:55-  :273°+o°.#.  #=50.4  cc. 

Thus  the  gas  evolved  from  5  cc.  of  the  spirit  of  nitrous  ether,  meas- 
ured at  o°  C.,  is  50.4  cc. 

The  next  step  in  the  calculation  is  to  find  how  much  ethyl  nitrite 
each  cc.  of  the  evolved  NO  represents,  i  liter  of  hydrogen  at  o°  C. 
and  normal  pressure  weighs  0.0896  gm. 

By  multiplying  this  weight  by  half  the  molecular  weight  of  NO, 
the  weight  of  1000  cc.  of  the  latter  gas  is  obtained;  this  will  be  found 
to  be  1.3396.  Now  if  1.3396  gms.  of  NO  measures  1000  cc.,  29.81 
gms.  will  measure  22^328.24  cc. 

1.3396:1000:  •  29.81:3;.  #=22,324.58. 


ASSAY  OF   NITRATES  685 

Then  if  22,324.58  cc.  of  NO  are  evolved  by,  and  consequently 
represent,  74.51  gms.  of  ethyl  nitrite,  as  the  equation  shows,  i  cc.  of 
NO  will  represent  0.0033482  gm.  of  pure  ethyl  nitrite. 

Now,  since  in  the  above  example  50.4  cc.  of  gas  were  evolved  at 
o°  C.,  the  5  cc.  of  spirit  of  nitrous  ether  examined  must  contain 


50.4X0.0033482  gm.  =  o.i6875  gm. 

of  pure  ethyl  nitrite. 

In  order  to  determine  the  percentage  strength,  the  weight  of  the 
spirit  taken  must  be  known.  This  may  be  found  by  multiplying  the 
measure  by  the  specific  gravity,  5  cc.Xo.823  =  4.ii5  gms.  Then 

4.115  gms.  .0.16875  gms.:  •  ioo:x.  x=4  per  cent. 

Amyl  Nitrite  is  a  liquid  containing  about  80  per  cent  of  amyl 
nitrite  (principally  iso-amyl  nitrite),  C5HuNO2  =  116.24,  together  with 
variable  quantities  of  undetermined  compounds. 

The  U.  S.  P.  assay  is  as  follows*  Transfer  about  3  cc.  of  the  amyl 
nitrite,  which  has  been  previously  shaken  with  0.5  gm.  of  potassium 
bicarbonate  and  carefully  decanted,  to  a  tared  100  cc.  measuring  flask, 
and  weigh  it  accurately;  add  alcohol  to  bring  the  volume  to  exactly 
100  cc.  10  cc.  of  this  alcoholic  solution  are  introduced  into  the  nitrom- 
eter as  directed  for  spirit  of  nitrous  ether;  10  cc.  of  potassium  iodid 

N 
T.  S.  and  10  cc.  of  —  H2SO4  V.  S.  are  then  added,  and  the  volume  of 

NO  generated,  measured  at  the  ordinary  indoor  temperature  (assumed 
to  be  at  or  near  25°  C.  or  77°  F.),  should  be  about  40  cc.  Each  cc.  at 
this  temperature  represents  0.0047851  gm.  of  pure  amyl  nitrite,  or  about 
2  per  cent. 

Sodium  Nitrite  (NaN  02=68.57).  This,  like  the  other  nitrites 
mentioned,  when  treated  with  potassium  iodid  and  sulphuric  acid, 
is  decomposed,  and  NO  is  given  off.  The  reaction  is  here  illustrated: 


A  molecule  of  NaNO2  (68.57)  evolves,  when  properly  treated,  one 
molecule  of  NO  (29.81). 

The  assay  process  is  as  follows:  Weigh  out  0.15  gm.  of  NaNO2, 
dissolve  it  in  about  5  cc.  of  water,  and  introduce  the  solution  into  a 
nitrometer.  This  is  followed  by  a  solution  of  i  gm.  of  KI  in  6  cc. 

N 
:>f  water  and  15  cc.  of  —  H2SO4.     The  gas  which  is  liberated  should 


686  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

measure  not  less  than  50  cc.  at  15°  C.  (59°  F.)  or  51.7  cc.  at  25°  C. 
(77°  F.),  corresponding  to  not  less  than  97.6  per  cent  of  the  pure  salt. 
Each  cc.  at  25°  C.  represents  0.0028227  gm.  and  at  o°  C.  0.0030813 
gm.  of  pure  NaNC>2. 

ESTIMATION  OF  NITRIC  ACID  IN  NITRATES 

This  may  also  be  effected  by  the  use  of  the  nitrometer. 

When  a  nitrate  is  shaken  up  with  an  excess  of  sulphuric  acid  and 
mercury,  the  nitrate  is  decomposed  and  NO  is  evolved,  as  seen  in 
the  following  equation: 


2)200.86  2)59.62 

100.43  29.81 

Thus  each  molecule  of  the  nitrate  radical  NOs  gives  off  a  molecule 
of  NO. 

Not  more  than  0.2  gm.  of  nitrate  should  be  taken  for  analysis, 
since,  if  this  quantity  is  exceeded,  the  volume  of  gas  evolved  will  be 
greater  than  the  instrument  can  conveniently  hold.  In  this  estimation 
the  nitrometer  is  filled  with  mercury  instead  of  brine;  the  nitrate  is 
dissolved  in  5  cc.  of  water,  introduced  into  the  nitrometer,  and  followed 
by  excess  of  strong  sulphuric  acid.  The  instrument  is  well  shaken  for 
some  time,  and  when  action  has  ceased  and  the  contents  have  cooled 
down  to  the  temperature  of  the  room,  the  level  is  adjusted  and  the 
volume  of  NO  read  off  and  calculated  in  the  usual  way. 


CHAPTER  LXIII 
HYDROGEN     DIOXID 

As  stated  in  a  previous  chapter,  hydrogen  dioxid  when  acted  upon 
by  an  acidulated  solution  of  potassium  permanganate,  is  decomposed 
and  oxygen  is  evolved.  One  half  of  this  oxygen  comes  from  the  dioxid 
and  the  other  half  from  the  permanganate. 

Therefore  if  i  cc.  of  the  dioxid  be  treated  in  this  way  and  20  cc. 
of  oxygen  are  evolved,  the  strength  of  the  solution  is  ten  volumes. 

The  nitrometer  may  be  used  for  this  estimation. 

This  instrument  is  charged  with  a  concentrated  solution  of  sodium 
sulphate  (which  in  this  case  is  better  than  brine),  and  i  cc.  of  the 
dioxid  introduced  from  the  funnel,  followed  by  excess  of  solution  of 
permanganate  acidulated  with  sulphuric  acid. 

This  latter  solution  should  be  of  such  strength  that  when  the 
reaction  is  completed,  the  solution  should  still  have  a  purple  color. 

The  reaction  is  thus  illustrated: 


By  the  use  of  Squibb1  s  Urea  Apparatus  (Fig.  100)  the  estimation 
may  be  easily  and  rapidly  made. 

Into  the  generating-bottle  is  put  about  30  cc.  of  a  strong,  acidu- 
lated solution  of  potassium  permanganate,  and  a  small  test-tube  con- 
taining i  cc.  of  H2C>2  is  carefully  introduced.  The  two  liquids  must 
not  be  allowed  to  come  in  contact. 

The  larger  flask  is  filled  with  water  or,  better,  a  solution  of  sodium 
sulphate;  the  connection  is  then  made  by  means  of  the  rubber  tube, 
and  the  generating-bottle  tipped  over  and  agitated  so  that  the  liquids 
will  mix  and  the  reaction  take  place. 

The  liberated  oxygen  then  passes  into  the  larger  bottle,  displacing 
an  equal  volume  of  water,  which  is  collected  and  measured.  Half 
of  this  volume  represents  the  volume  strength  of  the  H2O2. 

An  Improvised  Nitrometer  may  be  used.  The  author  has  found 
the  following  instrument  convenient: 

687 


A   MANUAL  OF   VOLUMETRIC  ANALYSIS 

To  the  bottom  of  an  ordinary  50-0:  burette  is  attached  a  suitable 
length  of  rubber  tubing,  to  the  other  end  of  which  is  attached  another 
burette  or  ungraduated  tube,  which  serves  as  a  control -tube. 

Into  the  top  of  the  burette  is  fitted  a  rubber  stopper,  through  which 
passes  a  short  glass  tube,  which  is  connected  by  means  of  a  rubber 
tube  to  a  generating  bottle  similar  to  that  used  with  Squibb' s  Urea 
Apparatus.  Into  the  control-tube  is  poured  the  solution  of  sodium 
sulphate,  sufficient  to  fill  the  burette  to  the  zero  mark  and  have  the 
surface  of  the  liquid  in  both  tubes  on  a  level. 

About  30  cc.  of  strong  permanganate  solution  acidulated  with  sul- 
phuric acid  are  now  placed  in  the  gen  era  ting -bottle,  and  then  \he 
small  test-tube  or  homeopathic  vial,  containing  exactly  i  cc.  of  hydrogen 
dioxid,  is  placed  in.  The  generat ing-bottle  is  then  stoppered  and 
agitated,  the  evolved  gas  passes  over,  and  forces  down  the  liquid 
in  the  burette;  the  control-tube  is  then  lowered  so  as  to  bring  the 
surfaces  of  the  liquid  in  both  tubes  on  a  level. 

The  reading  is  then  taken. 

Each  cc.  of  gas  represents  one  half  volume  of  oxygen  evolved  from 
the  dioxid  if  i  cc.  of  the  latter  is  used.  Each  cc.  of  oxygen  evolved 
from  i  cc.  of  the  dioxid  represents  also  0.001688  gm.  of  absolute 
H2O2,  or  0.0008  gm.  of  available  oxygen. 

Thus  if  from  i  cc.  of  the  solution  of  hydrogen  dioxid  20  cc.  of 
gas  are  evolved,  it  is  a  so-called  ten -volume  solution,  and  contains 
0.001688X20=0.03376  gm.  of  absolute  H2O2,  or  0.0008X20=0.016 
gm.  of  available  oxygen. 

According  to  Naylor  and  Dyer  (Trans.  Brit.  Ph.  Conf.,  1901,  339) 
the  gasometric  permanganate  method  is  unreliable,  because  under  the 
conditions  of  the  test  sulphuric  acid  added  to  the  brine  solution  in 
the  nitrometer  naturally  liberates  a  little  hydrochloric  acid,  and  this 
in  the  presence  of  permanganate  becomes  to  some  extent  decom- 
posed into  chlorin.  It  is  the  uncertainty  as  to  the  extent  to  which 
the  chlorin  is  absorbed  by  the  water,  which  renders  the  accuracy  of 
the  method  doubtful.  The  results  of  this  method  are  uniformly  too 
high,  whether  the  gas  be  collected  over  mercury,  over  saturated  mag- 
nesium sulphate,  or  over  brine,  and  in  the  latter  case  considerably 
higher.  But  when  the  dichromate  V.  S.  is  used  (without  acid),  closely 
concordant  results  are  obtained,  whether  the  gas  be  collected  over 
mercury,  or  the  other  solutions.  The  evolution  of  oxygen  by  the 
latter  method  is  slower  than  when  permanganate  is  used,  but  the 
oxygen  obtained  represents  the  volume  available  in  the  sample. 

In  the  Hypochlorite  Method,  the  nitrometer  is  filled  with  a  saturated 
solution  of  sodium  chlorid.  2  cc.  of  the  hydrogen  dioxid  are  admitted 
into  the  measuring  tube,  the  funnel  tube  filled  with  a  little  water, 
and  this  also  let  in,  then  20  cc.  of  the  chlorinated  lime  solution  intro- 


HYDROGEN  D 1 OX  ID 


689 


duced.     From  this  point  the  procedure  is  the  same  as  in  the  gasometric 
permanganate  method. 

Ca(ClO)2+2H2O2=O4+CaCl2+2H2O. 

The   presence  of  preservatives,  except   inorganic  ones,  gives  low 
results. 

The  Hypobromite  Method.  W.  M.  Dehn  (J.  A.  C.  S.,  xxix  (9), 
1315)  describes  an  accurate  and  rapid  determination  of  hydrogen 
dioxid  by  means  of  sodium  hypobromite,  using  a 
ureometer.  The  reaction  involved  is 

H2O2+NaBrO  =  NaBr+H2O-r-O2. 

The  apparatus  is  shown  in  Fig.  96.  The  follow- 
ing description  of  the  method  is  by  Dehn  from  the 
Journal  of  the  American  Chemical  Society.  "The 
stop-cock  E  is  opened  and  the  stop-cocks  D  and  F 
are  closed;  then  the  solution  of  sodium  hypobromite* 
is  poured  in  at  the  top  of  C  until  it  fills  the  tubes 
A  and  C  to  some  point  above  the  stop-cock  E. 

"The  stop-cock  E  is  then  closed  and  the  stop-  H 
cock  F  is  opened,  so  that  the  hypobromite  in  C  may 
run  down  to  the  constricted  portion ;  the  solution  in 
A  is  then  sustained  by  atmospheric  pressure.  The 
stopcock  D  (arranged  so  as  to  deliver  only  in  the 
two  directions  of  a  right  angle  triangle)  is  turned 
from  the  position  shown  in  the  figure  and  is  so 
controlled  that  B  may  first  be  washed  with  a  little 
of  the  hydrogen  peroxid  and  then  be  filled  with  the 
same  to  a  readable  height  on  the  scale.  Upon 
turning  D  so  as  to  admit  a  regulated  volume 
of  the  hydrogen  peroxid  solution  an  immediate  evaluation  of  oxygen 
results.  After  admitting  most  of  the  hypobromite  held  above  E,  and 
letting  stand  for  a  minute  or  two  so  as  to  drain  properly,  the  columns 
of  hypobromite  in  A  and  C  are  brought  to  the  same  level,  the  volume 
of  oxygen  is  read  off  and  its  weight  and  that  of  the  corresponding 
hydrogen  dioxid  are  calculated  by  the  usual  formulas." 

The  author  of  this  method  also  gives  the  following  table,  by  means 
of  which  the  cc.  of  oxygen,  under  various  conditions  of  temperature 


FIG.  96. 


*  This  solution  is  prepared  as  directed  under  Estimation  of  Urea,  except  that  it 
is  finally  diluted  with  an  equal  volume  of  water. 


690 


A    MANUAL   OF   VOLUMETRIC   ANALYSIS 


and  pressure,  may  be  calculated  into  milligrams  of  hydrogen  dioxid, 
and  claims  that  by  the  use  of  this  instrument,  this  hypobromite  method 
and  the  table  for  calculating  the  assay  of  hydrogen  dioxid,  is  not  only 
rapid  and  accurate  but  the  necessity  of  preparing  and  correcting 
standard  solutions  is  avoided  and  the  presence  of  the  usual  preserva- 
tives used  in  the  dioxid  solution  may  be  ignored. 

WEIGHT   IN   MILLIGRAMS    OF   H2O2    CORRESPONDING    TO    ONE 
CUBIC  CENTIMETER  OF  MOIST  OXYGEN 


//mm. 

728 

732 

736 

740 

744 

748 

4° 

.2664 

I  2734 

.2802 

[.2872 

.2942 

.3011 

8 

.2463 

1-2531 

2600 

.2669 

2736 

.2805 

12 

.2251 

.2317 

.2387 

.2454 

.2522 

.2589 

16 

.2044 

\2III 

.2178 

3 

.2245 

.2311 

.2378 

20 

.1817 

.1884 

.1948 

.2015 

.2080 

•2145 

24 

1583 

.1649 

.1719 

.1777 

.1843 

.1907 

28 

•1345 

.1411 

.1476 

^538 

.1603 

.1665 

32 

.1085 

1149 

1 

-1213 

-1275 

^338 

.1401 

36 

.0843 

.0905 

.0967 

.1030 

.1093 

•"55 

40 

] 

[,0605 

.0666 

.0725 

3 

.0786 

.0849 

.0909 

t 
/r 

nm 

752 

756 

760 

764 

768 

4° 

.3081 

[.3151 

.3222 

.3290 

3 

r-3359 

8 

.2876 

r.2944 

.3014 

.3081 

-3i50 

I 

2 

.2657 

2726 

.2823 

,2860 

.2928 

I 

6 

2444 

2512 

.2578 

.2946 

.2713 

2 

o 

.2213 

„ 

2279 

2345 

.2410 

.2475 

2 

4 

.1972 

.2036 

3 

.2100 

.2169 

.2230 

a 

8 

-I73I 

.1796 

-3 

.1857 

.1922 

.1986 

3 

2 

.1465 

.1528 

.1589 

.1562 

.1715 

3 

6 

.1214 

.1279 

3 

-I34I 

" 

.1402 

.1465 

A 

0 

.0971 

•i°33 

S3 

.1094 

-II55 

.1216 

CHAPTER  LXIV 


ESTIMATION  OF  SOLUBLE  CARBONATES  BY  THE  USE  OF  THE 

NITROMETER 

THE  nitrometer  may  be  used  for  estimating  ammonium  carbonate 
in  aromatic  spirit  of  ammonia. 

The  nitrometer  in  this  case  must  be  charged  with  mercury,  as 
the  liberated  CO2  is  soluble  in  aqueous  liquids. 

A  given  volume  of  the  spirit  is  introduced  into  the  nitrometer 
followed  by  an  excess  of  dilute  HC1,  and  the  evolved  gas  then  read 
off;  and  from  its  quantity  the  proportion  of  ammonium  carbonate 
may  be  calculated  by  applying  the  equation 

(NH4)2C03+  2HC1= 2NH4C1+H20+ CO2. 

*96  *44 


The  volume  of  gas  liberated  must  first  be  reduced  to  its  corre- 
sponding volume  at  o°  C. 

Each  cc.  of  CC>2  at  o°  C.  weighs  0.001966  gm.  Now  if  44  gms. 
of  CC>2  represent  96  gms.  of  normal  ammonium  carbonate,  how  much 
ammonium  carbonate  does  0.001966  gm.  of  CC>2  represent? 

44:96: :  0.001966  :#.  #=0.004289  gm. 

Thus  each  cc.  of  CC>2  at  normal  pressure  and  o°  C.  represents 
0.004289  gm.  of  (NH4)2CC>3,  approximately. 

*  The  atomic  weights  are  approximate. 

691 


CHAPTER  LXV 
ESTIMATION  OF  UREA  AND  URIC  ACID  IN  URINE 

THE  determination  of  urea  is  based  upon  the  fact  that  when  urea 
is  decomposed  by  an  alkaline  hypochlorite  or  hypobromite,  carbon 
dioxid  and  nitrogen  are  given  off,  as  the  equation  shows: 


The  liberated  N  may  be  measured,  and  from  its  quantity  the  quantity 
of  urea  calculated;  the  other  products  of  the  decomposition  go  into 
solution. 

The  hypobromite  solution  is  prepared  as  follows:  100  gms.  NaOH 
are  dissolved  in  250  cc.  of  water,  and  when  this  solution  has  become 
cold  25  cc.  of  bromin  are  added,  and  the  solution  kept  cold.  This 
solution  contains  sodium  hypobromite,  bromate,  and  hydroxid.  It 
readily  undergoes  decomposition,  and  should  therefore  always  be 
freshly  prepared  when  wanted  for  use.  To  15  cc.  of  the  NaOH  solu- 
tion add  i  cc.  of  bromin. 

The  solution  of  sodium  hypochlorite  is  generally  preferred  to  the 
hypobromite,  because  it  is  more  stable,  just  as  efficacious,  and  the 
disagreeable  handling  of  bromin  is  obviated. 

Various  forms  of  apparatus  have  been  devised  for  the  quantitative 
estimation  of  urea. 

The  simplest  of  these  is  probably  the  one  devised  by  Dr.  Chas.  A. 
Doremus.  (See  Fig.  97.) 

The  long  arm  of  the  ureometer  is  filled  with  the  hypobromite  solu- 
tion, and  then  i  cc.  of  the  urine  is  introduced  by  the  aid  of  the  pipette. 
The  pipette  is  introduced  through  the  bulb  as  far  as  it  will  go  in  the 
bend,  and  the  nipple  is  then  gently  but  steadily  compressed,  being 
careful  that  no  air  is  admitted. 

The  volume  of  the  liberated  gas  is  read  off  after  the  froth  has  sub- 
sided. 

The  ureometer  indicates,  according  to  its  graduation,  either  milli- 
grams of  urea  in  i  cc.  or  grains  of  urea  per  fluid  ounce  of  urine. 

692 


ESTIMATION  OF   UREA   AND    URIC  ACID  IN   URINE    693 


It  also  indicates  by  the  signs  -f,  N,  and  —  whether  the  urea  is 
present  in  an  increased,  normal,  or  decreased  quantity. 

Either  Knop's  or  Squibb's  solution,  or  Liquor  Sodae  Chloratae 
U.  S.  P.  may  be  used  in  this  instrument.  Knop's  solution  is  that 
described  above.  Squibb's  solution  contains  potassium  bromid  as 
well  as  bromin.  It  is  prepared  by  taking  an  equal  weight  of  bromin 
and  of  potassium  bromid,  and  adding  eight  times  as  many  cc.  of  water 
as  there  were  grams  of  bromin  taken.  For  use  mix  equal  volumes 
of  this  solution  with  the  sodium  hydroxid  solution  above  described. 


FIG.  97. 


FIG.  98. 


The  Hinds-Doremus  Ureometer.  This  apparatus,  which  is  shown 
in  Fig.  98,  is  capable  of  giving  more  exact  results  than  the  original 
apparatus,  because  the  i  cc.  of  urine  can  be  delivered  more  accu- 
rately. It  consists  of  a  bulb  with  an  upright  tube  a,  graduated  like 
the  original,  so  that  each  of  the  smallest  divisions  represents  o.ooi  gm. 
of  urea  in  the  urine  used.  The  lower  portion  of  this  tube  is  in  con- 
nection with  a  smaller  tube  c,  graduated  with  a  capacity  of  2  cc.; 
between  these  tubes  a  glass  stop-cock  is  situated.  Closing  the  stop- 
cock b,  Knop's  or  Squibb's  fluid  (diluted  one  half)  or  liquor  sodae 
chlorate  U.  S.  P.  are  introduced  into  tube  a  so  as  to  completely  fill 
it.  The  apparatus  is  then  placed  in  an  upright  position  and  the 
smaller  tube  c  is  filled  to  the  zero  mark  with  urine.  The  stop-cock 
is  then  turned  slowly  so  as  to  admit  gradually  i  cc.  of  the  urine  to 
tube  a.  After  fifteen  minutes  the  reading  is  taken.  If  the  reading 
be  0.015  and  the  amount  of  urine  taken  was  i  cc.,  then  multiplying 
by  100  gives  1.5  per  cent  of  urea. 


694  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

Another  Convenient  Form  of  Apparatus  is  a  tube  closed  at  one 
end  and  graduated  so  that  each  division  indicates  a  grain  of  urea  in 
a  fluid  ounce  of  urine,  when  i  cc.  of  urine  is  taken  for  the  estimation. 
(See  Fig.  99.) 

The  process  is  conducted  as  follows:  A  25  per  cent  solution 
of  KBr  is  introduced  to  the  fifth  division.  The  chlorinated 
soda  solution  is  then  added  to  the  fifteenth  or  twentieth 
division.  The  tube  is  now  inclined,  and  pure  water  care- 
fully poured  upon  the  liquid  so  that  it  will  float  on  top; 
i  cc.  of  urine  is  then  added  carefully,  so  that  it  will  not  mix 
with  the  reagents  below,  but  remain  in  the  water  at  the 
surface  of  the  fluid.  The  open  end  of  the  tube  is  then 
quickly  closed  with  the  thumb,  and  the  top  firmly  grasped 
in  the  right  hand.  The  tube  is  then  inverted,  and  the 
contents  well  mixed.  The  decomposition  which  takes  place 
is  usually  ended  in  five  minutes.  As  soon  as  the  effer- 
vescence has  ceased,  the  reading  is  taken  at  the  surface 
of  the  liquid.  The  tube  is  now  opened  under  water,  when 
the  column  of  fluid  in  the  tube  will  fall;  the  reading  is 
then  again  taken.  The  difference  between  the  two  read- 
ings gives  the  number  of  grains  of  urea  in  a  fluid  ounce 
FIG-  99-  of  the  urine. 

SquiWs  Urea  Apparatus  (Fig.  100)  is  a  very  simple  apparatus, 
and  can  be  easily  improvised  in  a  drug  store.  It  consists  of  two  wide- 
mouthed  bottles,  the  larger  of  which,  C,  capable  of  holding  about 
250  cc.,  is  fitted  with  a  rubber  stopper,  through  which  is  passed  sa 
curved  delivery-tube  and  a  short  straight  tube,  the  latter  connected 
by  a  piece  of  rubber  tubing  to  the  short  glass  tube  in  the  rubber 


FIG.  100. 


stopper  of  the  smaller  bottle  or  generating-bottle  B.     In  the  generating- 
bottle  is  a  small  test-tube  A. 

Into  the  test-tube  A  is  placed  5  cc.  of  urine,  and  into  the  smaller 
bottle  B  is  put  20  cc.  of  the  hypobromite  solution,  or  strong  liquor 
sodae  chloratae.  The  test-tube  is  then  placed  in  the  generating- 


ESTIMATION  OF   UREA   AND   URIC  ACID  IN   URINE    695 

bottle  B,  being  careful  that  the  urine  and  the  reagent  do  not  come  in 
contact.  The  larger  bottle  C  is  now  filled  with  water  and  the  two 
bottles  connected  by  the  rubber  tube,  the  larger  bottle  being  placed 
on  its  side  upon  a  block,  and  when  all  connections  are  tight,  the 
generating-bottle  is  shaken  so  that  the  urine  will  mix  with  the  reagent. 

Decomposition  takes  place,  and  the  generated  gas  passes  into  the 
bottle  C,  displacing  water,  which  is  caught  in  a  graduated  cylinder 
or  other  measuring  vessel.  The  volume  of  water  displaced  is  equiv- 
alent to  the  volume  of  gas  evolved. 

Each  cc.  of  nitrogen  gas  evolved  at  o°  C.  and  normal  pressure 
represents  0.0027  gm.  of  urea.  Then  by  multiplying  the  number  of 
cc.  evolved  by  this  number  the  quantity  of  urea  in  the  5  cc.  of  urine 
taken  is  ascertained. 

The  volume  of  gas  obtained  when  the  operation  is  conducted  at 
ordinary  temperatures  should  always  be  reduced  to  its  corresponding 
volume  at  o°  C.  and  760  mm. 

The  factor  0.0027  is  found  in  the  following  manner: 

1000  cc.  of  H  at  o°  C.  weigh  0.0896  gm.; 
1000  cc.  of  N  at  o°  C.  weigh  1.248  gms. 

By  the  equation  it  is  seen  that  59.65  gms.  of  urea  evolve  when 
decomposed  27.96  gms.  of  N. 


59.65  gms.  27.96  gms. 

Now  we  will  find  the  volume  occupied  by  28  gms.  of  N  at  o°  C. 
1.248  gms.  of  N=iooo  cc 

gms.       cc.        gms.  cc. 

i.  248:1000;  128:  x.  #=23237 

Thus  59.65  gms.  of  urea  evolve  23237  cc.  of  N;  i  cc.  of  N  thus 
represents  0.0027  gm-  °f  urea- 

By  Means  of  Sellier's  Urometer.*  In  order  to  obtain  reliable 
results  in  urea  determinations  according  to  Sellier,  it  is  necessary  to 
operate  upon  the  urine  clarified  by  means  of  lead  subacetate,  and 
with  freshly  prepared  bromin  solution,  in  the  presence  of  glucose. 

*Ph.  Ztg.,  48,  No.  62  (August  5,  1903),  627;  from  Chem.  Centralbl.,. 
1903,  II,  No.  4. 


696  A    MANUAL    OF    VOLUMETRIC    ANALYSIS 

The  bromin  solution  is  prepared  according  to  Moreigne's  formula:  100 
cc.  of  soda  solution  36  B.,  70  cc.  of  water,  and  10  cc.  of  bromin.  The 
reaction  is  conveniently  carried  out  by  the  aid  of  the  urometer 
(Fig.  101).  The  bulb  is  divided  by  a  central  partition  m  n 
into  separate  chambers  A  and  B,  communicating  with 
each  other,  and  with  the  graduated  tube  C  D,  at  m. 
In  use,  2  cc.  of  the  clarified  urine  and  i  cc.  of  25  per 
cent  glucose  solution  are  allowed  to  flow  along  the  wall 
into  A,  and  15  cc.  of  the  bromin  solution  in  the  same  way 
into  B.  The  stopper  is  inserted,  the  apparatus  placed 
into  water,  until  both  have  the  same  temperature,  and 
water  is  then  admitted  into  the  tube  up  to  the  o°  mark, 
by  lifting  the  stopper,  and  adjusted  to  the  level  of  the 
water  of  immersion.  The  reacting  materials  in  A  and 
B  are  then  mixed,  and  the  tube  raised  so  that  the  in- 
terior and  the  outer  level  of  the  water  shall  be  identical, 
observing  also  that  the  temperature  has  been  readjusted, 
FIG*IOI  wnen  the  volume  of  nitrogen  may  be  accurately  read  in 

cc.  values. 

Uric  Acid.  A.  F.  Dimmock  and  F.  W.  Branson  *  have  devised 
a  new  method  for  the  determination  of  uric  acid  in  urine,  which  has 
been  found  to  work  easily.  The  urine  (100  cc.)  is  warmed  to  about 
40°  C.,  and  then  saturated  with  ammonium  chlorid  (31  gms.),  the 
whole  being  well  shaken  in  a  graduated  flask  until  complete  solution 
of  the  ammonium  chlorid  is  effected  and  then  left  for  from  two  to  twelve 
hours  (preferably  the  latter)  for  the  complete  precipitation  and  sub- 
sidence of  the  ammonium  urate  formed.  The  supernatant  liquid 
is  decanted,  the  residual  ammonium  urate  collected  on  a  small  paper 
filter  (about  5.5  cm.  in  diameter)  and  carefully  washed  with  a  very 
dilute  solution  of  ammonia,  consisting  of  one  part  of  liq.  ammon.  fort, 
in  1000  parts  of  distilled  water,  until  the  filtrate  yields  only  a  slight 
precipitate  with  a  5  per  cent  solution  of  silver  nitrate  acidulated  with 
5  per  cent  of  nitric  acid — this  indicating  the  absence  of  an  appreciable 
amount  of  ammonium  chlorid.  The  precipitate,  with  the  filter-paper, 
is  then  placed  into  the  generating-bottle  shown  in  the  accompanying 
illustration  (Fig.  102)  of  the  nitrometer,  a  tube  containing  25  cc.  of 
hypobromite  solution  (100  gms.  sodium  hydrate,  250  cc.  water,  and 
22  cc.  bromin)  is  lowered  into  the  same  bottle  by  means  of  a  string, 
and,  having  immersed  the  latter  in  a  vessel  of  water  having  the  tem- 
perature of  the  room,  the  connection  may,  after  two  minutes  or  so,  be 
made  with  the  nitrometer;  the  reaction  being  established  by  tilting 

*  Trans.  Brit.  Pharm.  Conf.,  1903,  439. 


ESTIMATION  OF   UREA   AND    URIC   ACID   IN   URINE    697 

the  generating-bottle  so  that  the  reagent   may  flow  from  the  tube. 
The  operation  must,  of  course,  be  conducted  with  the  usual  precau- 


FlG.   IO2. 


tions  against  leakage  of  apparatus,  etc.,  which  are  explained  in  some 
detail.  The  method  is  available  for  urine  containing  from  I  in  1000 
to  i  in  10,000  uric  acid. 


INDEX 


PAGE 

Abbreviations .- ix 

Acetate  of  lime,  valuation  of 238 

Acetates,  metallic,  estimation  of 238 

Acetic  acid 104 

and  acetates 235 

table 236 

Acid,  acetic 104 

and  acetates 235 

Acid,  arsenous,  preparation  of  decinormal  solution  of 227 

Acid,  boric,  estimation  of 104,  240 

- —  carbolic,  assay  of 613 

—  carbonic  and  carbonates 244 

—  carbolic,  assay  of,  in  dressings 649,  651 

—  chloric,  iodometric  estimation  of . 301 

—  chromic 174 

estimation  of,  by  digestion  method 222 

estimation  of,  by  distillation  method 219 

—  citric 105 

—  hydrobromic 1 20 

assay  of  using  chromate  as  indicator 121 

—  hydrochloric 100,  1 20 

half  normal,  V.  S 70 

normal 65 

V.  S.  gravimetric  standardization  by  means  ot  silver  nitrate 68 

standardization  of 65 

standardization  by  means  of  borax 66 

standardization  by  means  of  calc-spar 69 

standardization  by  means  of  marble 70 

standardization  by  means  of  silver  nitrate  volumetrically 67 

standardization  of  by  means  of  specific  gravity 66 

—  hydrocyanic  assay,  using  chromate  as  indicator 127 

use  of  Porrier  Blue  as  indicator 127 

using  potassium  iodid  as  indicator 1 28 

estimation  by  Liebig's  method T  26 

—  hydriodic 1 20 

assay  of  by  sulphocyanate 1 23 

syrup  of 1 24 

699 


700  INDEX 

PAGE 

Acid,  hydrosulphuric,  estimation  of  by  permanganate 328 

estimation  of  by  means  of  iodin 3  29 

—  hypophosphorous  estimation 103,  167 

—  lactic 1 06 

—  nitric 103 

iodometric  estimation  of 301 

nitrometer  assay  of 686 

—  nitrous,  estimation  of,  in  chamber  acid 303 

estimation  of  by  means  of  permanganate 165,  302 

iodometric  estimation  of '. 303 

—  oxalic. 105 

estimation  with  permanganate 1 153 

normal 65 

—  phenylsulphate  solution 444 

—  phosphoric f 101 

estimation  of,  by  Gliickmann's  method 315 

—  estimation  of,  by  Pemberton's  new  method 317 

estimation  of,  by  Pemberton's  molybdic  method 316 

estimation  by  means  of  standard  uranium 312 

—  Stolba's  method 311 

—  salicylic 3  24 

assay  of,  in  dressings 651 

estimation  of,  by  means  of  bromin 325 

iodometric  estimation  of 324 

—  solution,  preparation  of  standard 64 

—  sulphanilic » 445 

—  sulphonic. 445 

—  sulphuric,  estimation  of 101,  331 

—  V.  S.,  gravimetric  standardization  by  means  of  barium  chlorid 73 

iodometric  standardization 71 

standardization  by  ammonium  sulphate  method 72 

standardization  by  means  of  sodium  carbonate 70 

standardization  by  means  of  sp.gr 73 

—  sulphurous,  estimation  of 197,  331 

—  tannic,  estimation  of 630 

—  tartaric 105 

—  uric,  in  urine 692,  696 

—  value  of  fats  and  oils 472 

Acidimetry 62,  94 

Acids,  estimation  of,  in  neutral  salts 1 08 

—  estimation  of,  by  neutralization 94 

—  haloid 1 20 

—  inorganic 99 

—  organic 104 

Aconite  extract,  assay  of 588 

—  fluid  extract,  assay  of 592 

—  root  assay  of 520 


INDEX  701 

PAGE 

Aconite  tincture,  assay  of. . . 588,  593 

Adams'  method  for  fat  in  milk 461 

Albumen  in  urine 672 

Albuminoid  ammonia,  estimation  of  in  water 441 

Alizarin 19 

Alkali  bicarbonate  and  carbonate  mixed 82 

—  carbonates 77 

—  earth,  hydroxids  and  carbonates,  mixed 93 

—  earths 91 

—  hydroxid  and  carbonate  mixed 8  r 

—  hydroxids,  estimation  of 74 

—  metals,  estimation  of,  in  their  salts 89 

—  standard  solutions 96 

—  V.  S.  containers  for 95 

Alkalies,  in  presence  of  sulphites 82 

—  organic  salts  of 83 

Alkalimetry 62,  64 

Alkaloidal  assay  by  immiscible  solvents 512 

new  method. 505 

—  salts,  general  method  of  procedure  in  estimating 507 

solubility  of  in  chloroform,  etc 514 

titration  of 503 

—  strength  of  scale  salts,  estimation  of 612 

Alkaloids,  behavior  of,  with  indicators 503 

—  estimation  of „ 498 

—  estimation  of  by  Gordon's  modified  alkalimetric  method 510 

—  estimation  of  by  Mayer's  reagent 505 

—  estimation  of  by  Wagner's  reagent 507 

—  extraction  of  from  drugs 512,  520 

Alum 335 

—  estimation  of  by  iodometric  method 337 

—  estimation  by  titration  with  barium  hydroxid 336 

Aluminum 335 

Ammonia 133 

—  albuminoid,  in  water 441 

—  aromatic  spirit  of 691 

—  estimation  of 338 

—  estimation  of,  in  water 440 

—  water 76 

Ammoniated  mercury,  assay  of 416 

Ammonium  bromid 116 

—  determination  of  the  amount  of  chlorid  present. 117 

—  carbonate 79 

—  chlorid 120 

estimation  of  in  ammonium  bromid 117 

—  molybdate  solution 447 

—  salts , 133 


702  INDEX 

PAGB 

Ammonium  salts,  estimation  of  by  distillation 338 

estimation  of  by  means  of  formaldehyde 339 

indirect  estimation  of 339 

Amyl  nitrite,  nitrometer  assay  of 685 

assay  of 659 

Analysis  by  neutralization 62 

—  by  oxidation  and  reduction 137 

—  by  precipitation iu 

—  statement  of 451 

Anthracene  Violet 20 

Antimonic  acid  and  its  salts 344 

Antimonous  chlorid,  estimation  of 341 

—  oxid,  estimation  of 195 

—  sulphid,  estimation  of 340 

estimation  of  by  ferric  sulphate  and  permanganate 344 

Antimony  and  potassium  tartrate,  estimation  of 196 

—  compounds,  estimation  of 195 

—  estimation  of  by  iodin 340 

—  estimation  of  by  means  of  dichromate  or  permanganate 342 

—  estimation  of  by  titration  with  standard  bromate  solution 341 

—  metallic,  estimation  of 340 

Apparatus,  on  the  use  of.  . 44 

—  used  in  volumetric  analyses 32 

Aromatic  spirit  of  ammonia 691 

Arsenates,  estimation  of  by  means  of  magnesia  mixture 348 

Arsenic  estimation  of  by  dichromate 346 

—  estimation  of  by  means  of  iodate.  . 347 

—  estimation  of  in  Paris  green.  Smith's  method 350 

—  estimation  of  in  small  quantities 349 

—  in  Paris  green,  estimation  of  Avery  and  Beans'  method 351 

—  oxid,  estimation  of,  after  reduction 348 

estimation  of  by  means  of  magnesia  mixture 348 

estimation  of  by  means  of  uranium 347 

Arsenous  acid,  estimation  of 1 78 

preparation  of  decinormal  solution  of 227 

—  compounds,  estimation  of 192 

—  iodid,  estimation  of 194 

—  oxid,  estimation  of 192 

—  —  estimation  of  by  distillation  with  chromic  and  hydrochloric  acids 348 

Assaying  drugs,  general  methods  of 515 

Aurin 28 

Automatic  burette 36 

Azolitmin 19 

Babcock  centrifugal  method 462 

Barium  compounds,  estimation  of 353 

—  dioxid,  estimation  with  permanganate 156,  161 


INDEX  703 

PAGB 
Barium,  estimation  of 1 78 

—  estimation  of,  by  means  of  dichromate 353 

—  estimation  of,  by  indirect  iodometric  method 354 

—  hydroxid  solution,  for  estimation  of  starch 484 

—  peroxid  estimation,  with  permanganate 161 

Baryta-water 484 

Beale's  filter 506 

Belladonna  extract,  assay  of , 589,  593 

—  fluid  extract,  assay  of 594,  596 

—  leaves,  assay  of 528 

—  plaster,  assay  of 598 

—  root,  assay  of 527 

—  tincture,  assay  of 589,  594 

Bicarbonate  of  sodium,  its  use  in  titrations  with  iodin 190 

Bichlorid  dressings,  assay  of 651 

Bink's  burette 33 

Bismuth,  estimation  of  by  precipitation  as  chromate 355 

—  estimation  of  by  precipitation  as  molybdate 357 

—  estimation  of  by  precipitation  as  oxalate 355 

—  estimation  of  by  precipitation  as  phosphate 356 

—  potassium  iodid  method  for  the  assay  of  galenicals 598 

Bisulphite,  sodium 201 

Bitter  almond  water 128 

Bleaching  powder 210 

estimation  of  by  means  of  arsenous  acid 229 

Blue,  gentian 27 

Blue,  Poirrier 27 

Borates v. .  132 

Borax,  estimation  of 240 

Boric  acid 104 

and  borates,  estimation  of 240 

detection  of  in  milk 466 

dressings,  assay  of 651 

estimation,  with  ferric  salicylate  indicator 241 

Bottle,  for  preserving  stannous  chlorid  V.  S 232 

Boyle's  law 680 

Brazil  wood  T.  S 19 

Bromates,  estimation  of  by  digestion  method 222,  223 

Bromid,  ammonium T  16 

—  lithium 119 

—  sodium ' 119 

—  strontium 119 

—  potassium 119 

—  zinc 119 

Bromids,  direct  titration  of,  with  chlorin  water 265 

—  estimation  of 115 

—  or  chlorids,  estimation  of  in  presence  of  sulphocyanate 268 


704  INDEX 

PAGE 

Bromin  absorption  number 478 

—  methods  of  estimation 260 

—  solution,  decinormal,  preparation  of 613 

—  value  of  fats  and  oils .' 478 

—  water 213 

Bunsen's  distilling  apparatus 215 

Bunsen  valve 143 

Burette,  glass  stop-cock .' 33 

-  The 32 

—  Mohr's 32 

-  Weight 59 

—  with  blue  striped  background 48 

—  with  enameled  sides 48 

Burettes,  various  forms  of 3 2,  39 

Butter  analysis  of 467 

—  estimation  of  volatile  acids  in 467 

Caffein,  estimation  of 509 

Calabar  bean,  assay  of 576 

Calchicum,  assay  of 542 

Calcium  chlorid,  standard  solution  of 449 

—  estimation  of,  by  precipitation  as  oxalate 359 

—  hypophosphate,  estimation 1 69 

—  salts,  estimation  as  oxalate 155 

estimation  with  permanganate 1 70 

Calculating  analyses,  methods  of 53 

Calibration  of  instruments 49 

Calx  chlorinata 210 

Carbon  monoxid  in  the  air 258 

Carbolic  acid,  assay  of 613 

dressing,  assay  of 649 

Carbonates 132 

—  estimation  of  by  the  nitrometer 691 

—  insoluble,  estimation 245 

—  of  the  alkalies,  estimation 244 

Carbonic  acid  and  carbonates 244 

gas 133 

in  the  atmosphere 246 

in  insoluble  carbonates 245 

in  natural  waters 251 

in  solutions  in  water 246 

C4B  (Poirrier  blue) 27 

Chamber  acid,  estimation  of  nitrous  acid  in 303 

Charles'  law 679 

Cherry  laurel  water 1 28 

Chloral  hydrate,  estimation  of 647 

Chlorates 133 


INDEX  705 

PAGE 

Chlorates,  bromates  and  iodates 262 

—  estimation  of  by  digestion  method 222,  223 

—  estimation  of  by  means  of  standard  potassium  iodate 264 

Chloric  acid,  iodometric  estimation  of 301 

Chlorid,  ammonium 1 20 

Chlorid  of  lime 210 

—  of  sodium 1 16 

Chlorids,  bromids,  and  iodids 260 

estimation  of,  in  the  presence  of  each  other 261 

—  estimation  of 115 

—  in  urine ; 663 

—  or  bromids,  estimation  of,  in  presence  of  sulphocyanate 268 

Chlorinated  lime 210 

estimation  of,  by  means  of  arsenous  acid 229 

Chlorin 202 

—  methods  of  estimation 260 

—  or  bromin 209 

—  water.  . 209 

estimation  of  by  means  of  arsenous  acid 229 

estimation  of  iodids  and  bromids  with 265 

Chloroform,  estimation  of 646 

Chlorometry 226 

Chromate 208 

—  potassium  T.  S 27 

Chromates 203 

Chromic  acid  and  chromates 174 

estimation  of  by  distillation  method 219 

estimation  of  by  digestion  method 222 

Chromophoric  theory  of  indicators 14 

Cinchona  bark,  assay  of "  531 

—  extract,  assay  of 589 

—  fluid  extract,  assay  of 600 

—  tincture,  assay  of 589 

Citrates  of  the  alkalies  and  alkali  earths 270 

Citric  acid 105 

and  citrates 270 

Cobaltic  oxid,  estimation  of  by  distillation  method 219 

Coca  fluid  extract,  assay  of 601 

—  leaves,  assay  of 537 

Cochineal  T.  S ,. 19 

Colostrum 458 

Compound  ethers,  estimation  of 656 

Congo-red 19 

Conium,  assay  of 544 

Copper,  acidimetric  estimation  of,  after  precipitation  with  cuprous  thiocyanate  372 

—  estimation  of,  as  sulphid 362 

—  estimation  of  (Fleitmann) 177 


706  INDEX 

PAGE 

Copper,  estimation  of  by  means  of  ferrocyanid 375 

—  estimation  of  by  means  of  potassium  cyanid 364 

—  estimation  of  by  means  of  stannous  chlorid 376 

—  estimation  of  by  means  of  thiocyanate 371 

—  estimation  of  by  means  of  thiocyanate  and  permanganate 373 

—  estimation  of  by  precipitation  as  cuprous  oxid 362 

—  estimation  of  by  precipitation  as  metallic  copper 363 

—  iodometric  estimation  of 366 

—  ores,  assay  of  by  Dulin's  process 366 

assay  of  by  Low's  process 369 

assay  of  by  Meade's  process 374 

assay  of  by  Parr's  process 375 

assay  of  by  Steinbeck's  process 365 

—  containing  iron,  assay  of 368 

—  iodometric  assay  of.    367 

—  solution,  estimation  of  cyanids  with 275 

Corallin 20,  28 

Cotton,  styptic,  assay  of 655 

Curcuma 29 

Cyanid,  potassium 1 29 

Cyanids 202 

—  assay  of  insoluble. 277 

—  estimation  of  by  modified  Kjeldahl  process 276 

—  estimation  of  with  copper  solution 275 

—  estimation  of  with  mercuric  chlorid 275 

—  titration  of  with  iodin  solution 274 

Cyanogen  and  its  compounds 272 

—  estimation  of. 125 

—  estimation  of  by  Liebig's  methodj 125 

Decem 52 

Decimillem 52 

Decinormal  oxalic  acid 65 

Diastase,  use  of  in  estimation  of  starch 487 

Diazo-sulphanilic  acid 24 

Bichromate,  analysis  by  means  of 1 78 

—  estimation  of 1 76 

—  method  of  titrating  with 183 

—  standard  solution  of 1 79 

—  use  of,  for  standardizing  sodium  thiosulphate  V.  S 204 

Digestion  flask 222 

—  methods 222 

Di-methylanilin 24 

Dioxid  of  hydrogen 213 

assay  of  gasometrically 687 

Direct  percentage  estimations 53 

Distillation  apparatus  for  alkali  iodids 220 


INDEX  707 

PACK 

Distillation  methods 214 

Dolomite,  estimation  of,  with  permanganate 156 

Dressings,  surgical,  assaying  of 649 

Drugs,  assaying  of 512,  520 

Eosin 20 

Eau  de  Javelle 212 

Emetine  (Kunz) 499 

Empirical  solutions 9 

End-point,  determination  of  when  using  Fehling's  solution 491 

—  reactions,  precision  in  determining  in  precipitation  analyses n  i 

Erdman's  float 47 

Erythrosin  B 21 

Esters,  estimation  of 656 

Ether,  nitrous,  estimation  of 657 

Ethers,  compound,  estimation  of .  .    656 

Expansion  and  contraction  of  fluids  by  changes  of  temperature 45 

Extract  of  aconite  root,  assay  of 588 

—  of  belladonna,  assay  of 589,  593 

—  of  cinchona,  assay  of 589 

—  of  hydrastis,  assay  of 603 

—  of  hyoscyamus,  assay  of 589 

—  of  nux  vomica,  assay  of 590,  605 

—  of  opium,  assay  of 608 

—  of  physostigma,  assay  of 610 

—  assay  of 584,  587,  592 

Factor,  how  to  find 56 

Factors  for  calculating  analyses 55 

Fat,  estimation  of  in  milk 459 

—  in  milk,  Adams'  method 461 

Babcock  centrifugal  method 462 

Werner-Schmid  method ; 462 

Fats,  acid  value  of 472 

—  and  oils,  bromin  value  of 478 

Hanus'  iodin  absorption  number 476 

iodin  absorption  number 474 

saponification  value  of 472 

volatile  fatty  acid  value 474 

Wijs'  iodin  absorption  number 477 

—  oils  and  waxes,  examination  of k 472 

Fatty  acids,  determination  of,  in  soap 480 

Fehling's  solution 489 

Ferric  alum  indicator r  23 

—  chlorid,  estimation  of,  by  U.  S.  P.  method 225 

estimation  with  permanganate 164 

—  salts - 203,  208 


708  INDEX 

PAGE 

Ferric  salts,  estimation  of 178 

estimation  of,  after  reduction  by  various  methods 225 

estimation  of  by  digestion  method 222,  224 

estimation  of  by  direct  titration  with  sodium  thiosulphate 380 

estimation  of  by  means  of  permanganate,  after  precipitation  as  ferrous 

sulphid 382 

estimation  of  by  means  of  permanganate,  after  reduction  with  stannous 

chlorid 382 

estimation  of  by  .means  of  stannous  chlorid 231 

estimation  of  by  U.  S.  P.  method 225 

estimation  with  permanganate  (after  reduction)  162 

reduction  of 163 

Ferricyanids 279 

—  potassium  T.  S 27 

—  iodometric  estimation  of 280 

Ferrocyanids 278 

Ferrous  carbonate,  estimation  of  by  dichromate 182 

—  estimation  of,  with  permanganate  V.  S 152 

—  iodid,  syrup  of j  24 

—  salts,  estimation  of 178 

estimation  of,  by  means  of  dichromate 181 

—  sulphate 184 

—  estimation  of,  with  permanganate,  V.  S 150 

Ferrum  reductum,  assay  of,  with  permanganate  V.  S 152 

Float,  Erdman's  and  others 47 

Fluid  extract  of  aconite,  assay  of 592 

of  belladonna,  assay  of 594,  596 

of  cinchona,  assay  of 600 

of  coca,  assay  of 601 

of  gelsemium,  assay  of 601 

of  hydrastis,  assay  of 601 

of  hyoscyanus,  assay  of 597 

of  ipecac,  assay  of 604 

of  nux  vomica,  assay  of 606,  607 

of  pilocarpus,  assay  of 61  T 

of  stramonium,  assay  of 597 

Fluid  extracts,  assay  of 584,  585,   586,   590,   591 

Formaldehyde,  estimation  of .  .    636 

—  detection  of  in  milk 466 

Fowler's  solution,  assay  of 193 

Free  halogens,  estimation  of  by  means  of  arsenous  acid 228 

Fresenius'  distilling  apparatus 216 

Galenical  preparations,  assay  of 584 

—  assay  of,  by  Farr  and  Wright's  method 590 

assay  of,  by  Katz's  method 586 

assay  of,  by  Kippenberger's  method 587 


INDEX  709 

PAGE 

Galenical  preparations,  assay  of,  by  Lloyd's  methods,* 584 

assay  of,  by  Lyons'  method 585 

assay  of,  by  potassium- bismuth  iodid  method 598 

assay  of,  by  Steiglitz's  method 586 

assay  of,  by  Thorn's  method. 598 

assay  of,  by  Thompson's  method 586 

—  assay  of,  by  Webster's  method 590 

Gallein 20 

Gay-Lussac's  burette ' 33 

degrees  for  expressing  the  value  of  chlorinated  lime 212 

Geissler's  burette 34 

Gelsemium,  fluid  extract,  assay  of 601 

General  methods  of  assaying  drugs 515 

—  principles 4 

Gentian  blue 27 

Gladding  method  for  boric  acid 242 

Glass  stop-cock  burette 33 

Glue  and  salt  solution,  preparation  of 631 

Glycerin,  determination  of,  in  soap 482 

—  estimation  of 623 

—  in  fluid  extracts,  estimation  of 628 

—  of  fats,  estimation  of 624 

Glycerol,  estimation  of 623 

Gold,  estimation  of 378 

Gordin's  method 517 

Gordin  and  Prescott's  method 518 

Graduated  cylinder 43 

Grain  system  of  volumetric  analysis. 51 

Grouvelle's  bleaching  fluid.  .  .    , 212 

Guide  for  the  selection  of  indicators 30 

Gunning  method  for  nitrogen 293 

Haberland's  method  for  the  valuation  of  acetate  of  lime.    239 

Halogens,  estimation  of,  by  means  of  arsenous  acid 228 

Haloid  acids 1 20 

—  salts,  estimation  of 115 

—  estimation  of,  by  Mohr's  method 115 

—  estimation  of,  with  chromate  indicator 115 

Hanus  number    476 

Hardness,  determination  of,  in  water 448 

—  of  water,  cause  of.  . 448 

Haematoxylin 20 

Hehner's  method  for  the  estimation  of  free  mineral  acids  in  vinegar 235 

Helianthin 24 

Hesse's  burette 34 

Hiibl's  number 474 

Hyde-powder 6?i 


710  INDEX 

PAGE 

Hydrastis  canadensis,  assay  of 545 

—  extract,  assay  of 603 

—  fluid  extract,  assay  of 602 

—  tincture,  assay  of 603 

Hydrazin,  use  of  in  estimation  of  iodates,  bromates,  and  hvpochlorites 265 

Hydriodic  acid 1 20 

assay  of,  by  the  sulphocyanate  method 1 2  ^ 

syrup  of 124 

Hydrobromic  acid 1 20 

assay,  using  chromate  as  indicator 121 

Hydrochloric  acid 100,  1 20 

effect  of  its  presence  when  titrating  with  permanganate 140 

half  normal  V  S    70 

normal.  .     65 

V  S.,  gravimetric  standardization  by  means  of  silver  nitrate 68 

standardization  by  means  of  borax 66 

standardization  by  means  of  calc-spar 68 

standardization  of,  by  means  of  specific  gravity 66 

volumetric  standardization  by  means  of  silver  nitrate 67 

Hydrocyanic  acid,  assay,  use  of  Poirrier  blue  as  indicator 127 

using  chromate  as  indicator 127 

using  potassium  iodid  as  indicator 1 28 

estimation  of,  by  Liebig's  method 1 26 

Hydrogen  dioxid 213,  309 

assay,  Kingzett's  - 213 

assay  of,  gasometrically 687 

concentration  by  heating 157 

estimation  of  volume  strength 159 

estimation  with  permanganate 156,  158 

iodometric  assay  of.  . 213 

reaction  with  permanganate 158 

"volume"  strength 157 

—  peroxid 309 

assay  of  gasometrically 687 

—  sulphid 202 

estimation  by  means  of  copper  sulphate 329 

estimation  of,  by  means  of  arsenous  acid 330 

estimation  of,  by  means  of  iodin 329 

estimation  of,  by  means  of  silver  nitrate 331 

estimation  of,  by  permanganate 3  20 

Hydroxylamin  as  a  reducing  agent  in  the  estimation  of  chlorates,  etc 26  ^ 

Hyoscyamus  extract,  assay  of 589 

—  fluid  extract,  assay  of ^97 

—  leaves,  assay  of 529 

—  tincture,  assay  of 589,  597 

Hypophosphite  calcium,  estimation 169 

Hypophosphorous  acid. "  . .  . .  103 


INDEX  711 


Hypophosphorous  acid,  and  hypophosphites,  estimation  with  permanganate.  .  167 
Hyposulphite,  sodium  (thiosulphate) 201 

—  solution,  Schiitzenberger's 308 

Imperial  gallon,  system  based  upon 52 

Indicator,  quantity  to  be  used  in  a  titration 18 

—  requirements  of  a  good 18 

Indicators 12 

—  classification  of 17 

Indirect  iodometric  estimations 208 

Indicators,  selection  of 17,  30 

—  theories  of 14 

Indirect  oxidation 185 

Indigo  solution,  preparation  of 631,  636 

Inorganic  acids 99 

Instruments,  calibration  of 49 

—  reading  of 46 

Interpretation  of  results,  in  water  analysis 451 

lodate,  potassium,  titrations  with  standard  solution  of 263 

lodates,  estimation  of,  by  digestion  method 222,  223 

lodeosin 21 

lodid,  estimation  of,  by  means  of  standard  potassium  iodate 264 

—  potassium 117 

—  sodium 1 20 

—  strontium 125 

—  syrup  of  ferrous 1 24 

—  zinc 125 

lodids,  direct  titration  of,  with  chlorin  water 265 

—  estimation  of 115 

—  estimation  of,  by  distillation  method 220 

—  estimation  of,  by  means  of  standard  potassium  bi-iodate 265 

—  estimation  of,  by  means  of  standard  potassium  dichromate 265 

—  estimation  of,  by  means  of  standard  potassium  permanganate 265 

—  estimation  of,  by  mercuric  chlorid  V.  S 134 

lodin  absorption  number  of  fats  and  oils 474 

—  action  of,  as  an  indirect  oxidizer 185 

—  empirical  solution  of 187 

—  free,  estimation  of 207 

estimation  of,  by  means  of  standard  potassium  iodate 264 

—  methods  of  estimation 260 

—  purification  of 186 

—  solution,  estimation  of  cyanids  with 274 

—  V.  S.,  standardization  by  means  of  arsenous  oxid 188 

standardization  of,  by  means  of  thiosulphate 187 

—  standard  solution  of 186 

- —  tincture  of 208 

Iodized  starch  test  paper 208 


712  INDEX 

PAGE 

lodometrie  assay  of  hydrogen  dioxid 213 

—  estimations,  indirect 208 

—  standardization  of  permanganate  V  S 146 

of  sulphuric  acid  V.  S 71 

lodoform,  in  dressings,  assay  of 653 

lodometry 202 

lonization  theory 13,   14,   15 

Ipecac  fluid  extract,  assay  of 604 

—  root,  assay  of 550 

Iron,  estimation  of 379 

—  estimation  of,  by  means  of  stannous  chlorid 231 

—  estimation  of,  in  liquor  ferri  albuminata 385 

—  estimation  of,  in  reduced  iron 383 

—  estimation  of,  metallic,  in  ferrum  reductum 152 

—  in  styptic  cotton,  estimation  of 655 

—  ores,  estimation  of  iron  in 386 

Jaborandi  leaves,  assay  of 577 

Javelle's  water 212 

Jodlbauer's  modification  of  Kjeldahl's  method 290 

Judgment,  passing  of,  in  water  analyses 451 

Kebler's  modification  of  the  Keller  method 517 

Keller's  method 515 

Kingzett's  method  for  the  assay  of  hydrogen  dioxid 213 

Kjeldahl  method,  as  adopted  by  the  A.  O.  A.  C 290 

for  nitrogen 287 

—  process,  estimation  of  cyanids  with 276 

Knapp's  method  for  the  estimation  of  sugar 495 

Knop's  solution  for  urea  estimation 693 

Koppeschaar's  solution 613 

Kottstorfer  number.  , 472 

Labarraque's  solution 212 

Laborde's  method  for  the  estimation  of  mercuric  salts 233 

Lacmoid 21 

—  paper 22 

Lacmus 22 

Lactic  acid 106 

Laudanum,  assay  of 609 

Law  of  Boyle 680 

—  of  Charles 679 

Lead,  estimation  of 1 78 

—  estimation  of,  by  means  of  a  standard  sulphate  solution 392 

—  estimation  of  by  means  of  dichromate 388 

—  estimation  of,  by  precipitation  as  oxalate 393 


INDEX  713 

PAGB 

Lead,  estimation  of,  by  precipitation  as  chromate,  and  digestion  with  ferrous 

sulphate 390 

—  estimation  of,  by  titration  with  dichromate,  and  ferrous  sulphate 390 

—  estimation  of,  iodometrically,  after  precipitation  as  chromate 392 

—  ores,  assay  of 391 

—  peroxid,  estimation  of  by  digestion  method 222 

estimation  of  by  distillation  method 219,  393 

Lemon  juice 270 

Lime  juice 270 

Liquor  acidi  arsenosi,  assay  of 193 

—  iodi  compositus 208 

—  potassii  arsenitis,  assay  of 193 

—  sodse  chlorinatse 212 

Liter  flasks 42 

Litharge,  assay  of 389 

Lithium  bromid 119 

—  carbonate 79 

—  citrate 88 

Litmus 22 

—  paper 23 

LugoFs  solution 208 

Luteol 24 

Lyons'  method 516 

Magnesia  mixture 311 

Magnesium,  estimation  of,  as  phosphate 394 

—  estimation  of  by  means  of,  sodium  arsenate 395 

Mandarin  orange 24 

Manganates 208 

Manganese  dioxid,  estimation  of 400 

estimation  of,  by  digestion  method 222 

estimation  of,  by  distillation  method 218 

estimation  of,  by  means  of  arsenous  acid 230 

estimation  with  permanganate 171 

—  estimation  of,  by  bismuthate  method 404 

—  estimation  of,  by  direct  titration  with  permanganate 401 

—  estimation  of,  by  distillation 399 

—  estimation  of,  by  means  of  ferricyanid 402 

—  estimation  of,  by  Volhard's  titration  method 401 

—  ores,  estimation  of  the  available  oxygen  in 397 

Mayer's  solution 5°6 

Meniscus - 46 

Mercuric  chlorid,  decinormal  V.  S 409 

estimation  of,  cyanids  with 275 

estimation  of,  in  colored  tablets,  etc 413 

in  dressings,  assay  of 651 

V.S i34 


714  INDEX 

PAGE 

Mercuric  cyanid,  analysis  of 415 

solution  for  the  estimation  of  sugar 495 

—  iodid  solution  for  the  estimation  of  sugar 496 

—  potassium  iodid  solution 506 

—  salts,  estimation  of,  by  means  of  ferrous  sulphate 410 

estimation  of,  by  means  of  stannous  chlorid 233 

Mercurous  compounds 202 

Mercury,  ammoniated,  assay  of 416 

—  estimation  of,  as  mercurous  chlorid 408 

—  estimation  of,  by  direct  titration  with  thiosulphate 411 

—  estimation  of,  by  iodin 408 

—  estimation  of,  by  means  of  dichromate 418 

—  estimation  of,  by  means  of  potassium  cyanid 411 

—  estimation  of,  by  potassium  iodid 409 

—  estimation  of,  in  its  organic  compounds 415 

Methyl  aurin 29 

Methyl  orange 24 

characters  of  a  good  article 25 

-  —  T.  S 25 

Milk,  adulterations  of 459 

—  analysis 457 

—  calculation  method  for  fat  in 463 

—  composition  of 457 

of  that  of  different  animals 457 

—  detection  of  boric  acid  in 466 

of  formaldehyde  in 466 

—  estimation  of,  ash 464 

—  estimation  of,  fat  in 459 

—  estimation  of,  fat  in,  by  Adams'  method         ...     : 461 

—  estimation  of,  fat  in.  by  Babcock  centrifugal  method 462 

—  estimation  of,  fat  in,  by  Werner-Schmid  method 462 

—  estimation  of,  total  proteids  in 465 

—  estimation  of,  total  solids  and  water  in     459 

—  specific  gravity  table  for 460 

—  sugar,  estimation  of,  in  milk 466 

Molybdate  ammonium  solution 447 

Mohr's  burette 32 

—  distilling  apparatus 217 

—  foot  burette 33 

—  method  for  estimating  acetic  acid  in  vinegar.  . 237 

—  salt,  standardization  of  permanganate  V.  S.  with 147 

Mydriatic  drugs,  assay  of 527 

Naphthalamin  hydrochlorate 445 

Naphthylammonium  chlorid 445 

Nessler's  solution 440 

Neutral  salts,  assay  of,  after  conversion  into  chlorid 132 


INDEX  715 


Neutralization  analysis 62 

Nitrate  potassium,  standard  solution  of 444 

Nitrates .t 133,  294 

—  by  modified  Kjeldahl  method 293 

—  by  the  Schlossing  method 299 

—  by  the  Street- Ulsch  method 298 

—  by  the  Ulsch  method 298 

—  by  the  zinc  iron  method 297 

—  estimation  of 178 

—  estimation  of,  in  water 444 

—  estimation  (Pe'.ouze  method) 172 

—  nitrometer  assay  of 686 

Nickel  oxid.  estimation  of,  by  distillation  method 219 

Nitric  acid i  o } 

—  and  nitrates 294 

in  nitrates,  nitrometer  assay  of 686 

—  iodometric  estimation  of 301 

Nitrite  of  amyl,  assay  of 659 

nitrometer,  assay  of 685 

• —  of  sodium,  nitrometer  assay  of 685 

standard  solution  of 445 

Nitrites,  estimation  of,  by  means  of  permanganate 302 

—  estimation  of  in  water 445 

—  estimation,  U.  S.  P.  method 166 

—  nitrometer  assay  of 683 

Nitrogen 133 

—  and  its  compounds 283 

—  in  nitrates,  by  modified  Kjeldahl  method 293 

—  in  organic  substances,  Gunning  method 293 

—  in  organic  substances,  the  Jodlbauer- Kjeldahl  method 290 

—  in  organic  substances,  the  Kjeldahl  method 287 

• —  in  organic  substances,  Ruffle  method 286 

—  in  organic  substances,  Will  and  Varrentrapp's  method 283 

Nitrometer,  improvised 687 

-  The 678 

Nitrous  acid  and  nitrites,  estimation  with  permanganate 165 

—  estimation  of,  in  chamber  acid 303 

estimation  of,  by  means  of  permanganate 302 

—  iodometric  estimation  of. 303 

—  ether,  estimation  of 657 

nitrometer  assay  of 683 

Normal  hydrochloric  acid. 65 

—  oxalic  acid.    . 65 

—  solution 6 

Nux  vomica,  assay  of 556 

—  extract,  assay  of 590,  605 

fluid  extract,  assay  of 606,  607 

tincture,  assay  of 590,  606,  607 


716  INDEX 

PAGE 

Oils  and  fats,  Harms  number , 476 

the  bromin  value  of 478 

Wijs',  iodin  absorption  number 477 

—  acid  value  of 472 

—  fats  and  waxes,  examination  of 472 

—  iodin  absorption  number 474 

—  saponification  value  of 472 

—  volatile  fatty  acid  value 474 

Oleomargarin,  detection  of.  .  > 471 

Opium,  assay  of 563 

—  extract,  assay  of 608 

—  tincture,  assay  of 609 

Organic  acids 104 

—  salts  of  alkalies 83 

of  alkalies  and  alkali  earths 133 

Oxalates,  estimation  with  permanganate 153 

Oxalic  acid.  .  * 105 

and  oxalates 305 

decinormal : 65 

estimation  with  permanganate 1 53 

normal 65 

Oxidation  analyses 137 

—  and  reduction  analysis 137 

—  indirect 185 

Oxidizing  agents 138 

Oxy-chlor-diphenyl-quinoxaline 24 

Oxygen  and  peroxids 307 

—  consuming  power  of  water 446 

—  dissolved  in  water 307 

—  dissolved  in  water,  estimation  of,  by  hyposulphite 308 

—  dissolved  in  water,  Schiitzenberger's  method 308 

Para-rosolic  acid 20 

—  sulpho-benzeneazo-dimethylaniline 24 

Paris  green,  estimation  of  arsenic  in,  by  Smith's  method 350 

Pavy's  solution 493 

Percentage  estimations 53 

Permanganate,  analyses  by 138 

—  direct  titrations , 141 

—  empirical  solution,  use  of 148 

—  indirect  titration  methods 1.41 

—  method  for  the  estimation  of  sugar 495 

—  preparation  of  decinormal  solution  of 142 

—  residual  titrations 141,  l67 

—  solution,  standardization  of,  for  use  in  estimating  nitrous  acid 302 

—  titration  methods I41 

—  use  of,  for  standardizing  sodium  thiosulphate 207 


INDEX  717 

PAGE 

Permanganate,  V.  S.,  direct  titrations  with 1 50 

iodometric  standardization  of 146 

standardization  by  means  of  oxalic  acid 144 

standardization  of,  by  means  of  iron 142 

standardization  of,  by  means  of  sodium  oxalate 148 

standardization  of,  with  ferrous  ammonium  sulphate 147 

standardization  of,  with  hydrogen  peroxid  in  the  nitrometer 148 

typical  analyses  with 150 

Peroxid  of  hydrogen 309 

—  of  hydrogen,  assay  of,  gasorhetrically 687 

—  of  lead,  estimation  of,  by  distillation 393 

—  of  sodium 310 

Peroxids 203,   208 

Personne's  method  for  iodids. 134 

Persulphates,  estimation  of  active  oxygen  in. 310 

Pettenkofer's  method  for  carbonic  acid  gas  in  the  atmosphere 246 

for  estimating  acetic  acid  in  vinegar 237 

Phenacetalin 26 

Phenol,  assay  of 613 

Phenolphthalein 26 

Phosphates 132 

—  estimation  of,  in  water 447 

—  estimation  of,  mixed  disodium  and  trisodium 3  20 

—  in  urine 663 

Phosphoric  acid IOi 

estimated  by  means  of  standard  uranium 312 

estimation  of,  by  Gliickmann's  method 315 

estimation  of,  by  Pemberton's  molybdic  method 316 

estimation  of,  by  Pemberton's  new  method 317 

' Stolba's  method ^ir 

Physostigma,  assay  of 575 

—  extract,  assay  of 6IO 

Pilocarpus  fluid  extract,  assay  of 61 1 

—  leaves,  assay  of 577 

Pinch  cocks 40 

Pipettes 40 

Pcenin 29 

Poirrier  blue  (C4B) 27 

Poirer's  orange  III 24 

Potassium  acetate 87 

—  bicarbonate 78 

—  bi-iodate,  standard  solution  of,  for  estimation  of  iodids 265 

use  of  for  standardizing  sodium  thiosulphate 206 

—  bismuth  iodid  method  for  assay  of  galenicals 598 

—  bitartrate 36 

—  bromate,  estimation  of,  by  the  digestion  method 224 

—  bromid 


718  INDEX 

PAGE 

Potassium  carbonate 78 

—  chlorate,  estimation  of  by  the  digestion  method 224 

—  chromate  T.  S 27 

—  citrate 87 

—  cyanid 1 29 

—  dichromate  as  an  oxidizing  agent 8 

as  a  precipitating  agent 7 

estimation  of 176 

estimation  of  by  distillation  method 219 

—  ferricyanid  T.  S 27 

—  hydroxid 74 

V.  S 96 

standardization  of,  by  means  of  potassium  bi-iodate 98 

standardization  by  means  of  potassium  bitartrate 97 

—  iodate  solution,  titrations  with 263 

—  iodid 117 

decinormal  V.  S 409 

determination  of  the  amount  of  chlorid  present 1 18 

—  permanganate,  analyses  by 138 

as  an  oxidizing  agent 8 

preparation  of  decinormal  solution  of 142 

V.  S..  iodometric  standardization 146 

standardization  by  means  of  oxalic  acid 144 

standardization  by  means  of  sodium  oxalate 148 

standardization  of,  by  means  of  iron    142 

• standardization  of,  with  Mohr's  salt 147 

standardization  with  hydrogen  peroxid  in  the  nitrometer 148 

—  sulphite 200 

—  sulphocyanate  V.  S 114 

—  tartrate r 84 

—  and  sodium  tartrate.  .    85 

Precipitation  analysis no 

precision  in  determining  end-reactions  in in 

titration  without  indicator  (Gay-Lussac) 1 20 

with  chromate  indicator  (Mohr's  method) 1 20 

Prollius'  fluid 512 

Puckner's  method 516 

Pyrogallo-phthalein 20 

Ramsay's  bleaching  fluid 212 

.Reading  of  graduated  instruments 46 

Reducing  agents 138,  202 

Reduction,  analysis  by 137 

—  methods,  involving  the  use  of  standard  arsenous  acid  solution 226 

involving  the  use  of  stannous  chlorid 231 

Reichert  number 474 

Resazurin « •  «  —  •  -  -  - 28 


INDEX  719 

PAGE 

Residual  titration 1 1 

Re-titration 1 1 

Rochelle  salt 85 

Rosolic  acid 20,   28 

Sachsse's  method  for  the  estimation  of  sugar 496 

Salicy  laies 3  24 

Salicylic  acid 324 

—  and  salicylates,  estimation  by  means  of  bromin 325 

—  dressings,  assay  of 651 

iodometric  estimation  of 324 

Saponification  value  of  fats  and  oils 472 

Standard  acid  solutions,  preparation  of 64 

Saturation  anal)4sis,  see  neutralization  analysis 62 

Scale  salts,  estimation  of  alkaloidal  strength  of 612 

Schlossing's  method  for  nitrates 299 

Scopola,  assay  of 530 

Selection  of  the  indicator  (Glaser) 17 

—  of  the  indicator,  guide  for  the 30 

Separators 513 

Silver,  estimation  of 417 

—  metallic,  and  alloys 132 

—  nitrate,  assay  of,  by  means  of  sodium  chlorid 130 

assay  of,  by  means  of  sulphocyanate 131 

V.  S.,  preparation  of 112 

—  salts,  assay  of 130 

Smith  method  for  estimating  boric  acid 240 

Soap,  analysis  of . 480 

—  determination  of  fatty  acids  in 480 

of  glycerin  in 482 

—  standard  solution  of 449 

Sodium  acetate ' 88 

—  and  potassium  tartrate 85 

—  benzoate 88 

—  bicarbonate 79 

its  use  in  titrations  with  iodin 190 

—  bisulphite 201 

—  bromid 119 

—  carbonate 79 

V.  S 92 

—  chlorid 116 

decinormal  solution 2 

preparation  of  pure 113 

V.  S 1 13 

Soxhlet  extraction  apparatus 461 

Sodium  hydroxid 75 

V.  S..  99 


720  INDEX 

PAGE 

Sodium  hyposulphite  (thiosulphate) 201 

—  iodid 1 20 

—  nitrite,  nitrometer  assay  of 685 

—  salicylate 88 

—  sulphite 200 

—  thiosulphate 8,  201 

decinormal  solution 203 

estimations  involving  use  of 202 

Solution  of  chlorinated  soda 212 

Soxhlet-Fehling  method  for  sugar 494 

Sparteine 499 

Spirit  of  nitrous  ether,  assay  of 657 

— nitrometer  assay  of 683 

Spirits  of  ammonia 76 

Squibb's  solution  for  urea  estimation 693 

—  urea  apparatus 694 

Standard  potassium  iodate  solution. 263 

—  silver  nitrate  V.  S 112 

—  sodium  chlorid  V.  S 113 

—  solutions 6 

Standardization  of  decinormal  sodium  chlorid 113 

—  of  hydrochloric  acid  V.  S.,  by  means  of  borax 66 

by  means  of  calc-spar 69 

by  means  of  silver  nitrate  (gravimetrically) 67 

by  means  of  silver  nitrate  (volumetrically) 68 

by  means  of  sp.gr 66 

—  of  permanganate  V.  S.,  by  the  iodometric  method 146 

by  means  of  iron 142 

by  means  of  ferrous-ammonium  sulphate 1 47 

by  means  of  oxalic  acid 144 

by  means  of  sodium  oxalate 148 

with  hydrogen  peroxid,  in  the  nitrometer 148 

—  of  potassium  hydroxid  V.  S.,  by  means  of  potassium  bi-iodate 97 

by  means  of  dichromate.  .     204 

by  means  of  permanganate 207 

by  means  of  potassium  bi-iodate 206 

Pharmacopoeial  method 205 

—  of  sulphuric  acid  V.  S.,  by  ammonium  sulphate  method 72 

(iodometrically) 71 

by  means  of  barium  chlorid 73 

by  means  of  sodium  carbonate 70 

by  means  of  sp.gr 73 

—  of  sulphuric  acid  by  Wenig's  method. 72 

—  of  thiosulphate  V.  S.,  by  means  of  iodin  V.  S 203 

Standards,  in  sanitary  water  analyses 455 

Stannous  chlorid,  estimation  of 421 

V.  S.,  preparation  of 23 2 


INDEX  T21 

PAGB 

Stannous  compounds 202 

Starch,  determination  of,  by  means  of  barium  hydroxid 484 

—  determination  of,  in  cereals 484 

—  estimation  of,  after  inversion  by  means  of  acid 486 

—  estimation  of,  after  inversion  by  means  of  diastase 487 

—  iodized,  test  paper  of 228 

—  solution 29 

application  of 189 

preparation  of 189 

preservation  of - 189 

Stramonium  fluid  extract,  assay  of.    597 

—  leaves,  assay  of 531 

—  tincture,  assay  of 597 

Street-Ulsch  method  for  nitrates.  . 298 

Strontium  bromid 119 

—  iodid 125 

—  salts,  estimation  of 419 

Strophanthus,  assay  of 579 

Styptic  cotton,  assay  of 655 

Sublimate  dressings,  assay  of 651 

Sugar,  estimation  pf ,  by  Knapp's  method 495 

—  estimation  of,  by  Pavy's  solution 493 

—  estimation  of,  by  permanganate  method 495 

—  estimation  of,  by  Sachsse's  method 496 

—  estimation  of,  by  Soxhlet-Fehling  method 494 

—  in  urin,  estimation  of 492 

Sugars,  estimation  of,  by  Fehling's  solution 489 

Sulphanilic  acid 445 

Sulphates 132 

—  estimation  of 1 78 

—  estimation  of,  by  means  of  barium  chlorid 331 

—  estimation  of,  by  precipitation  as  lead  sulphate 333 

—  estimation  of,  with  barium  chlorid  and  potassium  dichromate 333 

—  in  urine 664 

Sulphid,  estimation  of • 327 

—  hydrogen 202 

Sulphites 200 

—  estimation  of 198,  33  r 

Sulphocyanate  method 122 

-V.  S 114 

Sulphocyanates 281 

—  estimation  of,  by  means  of  cupric  sulphate 281 

Sulphonic  acid - 445 

Sulphur  and  its  compounds 326 

—  in  alkali  sulphids 327 

Sulphuric  acid 101 

estimation  of 331 


7?2  INDEX 

PAGE 

Sulphuric  acid,  normal  V.  S „ . .  70 

V.  S.,  standardization,  by  ammonium  sulphate  method 72 

gravimetric  standardization,  by  means  of  barium  chlorid 73 

iodometric  standardization 71 

standardization,  by  means  of  sp.gr 73 

standardization,  by  means  of  sodium  carbonate 70 

Sulphurous  acid  and  sulphites,  estimation  of 197 

estimation  of 198,  33 1 

measuring  of 1 99 

Surgical  dressings,  assaying  of 649 

Table  of  alkalies,  alkali  earths  and  acids,  showing  normal  factors,  etc 57 

—  of  decinormal  factors  for  alkaloids ^04 

—  of  factors  for  correction  for  barometric  pressure 68 1 

—  of  factors  for  temperature  corrections 68 1 

—  of  multiples.  .    xix 

—  of  substances,  estimated  by  means  of  standard  iodin  solution 201 

—  of  substances,  estimated  by  permanganate  or  dichromate 185 

—  of  substances,  estimated  by  precipitation . 135 

—  of  the  acids 106 

—  of  the  alkali  earth  metals 94 

—  of  the  elements  and  atomic  weights xx 

—  of  the  organic  salts  of  the  alkalies 89 

—  serving  as  a  guide  for  the  selection  of  a  suitable  indicator 30 

—  showing  amount  of  deviation  in  calibrated  burette 50 

—  showing  behavior  of  alkaloids,  with  indicators 503 

—  showing  color  changes  of  indicators 502 

—  showing  iodin  absorption  by  the  three  methods 477 

—  showing  iodin  factors  of  the  alkaloids 508 

—  showing  quantity  of  substance  to  be  taken  in  direct  percentage  estimations.  107 

—  showing  relative  expansion  and  contraction  of  water  at  different  tempera- 

tures   45 

—  showing  U.  S.  P.  requirement  as  to  saponification  number  of  fats  and  oils.  473 

—  showing  volume  of  o.ooi  gm.  of  carbon  dioxid  at  various  temperatures..  249 

—  showing  weight  of  H2O2,  corresponding  to  volume  of  oxygen 690 

Tannic  acid,  estimation  of 630 

Tannin,  estimation  of 630 

—  in  barks,  estimation  of 63 1 

—  in  cloves  and  allspice,  estimation  of. 637 

—  in  tea,  estimation  of 636 

—  in  wines,  estimation  of 636 

Tartaric  acid 105 

Tartar  emetic,  estimation  of 196,  341 

Tea,  estimation  of  tannin  in 636 

Tetra-bromo-resorcin-phthalein 20 

—  iodo-fluorescein 21 

Tetrazo-dipheny  1-naphtionate 19 


INDEX  723 

PAGE 

The  burette 32 

Thiosulphate,  sodium 201 

-V.S 203 

estimations  involving  the  use  of 202 

standardization  of,  by  means  of  dichromate 204 

standardization  of,  by  means  of  iodin  V.  S 203 

—  standardization  of,  by  means  of  permanganate 207 

—  standardization  of,  by  means  of  potassium  bi-iodate.  .  .     206 

Tin,  estimation  of  (Lowenthal) 177 

—  estimation  of,  by  means  of  potassium  dichromate.  .    423 

—  estimation  of,  by  titration  with  iodin  in  acid  solution 422 

—  estimation  by  titration  with  iodin  in  alkaline  solution 421 

—  indirect  titration  by  ferric  chlorid  and  permanganate. 422 

Tincture  of  aconite,  assay  of 593 

—  of  aconite  root,  assay  of 588 

—  of  belladonna,  assay  of 589,  594 

—  of  cinchona,  assay  of 589 

—  of  hydrastis,  assay  of 603 

—  of  hyoscyamus,  assay  of. 589,  597 

—  of  iodin 208 

—  of  nux  vomica,  assay  of 590,  606,  607 

—  of  opium,  assay  of 609 

—  of  stramonium,  assay  of 597 

Tinctures,  assay  of 584,  586,  590 

—  containing  chlorophyll,  assay  of 587 

Titer 6 

Titration,  residual 1 1 

Tobacco,  assay  of 580 

Total  solids,  estimation  of,  in  urine 662 

Triphenylrosanilin 27 

Tropaeolin  D 24 

—  O.O 29 

Turmeric  paper 29 

—  tincture 29 

Type  metal,  assay  of 341 

Ulsch  method  for  nitrates 298 

Urea  apparatus,  Squibb's 694 

—  estimation  of 692 

—  in  urine , 667 

Uric  acid,  estimation  of 692 

•  —  in  urine 667 

Urine,  albumen  in 672 

—  analysis  of 660 

—  bile  in 677 

—  blood  in 674 

—  chlorids  in 663 


724  INDEX 

PAT.E 

Urine,  composition  of 660 

—  estimation  of  sugar  in 492 

—  ethereal  sulphates  in 665 

—  phosphates  in 663 

—  pus  in 674 

—  sugar  in 674 

—  sulphates  in 664 

—  total  acidity  of.    : 66  ; 

—  urea  in 667,  69  : 

—  uric  acid  in 667,  692,  696 

Ureometer,  Doremus' 692 

—  Hinds-Doremus 693 

—  Sellier's 695 

Use  of  apparatus 44 

Veratrum,  assay  of 581 

Vicarious  methods — ' 58 

Vielhaber's  method  for  cyanogen 127 

Vinegar,  estimation  of 235 

—  estimation  of  acetic  acid  in -. 237 

—  estimation  of  free  mineral  acids  in. 235 

Volatile  fatty  acid,  value  of  oils 474 

Volhard's  method 1 20,  122 

—  solution 114 

Volumetric  analysis  by  weight 59 

—  —  without  a  burette 59 

without  weights  and  standard  solutions 58 

—  or  standard  solutions 6 

Wagner's  reagent,  estimation  of  alkaloids  with, . 507 

Water,  carbonic  acid  in,  Lunge-Trillich  method 255 

in,  Seyler  method 255 

—  determination  of  hardness 448 

—  estimation  of  chlorin  in 439 

—  estimation  of  free  ammonia  in 440 

—  estimation  of  nitrates  in 444 

—  estimation  of  nitrites  in 445 

—  estimation  of  nitrogen  in 444 

—  interpretation  of  results  of  analyses 451 

—  organic  and  volatile  matter 439 

—  oxygen-consuming  power  of 446 

—  phosphates  in 447 

—  sanitary  analysis  of 43  7 

—  total  solids  in 438 

Waxes,  fats  and  oils,  examination  of.  * 472 

Weber's  method  for  the  valuation  of  acetate  of  lime.  .  .  238 

Weighing  flask. 99 


INDEX  725 

PAGE 

Weight  burette 59 

—  and  measures  used  in  volumetric  analysis 51 

Wijs'  iodin  number 477 

Wild  cherry  bark,  assay  of 583 

Wilson's  bleaching  fluid 212 

Wines,  estimation  of  tannin  in 636 

Zinc  bromid 119 

—  estimation  of,  as  arsenate 435 

—  estimation  of,  as  ferrocyanid 430 

' —  estimation  of,  as  oxalate 425 

—  estimation  of,  as  sulphid,  by  means  of  ferric  salt  and  permanganate 428 

—  estimation  of,  as  sulphid,  using  iron  indicator 4^0 

—  estimation  of,  by  precipitation  as  phosphate 437 

—  estimation  of,  by  precipitation  with  standard  sodium  sulphid 427 

—  estimation  of,  indirect  method 426 

—  estimation  of,  in  the  presence  of  manganese 433 

—  metallic  estimation  of 202 

—  iodid 125 

—  iron  method  for  nitrates 297 


UNIVERSITY   CF   CALIFORNIA 


